linux-kernel - Re: [PATCH v3 2/5] sched/deadline: Fix reclaim inaccuracy with SMP

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Message-ID: <20230519121804.6c85a3ed@luca64>
Date:   Fri, 19 May 2023 12:18:04 +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 v3 2/5] sched/deadline: Fix reclaim inaccuracy with SMP

On Fri, 19 May 2023 11:56:21 +0200
luca abeni <luca.abeni@...tannapisa.it> wrote:

> Hi,
> 
> sorry for returning on this discussion, but there is something I still
> do not understand:
> 
> On Tue, 16 May 2023 11:08:18 -0400
> Vineeth Remanan Pillai <vineeth@...byteword.org> wrote:
> [...]  
> > I had tested this and it was reclaiming much less compared to the
> > first one. I had 3 tasks with reservation (3,100) and 3 cpus.  
> 
> So, just to confirm: here you have only 3 SCHED_DEADLINE tasks,
> scheduled on a root domain containing only 3 CPUs (dl_bw_cpus() return
> 3)... Right?
> So, the utilization of each task is 3/100 = 0.03 and Uextra is
> 1 - (0.03 * 3) / 3 = 0.97.

OK, sorry again... I found my error immediately after sending the email.
Uextra is computed as "Umax - ...", not "1 - ...".
So, I now understand where the 35% comes from.

I now _suspect_ the correct equation should be
	dq = -(max{u_i / Umax, (Umax - Uinact - Uextra)}) * dt
but I want to test it before wasting your time again; I'll write more
after performing some more tests.


			Luca

> And since all the tasks are always active, Uinact = 0...
> Is this understanding right?
> 
> If so:
> > With dq = -(max{u_i, (Umax - Uinact - Uextra)} / Umax) * dt (1)
> > TID[636]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 95.08
> > TID[635]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 95.07
> > TID[637]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 95.06
> > 
> > With dq = -(max{u_i, (1 - Uinact - Uextra)} / Umax) * dt (2)
> > TID[601]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 35.65
> > TID[600]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 35.65
> > TID[602]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 35.65  
> 
> Here, we should have
> 	dq = -(max{0.03, (1 - 0 - 0.97)} / Umax) * dt
> 	   = -(0.03 / Umax) * dt
> which reclaims up to Umax... So, the utilization should be 95%
> Since you measured 35.65%, it means that (1-Uextra) is much larger
> than 0.97... So, maybe you found some bug in the Uextra computation?
> 
> Can you try printing the extra_bw value, to check what happened?
> 
> 
> 
> 			Thanks,
> 				Luca
> 
> > 
> > As the task bandwidth goes higher, equation (2) reclaims more, but
> > equation (2) is a constant of 95% as long as number of tasks less
> > than cpus. If the number of tasks is more than cpus, eq (2) fares
> > better in reclaiming than eq (1)
> > 
> > eq (1) with 5 tasks (3,100)
> > TID[627]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.64
> > TID[626]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.64
> > TID[629]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.62
> > TID[628]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 29.00
> > TID[630]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.99
> > 
> > Here top shows 3 cpus in the range ~45 to 50% util
> > 
> > eq (2) with 5 tasks (3,100)
> > TID[667]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 57.20
> > TID[670]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 57.79
> > TID[668]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 57.11
> > TID[666]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 56.34
> > TID[669]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 55.82
> > 
> > And here top shows all 3 cpus with 95% util
> >   
> > > I'll write more about this later... And thanks for coping with all
> > > my comments!
> > >    
> > Thanks :-)
> > 
> > Vineeth  
>