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2019-09-16Merge branch 'sched-core-for-linus' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull scheduler updates from Ingo Molnar: - MAINTAINERS: Add Mark Rutland as perf submaintainer, Juri Lelli and Vincent Guittot as scheduler submaintainers. Add Dietmar Eggemann, Steven Rostedt, Ben Segall and Mel Gorman as scheduler reviewers. As perf and the scheduler is getting bigger and more complex, document the status quo of current responsibilities and interests, and spread the review pain^H^H^H^H fun via an increase in the Cc: linecount generated by scripts/get_maintainer.pl. :-) - Add another series of patches that brings the -rt (PREEMPT_RT) tree closer to mainline: split the monolithic CONFIG_PREEMPT dependencies into a new CONFIG_PREEMPTION category that will allow the eventual introduction of CONFIG_PREEMPT_RT. Still a few more hundred patches to go though. - Extend the CPU cgroup controller with uclamp.min and uclamp.max to allow the finer shaping of CPU bandwidth usage. - Micro-optimize energy-aware wake-ups from O(CPUS^2) to O(CPUS). - Improve the behavior of high CPU count, high thread count applications running under cpu.cfs_quota_us constraints. - Improve balancing with SCHED_IDLE (SCHED_BATCH) tasks present. - Improve CPU isolation housekeeping CPU allocation NUMA locality. - Fix deadline scheduler bandwidth calculations and logic when cpusets rebuilds the topology, or when it gets deadline-throttled while it's being offlined. - Convert the cpuset_mutex to percpu_rwsem, to allow it to be used from setscheduler() system calls without creating global serialization. Add new synchronization between cpuset topology-changing events and the deadline acceptance tests in setscheduler(), which were broken before. - Rework the active_mm state machine to be less confusing and more optimal. - Rework (simplify) the pick_next_task() slowpath. - Improve load-balancing on AMD EPYC systems. - ... and misc cleanups, smaller fixes and improvements - please see the Git log for more details. * 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (53 commits) sched/psi: Correct overly pessimistic size calculation sched/fair: Speed-up energy-aware wake-ups sched/uclamp: Always use 'enum uclamp_id' for clamp_id values sched/uclamp: Update CPU's refcount on TG's clamp changes sched/uclamp: Use TG's clamps to restrict TASK's clamps sched/uclamp: Propagate system defaults to the root group sched/uclamp: Propagate parent clamps sched/uclamp: Extend CPU's cgroup controller sched/topology: Improve load balancing on AMD EPYC systems arch, ia64: Make NUMA select SMP sched, perf: MAINTAINERS update, add submaintainers and reviewers sched/fair: Use rq_lock/unlock in online_fair_sched_group cpufreq: schedutil: fix equation in comment sched: Rework pick_next_task() slow-path sched: Allow put_prev_task() to drop rq->lock sched/fair: Expose newidle_balance() sched: Add task_struct pointer to sched_class::set_curr_task sched: Rework CPU hotplug task selection sched/{rt,deadline}: Fix set_next_task vs pick_next_task sched: Fix kerneldoc comment for ia64_set_curr_task ...
