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SubjectRe: [PATCH] sched/fair: Make cfs_rq::decay_counter non-atomic
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Kirill Tkhai <ktkhai@parallels.com> writes:

> We update decay_counter in update_cfs_rq_blocked_load()
> only. This function is always called with rq lock locked,
> so we can kill atomic actions.
>
> Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
> ---
> kernel/sched/fair.c | 11 ++++++-----
> kernel/sched/sched.h | 2 +-
> 2 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5f3b5a7..af990c4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2570,7 +2570,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
> static inline u64 __synchronize_entity_decay(struct sched_entity *se)
> {
> struct cfs_rq *cfs_rq = cfs_rq_of(se);
> - u64 decays = atomic64_read(&cfs_rq->decay_counter);
> + u64 decays = ACCESS_ONCE(cfs_rq->decay_counter);

This is called without rq lock held from migrate_task_rq_fair. (We could
technically change the atomic_add to atomic64_set(atomic64_read() + x),
but I don't know that that is a win) Now, we could do a
min_vruntime-style two-copy thing if this atomic usage is a hot spot on
32-bit, we just didn't bother initially.


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