Re: [PATCH] sched: Reduce contention in update_cfs_rq_blocked_load

[Yuyang Du <yuyang.du@intel.com>]

Hi Jason,

I am not sure whether you noticed my latest work: rewriting per entity load average

http://article.gmane.org/gmane.linux.kernel/1760754
http://article.gmane.org/gmane.linux.kernel/1760755
http://article.gmane.org/gmane.linux.kernel/1760757
http://article.gmane.org/gmane.linux.kernel/1760756

which simply does not track blocked load average at all. Are you interested in
testing the patchset with the workload you have? The comparison can also help
us understand the rewrite. Overall, per our tests, the overhead should be less,
and perf should be better.

Thanks,
Yuyang

On Mon, Aug 04, 2014 at 01:28:38PM -0700, Jason Low wrote:
> When running workloads on 2+ socket systems, based on perf profiles, the
> update_cfs_rq_blocked_load function constantly shows up as taking up a
> noticeable % of run time. This is especially apparent on an 8 socket
> machine. For example, when running the AIM7 custom workload, we see:
> 
>    4.18%        reaim  [kernel.kallsyms]        [k] update_cfs_rq_blocked_load
> 
> Much of the contention is in __update_cfs_rq_tg_load_contrib when we
> update the tg load contribution stats.  However, it turns out that in many
> cases, they don't need to be updated and "tg_contrib" is 0.
> 
> This patch adds a check in __update_cfs_rq_tg_load_contrib to skip updating
> tg load contribution stats when nothing needs to be updated. This reduces the
> cacheline contention that would be unnecessary. In the above case, with the
> patch, perf reports the total time spent in this function went down by more
> than a factor of 3x:
> 
>    1.18%        reaim  [kernel.kallsyms]        [k] update_cfs_rq_blocked_load
> 
> Signed-off-by: Jason Low <jason.low2@hp.com>
> ---
>  kernel/sched/fair.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bfa3c86..8d4cc72 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2377,6 +2377,9 @@ static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
>  	tg_contrib = cfs_rq->runnable_load_avg + cfs_rq->blocked_load_avg;
>  	tg_contrib -= cfs_rq->tg_load_contrib;
>  
> +	if (!tg_contrib)
> +		return;
> +
>  	if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
>  		atomic_long_add(tg_contrib, &tg->load_avg);
>  		cfs_rq->tg_load_contrib += tg_contrib;
> -- 
> 1.7.1
> 
> 
> 
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