2019-09-16Merge branch 'sched/rt' into sched/core, to pick up -rt changesIngo Molnar
Pick up the first couple of patches working towards PREEMPT_RT. Signed-off-by: Ingo Molnar <mingo@kernel.org>
2019-08-13rcu: Allow rcu_do_batch() to dynamically adjust batch sizesEric Dumazet
Bimodal behavior of rcu_do_batch() is not really suited to Google applications like gfe servers. When a process with millions of sockets exits, closing all files queues two rcu callbacks per socket. This eventually reaches the point where RCU enters an emergency mode, where rcu_do_batch() do not return until whole queue is flushed. Each rcu callback lasts at least 70 nsec, so with millions of elements, we easily spend more than 100 msec without rescheduling. Goal of this patch is to avoid the infamous message like following "need_resched set for > 51999388 ns (52 ticks) without schedule" We dynamically adjust the number of elements we process, instead of 10 / INFINITE choices, we use a floor of ~1 % of current entries. If the number is above 1000, we switch to a time based limit of 3 msec per batch, adjustable with /sys/module/rcutree/parameters/rcu_resched_ns Signed-off-by: Eric Dumazet <edumazet@google.com> [ paulmck: Forward-port and remove debug statements. ] Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Don't wake no-CBs GP kthread if timer posted under overloadPaul E. McKenney
When under overload conditions, __call_rcu_nocb_wake() will wake the no-CBs GP kthread any time the no-CBs CB kthread is asleep or there are no ready-to-invoke callbacks, but only after a timer delay. If the no-CBs GP kthread has a ->nocb_bypass_timer pending, the deferred wakeup from __call_rcu_nocb_wake() is redundant. This commit therefore makes __call_rcu_nocb_wake() avoid posting the redundant deferred wakeup if ->nocb_bypass_timer is pending. This requires adding a bit of ordering of timer actions. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Reduce __call_rcu_nocb_wake() leaf rcu_node ->lock contentionPaul E. McKenney
Currently, __call_rcu_nocb_wake() advances callbacks each time that it detects excessive numbers of callbacks, though only if it succeeds in conditionally acquiring its leaf rcu_node structure's ->lock. Despite the conditional acquisition of ->lock, this does increase contention. This commit therefore avoids advancing callbacks unless there are callbacks in ->cblist whose grace period has completed and advancing has not yet been done during this jiffy. Note that this decision does not take the presence of new callbacks into account. That is because on this code path, there will always be at least one new callback, namely the one we just enqueued. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Reduce nocb_cb_wait() leaf rcu_node ->lock contentionPaul E. McKenney
Currently, nocb_cb_wait() advances callbacks on each pass through its loop, though only if it succeeds in conditionally acquiring its leaf rcu_node structure's ->lock. Despite the conditional acquisition of ->lock, this does increase contention. This commit therefore avoids advancing callbacks unless there are callbacks in ->cblist whose grace period has completed. Note that nocb_cb_wait() doesn't worry about callbacks that have not yet been assigned a grace period. The idea is that the only reason for nocb_cb_wait() to advance callbacks is to allow it to continue invoking callbacks. Time will tell whether this is the correct choice. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Advance CBs after merge in rcutree_migrate_callbacks()Paul E. McKenney
The rcutree_migrate_callbacks() invokes rcu_advance_cbs() on both the offlined CPU's ->cblist and that of the surviving CPU, then merges them. However, after the merge, and of the offlined CPU's callbacks that were not ready to be invoked will no longer be associated with a grace-period number. This commit therefore invokes rcu_advance_cbs() one more time on the merged ->cblist in order to assign a grace-period number to these callbacks. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Avoid synchronous wakeup in __call_rcu_nocb_wake()Paul E. McKenney
When callbacks are in full flow, the common case is waiting for a grace period, and this grace period will normally take a few jiffies to complete. It therefore isn't all that helpful for __call_rcu_nocb_wake() to do a synchronous wakeup in this case. This commit therefore turns this into a timer-based deferred wakeup of the no-CBs grace-period kthread. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Print no-CBs diagnostics when rcutorture writer unduly delayedPaul E. McKenney
This commit causes locking, sleeping, and callback state to be printed for no-CBs CPUs when the rcutorture writer is delayed sufficiently for rcutorture to complain. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: EXP Check use and usefulness of ->nocb_lock_contendedPaul E. McKenney
