linux-kernel - Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and scheduling.

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Message-ID: <20201027165802.GA1005883@google.com>
Date:   Tue, 27 Oct 2020 12:58:02 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
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        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and
 scheduling.

On Mon, Oct 26, 2020 at 09:28:14AM +0100, Peter Zijlstra wrote:
> On Fri, Oct 23, 2020 at 05:31:18PM -0400, Joel Fernandes wrote:
> > BTW, as further optimization in the future, isn't it better for the
> > schedule() loop on 1 HT to select for all HT *even if* need_sync == false to
> > begin with?  i.e. no cookied tasks are runnable.
> > 
> > That way the pick loop in schedule() running on other HTs can directly pick
> > what was pre-selected for it via:
> >         if (rq->core->core_pick_seq == rq->core->core_task_seq &&
> >             rq->core->core_pick_seq != rq->core_sched_seq &&
> >             rq->core_pick)
> > .. which I think is more efficient. Its just a thought and may not be worth doing.
> 
> I'm not sure that works. Imagine a sibling doing a wakeup (or sleep)
> just after you done your core wide pick. Then it will have to repick and
> you end up with having to do 2*nr_smt picks instead of 2 picks.

For a workload that is having mostly runnable tasks (not doing lot of wakeup
/ sleep), maybe it makes sense.

Also if you have only cookied tasks and they are doing wake up / sleep, then
you have 2*nr_smt_picks anyway as the core picks constantly get invalidated,
AFAICS.

I guess in the current code, the assumptions are:
1. Most tasks are not cookied task
2. They can wake up and sleep a lot

I guess those are Ok assumptions though, but maybe we could document it.

thanks,

 - Joel