Re: [PATCH] sched: Reduce contention in update_cfs_rq_blocked_load

[Peter Zijlstra <peterz@infradead.org>]

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
> 

Supposedly adding something like __cacheline_aligned to
task_group::load_avg is also rumoured to help.