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Add bypass callback queueingPaul E. McKenney
Use of the rcu_data structure's segmented ->cblist for no-CBs CPUs takes advantage of unrelated grace periods, thus reducing the memory footprint in the face of floods of call_rcu() invocations. However, the ->cblist field is a more-complex rcu_segcblist structure which must be protected via locking. Even though there are only three entities which can acquire this lock (the CPU invoking call_rcu(), the no-CBs grace-period kthread, and the no-CBs callbacks kthread), the contention on this lock is excessive under heavy stress. This commit therefore greatly reduces contention by provisioning an rcu_cblist structure field named ->nocb_bypass within the rcu_data structure. Each no-CBs CPU is permitted only a limited number of enqueues onto the ->cblist per jiffy, controlled by a new nocb_nobypass_lim_per_jiffy kernel boot parameter that defaults to about 16 enqueues per millisecond (16 * 1000 / HZ). When that limit is exceeded, the CPU instead enqueues onto the new ->nocb_bypass. The ->nocb_bypass is flushed into the ->cblist every jiffy or when the number of callbacks on ->nocb_bypass exceeds qhimark, whichever happens first. During call_rcu() floods, this flushing is carried out by the CPU during the course of its call_rcu() invocations. However, a CPU could simply stop invoking call_rcu() at any time. The no-CBs grace-period kthread therefore carries out less-aggressive flushing (every few jiffies or when the number of callbacks on ->nocb_bypass exceeds (2 * qhimark), whichever comes first). This means that the no-CBs grace-period kthread cannot be permitted to do unbounded waits while there are callbacks on ->nocb_bypass. A ->nocb_bypass_timer is used to provide the needed wakeups. [ paulmck: Apply Coverity feedback reported by Colin Ian King. ] Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Atomic ->len field in rcu_segcblist structurePaul E. McKenney
Upcoming ->nocb_lock contention-reduction work requires that the rcu_segcblist structure's ->len field be concurrently manipulated, but only if there are no-CBs CPUs in the kernel. This commit therefore makes this ->len field be an atomic_long_t, but only in CONFIG_RCU_NOCB_CPU=y kernels. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Unconditionally advance and wake for excessive CBsPaul E. McKenney
When there are excessive numbers of callbacks, and when either the corresponding no-CBs callback kthread is asleep or there is no more ready-to-invoke callbacks, and when least one callback is pending, __call_rcu_nocb_wake() will advance the callbacks, but refrain from awakening the corresponding no-CBs grace-period kthread. However, because rcu_advance_cbs_nowake() is used, it is possible (if a bit unlikely) that the needed advancement could not happen due to a grace period not being in progress. Plus there will always be at least one pending callback due to one having just now been enqueued. This commit therefore attempts to advance callbacks and awakens the no-CBs grace-period kthread when there are excessive numbers of callbacks posted and when the no-CBs callback kthread is not in a position to do anything helpful. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Reduce ->nocb_lock contention with separate ->nocb_gp_lockPaul E. McKenney
The sleep/wakeup of the no-CBs grace-period kthreads is synchronized using the ->nocb_lock of the first CPU corresponding to that kthread. This commit provides a separate ->nocb_gp_lock for this purpose, thus reducing contention on ->nocb_lock. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Reduce contention at no-CBs invocation-done timePaul E. McKenney
Currently, nocb_cb_wait() unconditionally acquires the leaf rcu_node ->lock to advance callbacks when done invoking the previous batch. It does this while holding ->nocb_lock, which means that contention on the leaf rcu_node ->lock visits itself on the ->nocb_lock. This commit therefore makes this lock acquisition conditional, forgoing callback advancement when the leaf rcu_node ->lock is not immediately available. (In this case, the no-CBs grace-period kthread will eventually do any needed callback advancement.) Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Reduce contention at no-CBs registry-time CB advancementPaul E. McKenney
