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

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Message-ID: <982d866bd387964b47148b0492fe9aada3b9ae32.camel@infradead.org>
Date: Wed, 25 Sep 2024 16:34:57 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Suleiman Souhlal <ssouhlal@...ebsd.org>
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>,
 joelaf@...gle.com,  vineethrp@...gle.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.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 15:15 +0000, Suleiman Souhlal wrote:
> Yes, that's a good way to put it: The excess steal time isn't actually
> being stolen from anyone.
> And since it's not being stolen from anyone, isn't the right thing to do
> to drop it?

It's being stolen from the system, isn't it? Just not any specific
userspace process?

If we have separate "end of outgoing task" and "start of incoming task"
timestamps, surely the time between those two must be accounted
*somewhere*?

> There might still be extra steal time that doesn't exceed the current
> 'delta' from the race between reading the two values, that would still
> be erroneously accounted to the outgoing task, which this patch doesn't
> address, but we know that any steal > delta is from this race and should
> be dropped.

Well that's what we want the atomic paired read for :)

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