linux-kernel - Re: [PATCH 1/2] sched/deadline: accurate reclaim bandwidth for GRUB

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Message-ID: <20230509225417.61d36733@nowhere>
Date:   Tue, 9 May 2023 22:54:17 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Vineeth Remanan Pillai <vineeth@...byteword.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/deadline: accurate reclaim bandwidth for GRUB

On Tue, 9 May 2023 22:48:29 +0200
luca abeni <luca.abeni@...tannapisa.it> wrote:

> Hi,
> 
> On Tue, 9 May 2023 15:29:21 -0400
> Vineeth Remanan Pillai <vineeth@...byteword.org> wrote:
> [...]
> > > Is this understanding correct?    
> > Yes, the above two details are correct. In addition to that, I think
> > the existing equation had a small bug:
> > GRUB paper says, running time is depreciated as
> >    "dq = -U dt" where U is running_bw.
> > This is assuming that the whole cpu bandwidth could be reclaimed.
> > But in our case, we cap at Umax. So the equation should be
> >    "dq = -(U/Umax) dt"  
> 
> Yes, this is the approximation I was mentioning... Instead of using a
> division, I approximated it with a different equation using a sum.

Sorry, ignore this comment (and the following); I misinterpreted the
code (and my old notes).

I do not understand why the "max{}" doe not work well, I need to double
think about it.


			Luca