Currently, __call_rcu_nocb_wake() conditionally acquires the leaf rcu_node structure's ->lock, and only afterwards does rcu_advance_cbs_nowake() check to see if it is possible to advance callbacks without potentially needing to awaken the grace-period kthread. Given that the no-awaken check can be done locklessly, this commit reverses the order, so that rcu_advance_cbs_nowake() is invoked without holding the leaf rcu_node structure's ->lock and rcu_advance_cbs_nowake() checks the grace-period state before conditionally acquiring that lock, thus reducing the number of needless acquistions of the leaf rcu_node structure's ->lock. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Round down for number of no-CBs grace-period kthreadsPaul E. McKenney
Currently, when the square root of the number of CPUs is rounded down by int_sqrt(), this round-down is applied to the number of callback kthreads per grace-period kthreads. This makes almost no difference for large systems, but results in oddities such as three no-CBs grace-period kthreads for a five-CPU system, which is a bit excessive. This commit therefore causes the round-down to apply to the number of no-CBs grace-period kthreads, so that systems with from four to eight CPUs have only two no-CBs grace period kthreads. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Avoid ->nocb_lock capture by corresponding CPUPaul E. McKenney
A given rcu_data structure's ->nocb_lock can be acquired very frequently by the corresponding CPU and occasionally by the corresponding no-CBs grace-period and callbacks kthreads. In particular, these two kthreads will have frequent gaps between ->nocb_lock acquisitions that are roughly a grace period in duration. This means that any excessive ->nocb_lock contention will be due to the CPU's acquisitions, and this in turn enables a very naive contention-avoidance strategy to be quite effective. This commit therefore modifies rcu_nocb_lock() to first attempt a raw_spin_trylock(), and to atomically increment a separate ->nocb_lock_contended across a raw_spin_lock(). This new ->nocb_lock_contended field is checked in __call_rcu_nocb_wake() when interrupts are enabled, with a spin-wait for contending acquisitions to complete, thus allowing the kthreads a chance to acquire the lock. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Avoid needless wakeups of no-CBs grace-period kthreadPaul E. McKenney
Currently, the code provides an extra wakeup for the no-CBs grace-period kthread if one of its CPUs is generating excessive numbers of callbacks. But satisfying though it is to wake something up when things are going south, unless the thing being awakened can actually help solve the problem, that extra wakeup does nothing but consume additional CPU time, which is exactly what you don't want during a call_rcu() flood. This commit therefore avoids doing anything if the corresponding no-CBs callback kthread is going full tilt. Otherwise, if advancing callbacks immediately might help and if the leaf rcu_node structure's lock is immediately available, this commit invokes a new variant of rcu_advance_cbs() that advances callbacks only if doing so won't require awakening the grace-period kthread (not to be confused with any of the no-CBs grace-period kthreads). Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Make __call_rcu_nocb_wake() safe for many callbacksPaul E. McKenney
It might be hard to imagine having more than two billion callbacks queued on a single CPU's ->cblist, but someone will do it sometime. This commit therefore makes __call_rcu_nocb_wake() handle this situation by upgrading local variable "len" from "int" to "long". Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Never downgrade ->nocb_defer_wakeup in wake_nocb_gp_defer()Paul E. McKenney
Currently, wake_nocb_gp_defer() simply stores whatever waketype was passed in, which can result in a RCU_NOCB_WAKE_FORCE being downgraded to RCU_NOCB_WAKE, which could in turn delay callback processing. This commit therefore adds a check so that wake_nocb_gp_defer() only updates ->nocb_defer_wakeup when the update increases the forcefulness, thus avoiding downgrades. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Enable re-awakening under high callback loadPaul E. McKenney
The __call_rcu_nocb_wake() function and its predecessors set ->qlen_last_fqs_check to zero for the first callback and to LONG_MAX / 2 for forced reawakenings. The former can result in a too-quick reawakening when there are many callbacks ready to invoke and the latter prevents a second reawakening. This commit therefore sets ->qlen_last_fqs_check to the current number of callbacks in both cases. While in the area, this commit also moves both assignments under ->nocb_lock. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nohz: Turn off tick for offloaded CPUsPaul E. McKenney
