Re: [PATCH] sched: Reduce contention in update_cfs_rq_blocked_load

[Jason Low <jason.low2@hp.com>]

On Mon, 2014-08-04 at 13:52 -0700, bsegall@google.com wrote:
> Jason Low <jason.low2@hp.com> writes:
> 
> > 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>
> Reviewed-by: Ben Segall <bsegall@google.com>
> 
> That said, it might be better to remove force_update for this function,
> or make it just reduce the minimum to /64 or something. If the test is
> easy to run it would be good to see what it's like just removing the
> force_update param for this function to see if it's worth worrying
> about or if the zero case catches ~all the perf gain.

Sure, I can test that out too. I did notice when running another AIM7
workload that !zero was the more common case, so this has the potential
to further reduce contention.

>  Paul, your thoughts?