From c628db0a6831f80e89873ee44f1b40e3ab3216c6 Mon Sep 17 00:00:00 2001 From: Shrikanth Hegde Date: Mon, 25 Mar 2024 11:15:04 +0530 Subject: sched/fair: Check root_domain::overload value before update The root_domain::overload flag is 1 when there's any rq in the root domain that has 2 or more running tasks. (Ie. it's overloaded.) The root_domain structure itself is a global structure per cpuset island. The ::overload flag is maintained the following way: - Set when adding a second task to the runqueue. - It is cleared in update_sd_lb_stats() during load balance, if none of the rqs have 2 or more running tasks. This flag is used during newidle balance to see if its worth doing a full load balance pass, which can be an expensive operation. If it is set, then newidle balance will try to aggressively pull a task. Since commit: 630246a06ae2 ("sched/fair: Clean-up update_sg_lb_stats parameters") ::overload is being written unconditionally, even if it has the same value. The change in value of this depends on the workload, but on typical workloads, it doesn't change all that often: a system is either dominantly overloaded for substantial amounts of time, or not. Extra writes to this semi-global structure cause unnecessary overhead, extra bus traffic, etc. - so avoid it as much as possible. Perf probe stats show that it's worth making this change (numbers are with patch applied): 1M probe:sched_balance_newidle_L38 139 probe:update_sd_lb_stats_L53 <====== 1->0 writes 129K probe:add_nr_running_L12 74 probe:add_nr_running_L13 <====== 0->1 writes 54K probe:update_sd_lb_stats_L50 <====== reads These numbers prove that actual change in the ::overload value is (much) less frequent: L50 is much larger at ~54,000 accesses vs L53+L13 of 139+74. [ mingo: Rewrote the changelog. ] Signed-off-by: Shrikanth Hegde Signed-off-by: Ingo Molnar Reviewed-by: Qais Yousef Reviewed-by: Vincent Guittot Cc: Mel Gorman Link: https://lore.kernel.org/r/20240325054505.201995-2-sshegde@linux.ibm.com --- kernel/sched/fair.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'kernel/sched') diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 38462305bc39..600fdde1e7ac 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -10657,7 +10657,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd if (!env->sd->parent) { /* update overload indicator if we are at root domain */ - WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD); + if (READ_ONCE(env->dst_rq->rd->overload) != (sg_status & SG_OVERLOAD)) + WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD); /* Update over-utilization (tipping point, U >= 0) indicator */ set_rd_overutilized_status(env->dst_rq->rd, -- cgit v1.2.3