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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 27 Sep 2023 10:00:11 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Chen Yu <yu.c.chen@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Tim Chen <tim.c.chen@intel.com>, Aaron Lu <aaron.lu@intel.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        "Gautham R . Shenoy" <gautham.shenoy@amd.com>,
        linux-kernel@vger.kernel.org, Chen Yu <yu.chen.surf@gmail.com>
Subject: Re: [PATCH 0/2] Introduce SIS_CACHE to choose previous CPU during
 task wakeup
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References: <cover.1695704179.git.yu.c.chen@intel.com>
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* Chen Yu <yu.c.chen@intel.com> wrote:

> When task p is woken up, the scheduler leverages select_idle_sibling()
> to find an idle CPU for it. p's previous CPU is usually a preference
> because it can improve cache locality. However in many cases, the
> previous CPU has already been taken by other wakees, thus p has to
> find another idle CPU.
> 
> Inhibit the task migration while keeping the work conservation of
> scheduler could benefit many workloads. Inspired by Mathieu's
> proposal to limit the task migration ratio[1], this patch considers
> the task average sleep duration. If the task is a short sleeping one,
> then tag its previous CPU as cache hot for a short while. During this
> reservation period, other wakees are not allowed to pick this idle CPU
> until a timeout. Later if the task is woken up again, it can find its
> previous CPU still idle, and choose it in select_idle_sibling().

Yeah, so I'm not convinced about this at this stage.

By allowing a task to basically hog a CPU after it has gone idle already,
however briefly, we reduce resource utilization efficiency for the sake
of singular benchmark workloads.

In a mixed environment the cost of leaving CPUs idle longer than necessary
will show up - and none of these benchmarks show that kind of side effect
and indirect overhead.

This feature would be a lot more convincing if it tried to measure overhead
in the pathological case, not the case it's been written for.

Thanks,

	Ingo