Re: [PATCH 1/2] sched/fair: Check rd->overload value before update

[Vincent Guittot <vincent.guittot@linaro.org>]

On Wed, 13 Mar 2024 at 16:02, Shrikanth Hegde <sshegde@linux.ibm.com> wrote:
>
> Overload is an indicator used to inform if there is any rq in the root
> domain has 2 or more running tasks. Root domain is a global structure per
> cpuset island.
>
> Overload status is updated when adding a task to the runqueue.
> It is cleared in update_sd_lb_stats during load balance, if none of the
> rq has 2 or more running task. It is used during to newidle
> balance to see if its worth doing load balance. If it is set, then
> newidle balance will try aggressively to pull a task.
>
> Since commit 630246a06ae2 ("sched/fair: Clean-up update_sg_lb_stats
> parameters"), overload is being updated unconditionally. The change in
> value of this depends on the workload. On typical workloads, it is
> likely that this value doesn't change often. Write causes the
> cacheline to be invalidated. This would cause true sharing when the same
> variable is accessed in newidle_balance. On large systems this would
> cause more CPU bus traffic.
>
> Perf probes prove that actual change in value is less often. So it would
> be better to check the value before updating it. Detailed perf probes
> and annotation can be found in the cover letter. CPU bus traffic reduces

the cover letter will not be merged. It's better to put the figures
here if you want to mention them

> with the patch.
>
> Fixes: 630246a06ae2 ("sched/fair: Clean-up update_sg_lb_stats parameters")

I don't think it's worth a Fixes tag. This is an improvement more than
a fix as the current code works fine but generates more bus traffic

With these minor changes in the commit message:

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02d4d224b436..eeebadd7d9ae 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10621,7 +10621,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>
>         if (!env->sd->parent) {
>                 /* update overload indicator if we are at root domain */
> -               WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD);
> +               if (READ_ONCE(env->dst_rq->rd->overload) != (sg_status & SG_OVERLOAD))
> +                       WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD);
>
>                 /* Update over-utilization (tipping point, U >= 0) indicator */
>                 set_rd_overutilized_status(env->dst_rq->rd,
> --
> 2.39.3
>