Historically, no-CBs CPUs allowed the scheduler-clock tick to be unconditionally disabled on any transition to idle or nohz_full userspace execution (see the rcu_needs_cpu() implementations). Unfortunately, the checks used by rcu_needs_cpu() are defeated now that no-CBs CPUs use ->cblist, which might make users of battery-powered devices rather unhappy. This commit therefore adds explicit rcu_segcblist_is_offloaded() checks to return to the historical energy-efficient semantics. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Suppress uninitialized false-positive in nocb_gp_wait()Paul E. McKenney
Some compilers complain that wait_gp_seq might be used uninitialized in nocb_gp_wait(). This cannot actually happen because when wait_gp_seq is uninitialized, needwait_gp must be false, which prevents wait_gp_seq from being used. But this analysis is apparently beyond some compilers, so this commit adds a bogus initialization of wait_gp_seq for the sole purpose of suppressing the false-positive warning. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Use build-time no-CBs check in rcu_pending()Paul E. McKenney
Currently, rcu_pending() invokes rcu_segcblist_is_offloaded() even in CONFIG_RCU_NOCB_CPU=n kernels, which cannot possibly be offloaded. Given that rcu_pending() is on a fastpath, it makes sense to check for CONFIG_RCU_NOCB_CPU=y before invoking rcu_segcblist_is_offloaded(). This commit therefore makes this change. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Use build-time no-CBs check in rcu_core()Paul E. McKenney
Currently, rcu_core() invokes rcu_segcblist_is_offloaded() each time it needs to know whether the current CPU is a no-CBs CPU. Given that it is not possible to change the no-CBs status of a CPU after boot, and given that it is not possible to even have no-CBs CPUs in CONFIG_RCU_NOCB_CPU=n kernels, this repeated runtime invocation wastes CPU. This commit therefore created a const on-stack variable to allow this check to be done only once per rcu_core() invocation. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Use build-time no-CBs check in rcu_do_batch()Paul E. McKenney
Currently, rcu_do_batch() invokes rcu_segcblist_is_offloaded() each time it needs to know whether the current CPU is a no-CBs CPU. Given that it is not possible to change the no-CBs status of a CPU after boot, and given that it is not possible to even have no-CBs CPUs in CONFIG_RCU_NOCB_CPU=n kernels, this per-callback invocation wastes CPU. This commit therefore created a const on-stack variable to allow this check to be done only once per rcu_do_batch() invocation. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Remove obsolete nocb_gp_head and nocb_gp_tail fieldsPaul E. McKenney
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Remove obsolete nocb_cb_tail and nocb_cb_head fieldsPaul E. McKenney
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Remove obsolete nocb_q_count and nocb_q_count_lazy fieldsPaul E. McKenney
This commit removes the obsolete nocb_q_count and nocb_q_count_lazy fields, also removing rcu_get_n_cbs_nocb_cpu(), adjusting rcu_get_n_cbs_cpu(), and making rcutree_migrate_callbacks() once again disable the ->cblist fields of offline CPUs. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Remove obsolete nocb_head and nocb_tail fieldsPaul E. McKenney
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Use rcu_segcblist for no-CBs CPUsPaul E. McKenney
Currently the RCU callbacks for no-CBs CPUs are queued on a series of ad-hoc linked lists, which means that these callbacks cannot benefit from "drive-by" grace periods, thus suffering needless delays prior to invocation. In addition, the no-CBs grace-period kthreads first wait for callbacks to appear and later wait for a new grace period, which means that callbacks appearing during a grace-period wait can be delayed. These delays increase memory footprint, and could even result in an out-of-memory condition. This commit therefore enqueues RCU callbacks from no-CBs CPUs on the rcu_segcblist structure that is already used by non-no-CBs CPUs. It also restructures the no-CBs grace-period kthread to be checking for incoming callbacks while waiting for grace periods. Also, instead of waiting for a new grace period, it waits for the closest grace period that will cause some of the callbacks to be safe to invoke. All of these changes reduce callback latency and thus the number of outstanding callbacks, in turn reducing the probability of an out-of-memory condition. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Leave ->cblist enabled for no-CBs CPUsPaul E. McKenney
As a first step towards making no-CBs CPUs use the ->cblist, this commit leaves the ->cblist enabled for these CPUs. The main reason to make no-CBs CPUs use ->cblist is to take advantage of callback numbering, which will reduce the effects of missed grace periods which in turn will reduce forward-progress problems for no-CBs CPUs. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Allow lockless use of rcu_segcblist_empty()Paul E. McKenney
