Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 8 Jan 2021 20:49:33 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        "Li, Aubrey" <aubrey.li@linux.intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Jiang Biao <benbjiang@gmail.com>
Subject: Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost
 accounting
Message-ID: <X/i3TbRRS9chYb3l@hirez.programming.kicks-ass.net>
References: <20201214164822.402812729@infradead.org>
 <20201214170017.877557652@infradead.org>
 <c4e31235-e1fb-52ac-99a8-ae943ee0de54@linux.intel.com>
 <20201215075911.GA3040@hirez.programming.kicks-ass.net>
 <20210108102738.GB3592@techsingularity.net>
 <CAKfTPtD5R1S=rwp9C-jyMg8bAB-37FCe3qrqad9KEeyR7mOmkw@mail.gmail.com>
 <20210108144058.GD3592@techsingularity.net>
 <CAKfTPtCGCmCv0yXSUmYUh6=8uzd0n9xFPqC0cW4sm-FqDvjvCQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAKfTPtCGCmCv0yXSUmYUh6=8uzd0n9xFPqC0cW4sm-FqDvjvCQ@mail.gmail.com>
Precedence: bulk

On Fri, Jan 08, 2021 at 04:10:51PM +0100, Vincent Guittot wrote:
> Another thing that worries me, is that we use the avg_idle of the
> local cpu, which is obviously not idle otherwise it would have been
> selected, to decide how much time we should spend on looking for
> another idle CPU. I'm not sure that's the right metrics to use
> especially with a possibly stalled value.

The thinking was that if this CPU has little idle time, this CPU should
not spend a lot of time searching...

That is; if we spend more time looking for places to run, than we have
idle time, we're loosing cycles we could've ran (supposedly useful) work.

The only counter argument is tail latency.