linux-kernel - Re: [PATCH v2 5/7] x86/sched: Remove SD_ASYM

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Date:   Wed, 14 Dec 2022 08:59:00 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Ionela Voinescu <ionela.voinescu@....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>,
        Valentin Schneider <vschneid@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v2 5/7] x86/sched: Remove SD_ASYM_PACKING from the "SMT"
 domain

On Thu, Dec 08, 2022 at 04:03:04PM +0000, Ionela Voinescu wrote:
> Hi Ricardo,

Hi Ionela,

Thank you very much for your feedback!

> 
> On Tuesday 22 Nov 2022 at 12:35:30 (-0800), Ricardo Neri wrote:
> > There is no difference between any of the SMT siblings of a physical core.
> > asym_packing load balancing is not needed among siblings.
> > 
> > When balancing load among physical cores, the scheduler now considers the
> > state of the siblings when checking the priority of a CPU.
> > 
> > Cc: Ben Segall <bsegall@...gle.com>
> > Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > Cc: Len Brown <len.brown@...el.com>
> > Cc: Mel Gorman <mgorman@...e.de>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Tim C. Chen <tim.c.chen@...el.com>
> > Cc: Valentin Schneider <vschneid@...hat.com>
> > Cc: x86@...nel.org
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> > ---
> > Changes since v1:
> >  * Introduced this patch.
> > ---
> >  arch/x86/kernel/smpboot.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index 3f3ea0287f69..c3de98224cb4 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -545,7 +545,7 @@ static int x86_core_flags(void)
> >  #ifdef CONFIG_SCHED_SMT
> >  static int x86_smt_flags(void)
> >  {
> > -	return cpu_smt_flags() | x86_sched_itmt_flags();
> > +	return cpu_smt_flags();
> 
> Based on:
> 
> kernel/sched/topology.c:
> sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
> rcu_assign_pointer(per_cpu(sd_asym_packing, cpu), sd);
> 
> and described at:
> 
> include/linux/sched/sd_flags.h:
> /*
>  * Place busy tasks earlier in the domain
>  *
>  * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
>  *               up, but currently assumed to be set from the base domain
>  *               upwards (see update_top_cache_domain()).
>  * NEEDS_GROUPS: Load balancing flag.
>  */
> SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
> 
> doesn't your change result in sd_asym_packing being NULL?

Yes. This is a good catch. Thanks!

> 
> The SD_ASYM_PACKING flag requires all children of a domain to have it set
> as well. So having SMT not setting the flag, while CLUSTER and MC having
> set the flag would result in a broken topology, right?

I'd say that highest_flag_domain(..., flag) requires all children to have
`flag`, but clearly the comment you quote allows for SD_ASYM_PACKING to
be located in upper domains.

Perhaps this can be fixed with a variant of highest_flag_domain() that do
not require all children to have the flag?

Thanks and BR,
Ricardo