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Date:   Mon, 7 Dec 2020 15:42:16 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Ziljstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Linux-ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH 0/4] Reduce worst-case scanning of runqueues in
 select_idle_sibling
Message-ID: <20201207154216.GE3371@techsingularity.net>
References: <20201207091516.24683-1-mgorman@techsingularity.net>
 <CAKfTPtC9At0Oej+u6-mtBdV6_vhFiNJGPQ-BFQc7RpUtDDixVA@mail.gmail.com>
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On Mon, Dec 07, 2020 at 04:04:41PM +0100, Vincent Guittot wrote:
> On Mon, 7 Dec 2020 at 10:15, Mel Gorman <mgorman@techsingularity.net> wrote:
> >
> > This is a minimal series to reduce the amount of runqueue scanning in
> > select_idle_sibling in the worst case.
> >
> > Patch 1 removes SIS_AVG_CPU because it's unused.
> >
> > Patch 2 improves the hit rate of p->recent_used_cpu to reduce the amount
> >         of scanning. It should be relatively uncontroversial
> >
> > Patch 3-4 scans the runqueues in a single pass for select_idle_core()
> >         and select_idle_cpu() so runqueues are not scanned twice. It's
> >         a tradeoff because it benefits deep scans but introduces overhead
> >         for shallow scans.
> >
> > Even if patch 3-4 is rejected to allow more time for Aubrey's idle cpu mask
> 
> patch 3 looks fine and doesn't collide with Aubrey's work. But I don't
> like patch 4  which manipulates different cpumask including
> load_balance_mask out of LB and I prefer to wait for v6 of Aubrey's
> patchset which should fix the problem of possibly  scanning twice busy
> cpus in select_idle_core and select_idle_cpu
> 

Seems fair, we can see where we stand after V6 of Aubrey's work.  A lot
of the motivation for patch 4 would go away if we managed to avoid calling
select_idle_core() unnecessarily. As it stands, we can call it a lot from
hackbench even though the chance of getting an idle core are minimal.

Assuming I revisit it, I'll update the schedstat debug patches to include
the times select_idle_core() starts versus how many times it fails and
see can I think of a useful heuristic.

I'll wait for more review on patches 1-3 and if I hear nothing, I'll
resend just those.

Thanks Vincent.

-- 
Mel Gorman
SUSE Labs