linux-kernel - Re: [PATCH v4 00/12] sched: Avoid unnecessary migrations within SMT domains

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Message-ID: <20230519000335.GB24449@ranerica-svr.sc.intel.com>
Date:   Thu, 18 May 2023 17:03:35 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Cc:     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>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        naveen.n.rao@...ux.vnet.ibm.com,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 00/12] sched: Avoid unnecessary migrations within SMT
 domains

On Fri, May 12, 2023 at 11:53:48PM +0530, Shrikanth Hegde wrote:
> 
> 
> On 4/29/23 9:02 PM, Peter Zijlstra wrote:
> > On Thu, Apr 06, 2023 at 01:31:36PM -0700, Ricardo Neri wrote:
> >> Hi,
> >>
> >> This is v4 of this series. Previous versions can be found here [1], [2],
> >> and here [3]. To avoid duplication, I do not include the cover letter of
> >> the original submission. You can read it in [1].
> >>
> >> This patchset applies cleanly on today's master branch of the tip tree.
> >>
> >> Changes since v3:
> >>
> >> Nobody liked the proposed changes to the setting of prefer_sibling.
> >> Instead, I tweaked the solution that Dietmar proposed. Now the busiest
> >> group, not the local group, determines the setting of prefer_sibling.
> >>
> >> Vincent suggested improvements to the logic to decide whether to follow
> >> asym_packing priorities. Peter suggested to wrap that in a helper function.
> >> I added sched_use_asym_prio().
> >>
> >> Ionela found that removing SD_ASYM_PACKING from the SMT domain in x86
> >> rendered sd_asym_packing NULL in SMT cores. Now highest_flag_domain()
> >> does not assume that all child domains have the requested flag.
> >>
> >> Tim found that asym_active_balance() needs to also check for the idle
> >> states of the SMT siblings of lb_env::dst_cpu. I added such check.
> >>
> >> I wrongly assumed that asym_packing could only be used when the busiest
> >> group had exactly one busy CPU. This broke asym_packing balancing at the
> >> DIE domain. I limited this check to balances between cores at the MC
> >> level.
> >>
> >> As per suggestion from Dietmar, I removed sched_asym_smt_can_pull_tasks()
> >> and placed its logic in sched_asym(). Also, sched_asym() uses
> >> sched_smt_active() to skip checks when not needed.
> >>
> >> I also added a patch from Chen Yu to enable asym_packing balancing in
> >> Meteor Lake, which has CPUs of different maximum frequency in more than
> >> one die.
> > 
> > Is the actual topology of Meteor Lake already public? This patch made me
> > wonder if we need SCHED_CLUSTER topology in the hybrid_topology thing,
> > but I can't remember (one of the raisins why the endless calls are such
> > a frigging waste of time) and I can't seem to find the answer using
> > Google either.
> > 
> >> Hopefully, these patches are in sufficiently good shape to be merged?
> > 
> > Changelogs are very sparse towards the end and I had to reverse engineer
> > some of it which is a shame. But yeah, on a first reading the code looks
> > mostly ok. Specifically 8-10 had me WTF a bit and only at 11 did it
> > start to make a little sense. Mostly they utterly fail to answer the
> > very fundament "why did you do this" question.
> > 
> > Also, you seem to have forgotten to Cc our friends from IBM such that
> > they might verify you didn't break their Power7 stuff -- or do you have
> > a Power7 yourself to verify and forgot to mention that?
> 
> Very good patch series in addressing asym packing. Interesting discussions as
> well. Took me quite sometime to get through to understand and do a little bit
> of testing.
> 
> Tested this patch a bit on power7 with qemu. Tested with SMT=4. sched domains
> show ASYM_PACKING present only for SMT domain.
> 
> We don't see any regressions/gain due to patch. SMT priorities are honored when
> tasks are scheduled and load_balanced.

Thank you very much for your review and testing! Would you mind sharing the
qemu command you use? I would like to test my future patches on power7.

BR,
Ricardo