linux-kernel - Re: [PATCH 0/1] Reduce cost of accessing tg->load

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Message-ID: <20e21145-4fef-33b5-f41c-5bfd66a0281e@amd.com>
Date:   Fri, 25 Aug 2023 16:03:20 +0530
From:   Swapnil Sapkal <Swapnil.Sapkal@....com>
To:     Aaron Lu <aaron.lu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Nitin Tekchandani <nitin.tekchandani@...el.com>,
        Yu Chen <yu.c.chen@...el.com>,
        Waiman Long <longman@...hat.com>,
        Deng Pan <pan.deng@...el.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        David Vernet <void@...ifault.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] Reduce cost of accessing tg->load_avg

Hello Aaron,

On 8/23/2023 11:38 AM, Aaron Lu wrote:
> RFC v2 -> v1:
> - drop RFC;
> - move cfs_rq->last_update_tg_load_avg before cfs_rq->tg_load_avg_contrib;
> - add Vincent's reviewed-by tag.
> 
> RFC v2:
> Nitin Tekchandani noticed some scheduler functions have high cost
> according to perf/cycles while running postgres_sysbench workload.
> I perf/annotated the high cost functions: update_cfs_group() and
> update_load_avg() and found the costs were ~90% due to accessing to
> tg->load_avg. This series is an attempt to reduce the overhead of
> the two functions.
>                                                                                                                         
> Thanks to Vincent's suggestion from v1, this revision used a simpler way
> to solve the overhead problem by limiting updates to tg->load_avg to at
> most once per ms. Benchmark shows that it has good results and with the
> rate limit in place, other optimizations in v1 don't improve performance
> further so they are dropped from this revision.
> 

I have tested this series alongside Mathieu's changes. You can find the
report here: https://lore.kernel.org/all/f6dc1652-bc39-0b12-4b6b-29a2f9cd8484@amd.com/

Tested-by: Swapnil Sapkal <Swapnil.Sapkal@....com>

> Aaron Lu (1):
>    sched/fair: ratelimit update to tg->load_avg
> 
>   kernel/sched/fair.c  | 13 ++++++++++++-
>   kernel/sched/sched.h |  1 +
>   2 files changed, 13 insertions(+), 1 deletion(-)
> 
--
Thanks and Regards,
Swapnil