linux-kernel - Re: [PATCH v2] sched: Don't try to catch up excess steal time.

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Message-ID: <545fe60a5c9e8bd93a5b36b19a0af379a6170637.camel@infradead.org>
Date: Wed, 25 Sep 2024 15:02:57 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Joel Fernandes <joelaf@...gle.com>
Cc: Suleiman Souhlal <suleiman@...gle.com>, Ingo Molnar <mingo@...hat.com>, 
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
 <dietmar.eggemann@....com>,  Steven Rostedt <rostedt@...dmis.org>, Ben
 Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Valentin
 Schneider <vschneid@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>,
 vineethrp@...gle.com,  linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 ssouhlal@...ebsd.org, Srikar Dronamraju <srikar@...ux.ibm.com>, Sean
 Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2] sched: Don't try to catch up excess steal time.

On Wed, 2024-09-25 at 09:24 -0400, Joel Fernandes wrote:
> > 
> > In
> > https://lore.kernel.org/all/20240522001817.619072-22-dwmw2@infradead.org/
> > I put a limit on the amount of steal time carried forward from one
> > timeslice to the next, as it was misbehaving when a bad hypervisor
> > reported negative steal time. But I don't think the limit should be
> > zero.
> 
> Yea, this is the solution I was thinking but I don’t see any limits
> to steal time in the current code. That is what Suleiman is trying to
> fix.
> 
> So why dont we cap the maximum steal time accrued any more (I.e. what
> happened to the code in your patch, was it deleted for another
> reason?).
> 

My patch was never merged. It was part of an exploratory RFC series
working on various KVM clock problems, one of which *fixed* the problem
of steal time going backwards in the hypervisor, and that guest patch
was an attempt to work around that bug in existing KVM. I never did
split it out and send it on its own.

> My impression is your old patch is exactly what we need as Suleiman
> is seeing an issue with suspend causing very very long steal times in
> virtual Machines, but I will let him speak for himself.

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