linux-kernel - Re: [PATCH -tip 09/32] sched/fair: Snapshot the min

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Date:   Tue, 24 Nov 2020 10:31:49 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Vineeth Pillai <viremana@...ux.microsoft.com>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.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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        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 09/32] sched/fair: Snapshot the min_vruntime of CPUs
 on force idle

On Mon, Nov 23, 2020 at 07:31:31AM -0500, Vineeth Pillai wrote:
> Hi Balbir,
> 
> On 11/22/20 6:44 AM, Balbir Singh wrote:
> > 
> > This seems cumbersome, is there no way to track the min_vruntime via
> > rq->core->min_vruntime?
> Do you mean to have a core wide min_vruntime? We had a
> similar approach from v3 to v7 and it had major issues which
> broke the assumptions of cfs. There were some lengthy
> discussions and Peter explained in-depth about the issues:
> 
> https://lwn.net/ml/linux-kernel/20200506143506.GH5298@hirez.programming.kicks-ass.net/
> https://lwn.net/ml/linux-kernel/20200515103844.GG2978@hirez.programming.kicks-ass.net/
>

One of the equations in the link is

">From which immediately follows that:

          T_k + T_l
  S_k+l = ---------                                       (13)
          W_k + W_l

On which we can define a combined lag:

  lag_k+l(i) := S_k+l - s_i                               (14)

And that gives us the tools to compare tasks across a combined runqueue.
"

S_k+l reads like rq->core->vruntime, but it sounds like the equivalent
of rq->core->vruntime is updated when we enter forced idle as opposed to
all the time.

Balbir