linux-kernel - Re: [PATCH v3 1/2] sched/fair: Add EAS checks before updating overutilized

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Date: Fri, 1 Mar 2024 14:56:34 +0530
From: Srikar Dronamraju <srikar@...ux.ibm.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org,
        yu.c.chen@...el.com, dietmar.eggemann@....com,
        linux-kernel@...r.kernel.org, nysal@...ux.ibm.com,
        aboorvad@...ux.ibm.com, vschneid@...hat.com, pierre.gondois@....com,
        morten.rasmussen@....com, qyousef@...alina.io
Subject: Re: [PATCH v3 1/2] sched/fair: Add EAS checks before updating
 overutilized

* Shrikanth Hegde <sshegde@...ux.ibm.com> [2024-02-29 16:10:09]:

> Overutilized field of root domain is only used for EAS(energy aware scheduler)
> to decide whether to do load balance or not. It is not used if EAS
> not possible.
> 
> Currently enqueue_task_fair and task_tick_fair accesses, sometime updates
> this field. In update_sd_lb_stats it is updated often. This causes cache
> contention due to true sharing and burns a lot of cycles. overload and
> overutilized are part of the same cacheline. Updating it often invalidates
> the cacheline. That causes access  to overload to slow down due to
> false sharing. Hence add EAS check before accessing/updating this field.
> EAS check is optimized at compile time or it is a static branch.
> Hence it shouldn't cost much.
> 
> With the patch, both enqueue_task_fair and newidle_balance don't show
> up as hot routines in perf profile.
> 
> 6.8-rc4:
> 7.18%  swapper          [kernel.vmlinux]              [k] enqueue_task_fair
> 6.78%  s                [kernel.vmlinux]              [k] newidle_balance
> +patch:
> 0.14%  swapper          [kernel.vmlinux]              [k] enqueue_task_fair
> 0.00%  swapper          [kernel.vmlinux]              [k] newidle_balance
> 
> Minor change: trace_sched_overutilized_tp expect that second argument to
> be bool. So do a int to bool conversion for that.
> 
> Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>

Looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@...ux.ibm.com>
-- 
Thanks and Regards
Srikar Dronamraju