sched/fair: Disable the task group load_avg update for the root_task_group
Currently, the update_tg_load_avg() function attempts to update the tg's load_avg value whenever the load changes even for root_task_group where the load_avg value will never be used. This patch will disable the load_avg update when the given task group is the root_task_group. Running a Java benchmark with noautogroup and a 4.3 kernel on a 16-socket IvyBridge-EX system, the amount of CPU time (as reported by perf) consumed by task_tick_fair() which includes update_tg_load_avg() decreased from 0.71% to 0.22%, a more than 3X reduction. The Max-jOPs results also increased slightly from 983015 to 986449. Signed-off-by: Waiman Long <Waiman.Long@hpe.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Ben Segall <bsegall@google.com> Cc: Douglas Hatch <doug.hatch@hpe.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Morten Rasmussen <morten.rasmussen@arm.com> Cc: Paul Turner <pjt@google.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Scott J Norton <scott.norton@hpe.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Yuyang Du <yuyang.du@intel.com> Link: http://lkml.kernel.org/r/1449081710-20185-4-git-send-email-Waiman.Long@hpe.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -2709,6 +2709,12 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq, int force)
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{
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{
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long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
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long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
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/*
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* No need to update load_avg for root_task_group as it is not used.
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*/
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if (cfs_rq->tg == &root_task_group)
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return;
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if (force || abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
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if (force || abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
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atomic_long_add(delta, &cfs_rq->tg->load_avg);
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atomic_long_add(delta, &cfs_rq->tg->load_avg);
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cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg;
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cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg;
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