Currently, rcu_segcblist_empty() assumes that the callback list is not being changed by other CPUs, but upcoming changes will require it to operate locklessly. This commit therefore adds the needed READ_ONCE() call, along with the WRITE_ONCE() calls when updating the callback list's ->head field. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Allow lockless use of rcu_segcblist_restempty()Paul E. McKenney
Currently, rcu_segcblist_restempty() assumes that the callback list is not being changed by other CPUs, but upcoming changes will require it to operate locklessly. This commit therefore adds the needed READ_ONCE() calls, along with the WRITE_ONCE() calls when updating the callback list. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Remove deferred wakeup checks for extended quiescent statesPaul E. McKenney
The idea behind the checks for extended quiescent states at the end of __call_rcu_nocb() is to handle cases where call_rcu() is invoked directly from within an extended quiescent state, for example, from the idle loop. However, this will result in a timer-mediated deferred wakeup, which will cause the needed wakeup to happen within a jiffy or thereabouts. There should be no forward-progress concerns, and if there are, the proper response is to exit the extended quiescent state while executing the endless blast of call_rcu() invocations, for example, using RCU_NONIDLE(). Given the more realistic case of an isolated call_rcu() invocation, there should be no problem. This commit therefore removes the checks for invoking call_rcu() within an extended quiescent state for on no-CBs CPUs. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Check for deferred nocb wakeups before nohz_full early exitPaul E. McKenney
In theory, a timer is used to defer wakeups of no-CBs grace-period kthreads when the wakeup cannot be done safely directly from the call_rcu(). In practice, the one-jiffy delay is not always consistent with timely callback invocation under heavy call_rcu() loads. Therefore, there are a number of checks for a pending deferred wakeup, including from the scheduling-clock interrupt. Unfortunately, this check follows the rcu_nohz_full_cpu() early exit, which renders it useless on such CPUs. This commit therefore moves the check for the pending deferred no-CB wakeup to precede the rcu_nohz_full_cpu() early exit. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Make rcutree_migrate_callbacks() start at leaf rcu_node structurePaul E. McKenney
Because rcutree_migrate_callbacks() is invoked infrequently and because an exact snapshot of the grace-period state might save some callbacks a second trip through a grace period, this function has used the root rcu_node structure. However, this safe-second-trip optimization happens only if rcutree_migrate_callbacks() races with grace-period initialization, so it is not worth the added mental load. This commit therefore makes rcutree_migrate_callbacks() start with the leaf rcu_node structures, as is done elsewhere. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Add checks for offloaded callback processingPaul E. McKenney
This commit is a preparatory patch for offloaded callbacks using the same ->cblist structure used by non-offloaded callbacks. It therefore adds rcu_segcblist_is_offloaded() calls where they will be needed when !rcu_segcblist_is_enabled() no longer flags the offloaded case. It also adds checks in rcu_do_batch() to ensure that there are no missed checks: Currently, it should not be possible for offloaded execution to reach rcu_do_batch(), though this will change later in this series. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Use separate flag to indicate offloaded ->cblistPaul E. McKenney
RCU callback processing currently uses rcu_is_nocb_cpu() to determine whether or not the current CPU's callbacks are to be offloaded. This works, but it is not so good for cache locality. Plus use of ->cblist for offloaded callbacks will greatly increase the frequency of these checks. This commit therefore adds a ->offloaded flag to the rcu_segcblist structure to provide a more flexible and cache-friendly means of checking for callback offloading. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Use separate flag to indicate disabled ->cblistPaul E. McKenney
NULLing the RCU_NEXT_TAIL pointer was a clever way to save a byte, but forward-progress considerations would require that this pointer be both NULL and non-NULL, which, absent a quantum-computer port of the Linux kernel, simply won't happen. This commit therefore creates as separate ->enabled flag to replace the current NULL checks. [ paulmck: Add include files per 0day test robot and -next. ] Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Print gp/cb kthread hierarchy if dump_treePaul E. McKenney
