linux-kernel - Re: [PATCH v8 1/9] sched/fair: fix unfairness at wakeup

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Message-ID: <Y3SefaraLvX2hPYj@ziqianlu-desk2>
Date:   Wed, 16 Nov 2022 16:25:33 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
CC:     Joel Fernandes <joel@...lfernandes.org>, <mingo@...hat.com>,
        <peterz@...radead.org>, <juri.lelli@...hat.com>,
        <dietmar.eggemann@....com>, <rostedt@...dmis.org>,
        <bsegall@...gle.com>, <mgorman@...e.de>, <bristot@...hat.com>,
        <vschneid@...hat.com>, <linux-kernel@...r.kernel.org>,
        <parth@...ux.ibm.com>, <qyousef@...alina.io>,
        <chris.hyser@...cle.com>, <patrick.bellasi@...bug.net>,
        <David.Laight@...lab.com>, <pjt@...gle.com>, <pavel@....cz>,
        <tj@...nel.org>, <qperret@...gle.com>,
        <tim.c.chen@...ux.intel.com>, <joshdon@...gle.com>, <timj@....org>,
        <kprateek.nayak@....com>, <yu.c.chen@...el.com>,
        <youssefesmat@...omium.org>, <riel@...hat.com>
Subject: Re: [PATCH v8 1/9] sched/fair: fix unfairness at wakeup

On Mon, Nov 14, 2022 at 12:05:18PM +0100, Vincent Guittot wrote:
> On Mon, 14 Nov 2022 at 04:06, Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > Hi Vincent,
> >
> > On Thu, Nov 10, 2022 at 06:50:01PM +0100, Vincent Guittot wrote:

... ...

> > > +static inline unsigned long  get_latency_max(void)
> > > +{
> > > +     unsigned long thresh = get_sched_latency(false);
> > > +
> > > +     thresh -= sysctl_sched_min_granularity;
> >
> > Could you clarify, why are you subtracting sched_min_granularity here? Could
> > you add some comments here to make it clear?
> 
> If the waking task failed to preempt current it could to wait up to
> sysctl_sched_min_granularity before preempting it during next tick.

check_preempt_tick() compares vdiff/delta between the leftmost se and
curr against curr's ideal_runtime, it doesn't use thresh here or the
adjusted wakeup_gran, so I don't see why reducing thresh here can help
se to preempt curr during next tick if it failed to preempt curr in its
wakeup path.

I can see reducing thresh here with whatever value can help the waking
se to preempt curr in wakeup_preempt_entity() though, because most
likely the waking se's vruntime is cfs_rq->min_vruntime -
sysctl_sched_latency/2 and curr->vruntime is near cfs_rq->min_vruntime
so vdiff is about sysctl_sched_latency/2, which is the same value as
get_sched_latency(false) and when thresh is reduced some bit, then vdiff
in wakeup_preempt_entity() will be larger than gran and make it possible
to preempt.

So I'm confused by your comment or I might misread the code.