linux-kernel - Re: [PATCH 2/4] x86: Fix CPUIDLE_FLAG_IRQ

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Date:   Wed, 15 Nov 2023 10:57:44 -0500
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "Rafael J . Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH 2/4] x86: Fix CPUIDLE_FLAG_IRQ_ENABLE leaking timer
 reprogram

Le Wed, Nov 15, 2023 at 04:52:32PM +0100, Peter Zijlstra a écrit :
> On Wed, Nov 15, 2023 at 10:13:23AM -0500, Frederic Weisbecker wrote:
> > From: Peter Zijlstra <peterz@...radead.org>
> > 
> > intel_idle_irq() re-enables IRQs very early. As a result, an interrupt
> > may fire before mwait() is eventually called. If such an interrupt queues
> > a timer, it may go unnoticed until mwait returns and the idle loop
> > handles the tick re-evaluation. And monitoring TIF_NEED_RESCHED doesn't
> > help because a local timer enqueue doesn't set that flag.
> > 
> > The issue is mitigated by the fact that this idle handler is only invoked
> > for shallow C-states when, presumably, the next tick is supposed to be
> > close enough. There may still be rare cases though when the next tick
> > is far away and the selected C-state is shallow, resulting in a timer
> > getting ignored for a while.
> > 
> > Fix this with using sti_mwait() whose IRQ-reenablement only triggers
> > upon calling mwait(), dealing with the race while keeping the interrupt
> > latency within acceptable bounds.
> > 
> > Fixes: c227233ad64c (intel_idle: enable interrupts before C1 on Xeons)
> > Not-yet-signed-off-by: Peter Zijlstra <peterz@...radead.org>
> 
> Feel free to change to normal SOB, I'm assuming it actually compiles and
> works by now :-)

Not sure, I might have tested it at some point ;-)

Thanks!