Date:   Wed, 1 Feb 2023 17:06:01 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Roman Kagan <rkagan@amazon.de>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Zhang Qiao <zhangqiao22@huawei.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Waiman Long <longman@redhat.com>,
        Ben Segall <bsegall@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being placed
Message-ID: <Y9qN6TEY/U4a8nks@hirez.programming.kicks-ass.net>
References: <20230130122216.3555094-1-rkagan@amazon.de>
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On Mon, Jan 30, 2023 at 01:22:16PM +0100, Roman Kagan wrote:
> From: Zhang Qiao <zhangqiao22@huawei.com>
> 
> When a scheduling entity is placed onto cfs_rq, its vruntime is pulled
> to the base level (around cfs_rq->min_vruntime), so that the entity
> doesn't gain extra boost when placed backwards.
> 
> However, if the entity being placed wasn't executed for a long time, its
> vruntime may get too far behind (e.g. while cfs_rq was executing a
> low-weight hog), which can inverse the vruntime comparison due to s64
> overflow.  This results in the entity being placed with its original
> vruntime way forwards, so that it will effectively never get to the cpu.
> 
> To prevent that, ignore the vruntime of the entity being placed if it
> didn't execute for much longer than the characteristic sheduler time
> scale.
> 
> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
> [rkagan: formatted, adjusted commit log, comments, cutoff value]
> Co-developed-by: Roman Kagan <rkagan@amazon.de>
> Signed-off-by: Roman Kagan <rkagan@amazon.de>

Thanks!