linux-kernel - Re: [PATCH -tip 15/32] sched: Improve snapshotting of min

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Message-ID: <20201124170744.GD1021337@google.com>
Date:   Tue, 24 Nov 2020 12:07:44 -0500
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>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        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, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 15/32] sched: Improve snapshotting of min_vruntime
 for CGroups

Hi Peter,

On Tue, Nov 24, 2020 at 11:27:41AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 06:19:45PM -0500, Joel Fernandes (Google) wrote:
> > A previous patch improved cross-cpu vruntime comparison opertations in
> > pick_next_task(). Improve it further for tasks in CGroups.
> > 
> > In particular, for cross-CPU comparisons, we were previously going to
> > the root-level se(s) for both the task being compared. That was strange.
> > This patch instead finds the se(s) for both tasks that have the same
> > parent (which may be different from root).
> > 
> > A note about the min_vruntime snapshot and force idling:
> > Abbreviations: fi: force-idled now? ; fib: force-idled before?
> > During selection:
> > When we're not fi, we need to update snapshot.
> > when we're fi and we were not fi, we must update snapshot.
> > When we're fi and we were already fi, we must not update snapshot.
> > 
> > Which gives:
> >         fib     fi      update?
> >         0       0       1
> >         0       1       1
> >         1       0       1
> >         1       1       0
> > So the min_vruntime snapshot needs to be updated when: !(fib && fi).
> > 
> > Also, the cfs_prio_less() function needs to be aware of whether the core
> > is in force idle or not, since it will be use this information to know
> > whether to advance a cfs_rq's min_vruntime_fi in the hierarchy. So pass
> > this information along via pick_task() -> prio_less().
> 
> Hurmph.. so I'm tempted to smash a bunch of patches together.
> 
>  2 <- 3 (already done - bisection crashes are daft)
>  6 <- 11
>  7 <- {10, 12}
>  9 <- 15
> 
> I'm thinking that would result in an easier to read series, or do we
> want to preserve this history?
> 
> (fwiw, I pulled 15 before 13,14, as I think that makes more sense
> anyway).

Either way would be Ok with me, I would suggest retaining the history though
so that the details in the changelog are preserved of the issues we faced,
and in the future we can refer back to them.

thanks,

 - Joel