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

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Date:   Tue, 17 Jan 2023 04:49:28 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Valentin Schneider <vschneid@...hat.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 Mon, Jan 16, 2023 at 07:07:54PM +0000, Valentin Schneider wrote:
> 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;

Agreed. This would also make the code easier to understand. It should not change
the current behavior either; except (i.e., fix) for the busiest->group_weight = 2
vs local->group_weight = 1 I raised.

> 
> 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.

Agreed.

> 
> 
> > 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.

Then all looks good with your suggestion. I'll include a patch in my series.

Thanks and BR,
Ricardo