This commit causes the no-CBs grace-period/callback hierarchy to be printed to the console when the dump_tree kernel boot parameter is set. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Rename rcu_nocb_leader_stride kernel boot parameterPaul E. McKenney
This commit changes the name of the rcu_nocb_leader_stride kernel boot parameter to rcu_nocb_gp_stride in order to account for the new distinction between callback and grace-period no-CBs kthreads. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Rename and document no-CB CB kthread sleep trace eventPaul E. McKenney
The nocb_cb_wait() function traces a "FollowerSleep" trace_rcu_nocb_wake() event, which never was documented and is now misleading. This commit therefore changes "FollowerSleep" to "CBSleep", documents this, and updates the documentation for "Sleep" as well. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Rename rcu_organize_nocb_kthreads() local variablePaul E. McKenney
This commit renames rdp_leader to rdp_gp in order to account for the new distinction between callback and grace-period no-CBs kthreads. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Rename wake_nocb_leader_defer() to wake_nocb_gp_defer()Paul E. McKenney
This commit adjusts naming to account for the new distinction between callback and grace-period no-CBs kthreads. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Rename __wake_nocb_leader() to __wake_nocb_gp()Paul E. McKenney
This commit adjusts naming to account for the new distinction between callback and grace-period no-CBs kthreads. While in the area, it also updates local variables. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Rename wake_nocb_leader() to wake_nocb_gp()Paul E. McKenney
This commit adjusts naming to account for the new distinction between callback and grace-period no-CBs kthreads. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Rename nocb_follower_wait() to nocb_cb_wait()Paul E. McKenney
This commit adjusts naming to account for the new distinction between callback and grace-period no-CBs kthreads. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-08-13rcu/nocb: Provide separate no-CBs grace-period kthreadsPaul E. McKenney
Currently, there is one no-CBs rcuo kthread per CPU, and these kthreads are divided into groups. The first rcuo kthread to come online in a given group is that group's leader, and the leader both waits for grace periods and invokes its CPU's callbacks. The non-leader rcuo kthreads only invoke callbacks. This works well in the real-time/embedded environments for which it was intended because such environments tend not to generate all that many callbacks. However, given huge floods of callbacks, it is possible for the leader kthread to be stuck invoking callbacks while its followers wait helplessly while their callbacks pile up. This is a good recipe for an OOM, and rcutorture's new callback-flood capability does generate such OOMs. One strategy would be to wait until such OOMs start happening in production, but similar OOMs have in fact happened starting in 2018. It would therefore be wise to take a more proactive approach. This commit therefore features per-CPU rcuo kthreads that do nothing but invoke callbacks. Instead of having one of these kthreads act as leader, each group has a separate rcog kthread that handles grace periods for its group. Because these rcuog kthreads do not invoke callbacks, callback floods on one CPU no longer block callbacks from reaching the rcuc callback-invocation kthreads on other CPUs. This change does introduce additional kthreads, however: 1. The number of additional kthreads is about the square root of the number of CPUs, so that a 4096-CPU system would have only about 64 additional kthreads. Note that recent changes decreased the number of rcuo kthreads by a factor of two (CONFIG_PREEMPT=n) or even three (CONFIG_PREEMPT=y), so this still represents a significant improvement on most systems. 2. The leading "rcuo" of the rcuog kthreads should allow existing scripting to affinity these additional kthreads as needed, the same as for the rcuop and rcuos kthreads. (There are no longer any rcuob kthreads.) 3. A state-machine approach was considered and rejected. Although this would allow the rcuo kthreads to continue their dual leader/follower roles, it complicates callback invocation and makes it more difficult to consolidate rcuo callback invocation with existing softirq callback invocation. The introduction of rcuog kthreads should thus be acceptable. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>