linux-kernel - Re: [PATCH v2 1/7] sched/fair: Generalize asym

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Message-ID: <xhsmhtu0qi9h1.mognet@vschneid.remote.csb>
Date:   Mon, 16 Jan 2023 19:07:54 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v2 1/7] sched/fair: Generalize asym_packing logic for
 SMT local sched group

On 15/01/23 20:05, Ricardo Neri wrote:
>> >
>> > It should be set on any topology level below the NUMA ones, we do remove it
>> > on SD_ASYM_CPUCAPACITY levels because this used to interfere with misfit
>> > balancing (it would override the group_type), things are a bit different
>> > since Vincent's rewrite of load_balance() but I think we still want it off
>> > there.
>
> Your comment got me thinking. Whose child sched domain wants prefer_sibling?
> It sounds to me that is busiest's. I could not think of any reason of *having*
> to use the flags of the local group.
>

Hm, given that on systems without SD_ASYM_CPUCAPACITY, SD_PREFER_SIBLING is
set all the way from SMT up to the last !NUMA domain, should we just get
rid of the child/parent weirdness of SD_PREFER_SIBLING and stick with the
flags we are given at the level we're balancing at?

i.e.

        sds->prefer_sibling = env->sd & SD_PREFER_SIBLING;

Unless I'm reading this wrong, this also eliminates the effect of
SD_PREFER_SIBLING on the first NUMA level - DIE level has SD_PREFER_SIBLING
set, but we don't necessarily want to evenly spread things out when accross
NUMA nodes.


> We can use the flags of the sched group (as per 16d364ba6ef2 ("sched/topology:
> Introduce sched_group::flags"), these are the flags of the child domain).
>
> The patch below works for me and I don't have to even the number of busy CPUs.
> It should not interfere with misfit balancing either:
>

We remove that flag on systems where misfit balancing happens anyway, so
that's safe vs. SD_PREFER_SIBLING.