linux-kernel - Re: [PATCH v3 1/2] x86/tsc: add a timer to make sure tsc

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Date:   Tue, 15 Mar 2022 09:33:08 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     tglx@...utronix.de, andi.kleen@...el.com, bp@...en8.de,
        dave.hansen@...el.com, hpa@...or.com, len.brown@...el.com,
        linux-kernel@...r.kernel.org, mingo@...hat.com, paulmck@...nel.org,
        peterz@...radead.org, rui.zhang@...el.com, tim.c.chen@...el.com,
        x86@...nel.org, mtosatti@...hat.com, frederic@...nel.org
Subject: Re: [PATCH v3 1/2] x86/tsc: add a timer to make sure tsc_adjust is
 always checked

Hi Nicolas,

Thanks for raising it.

On Mon, Mar 14, 2022 at 06:52:07PM +0100, Nicolas Saenz Julienne wrote:
> Hi Feng, Thomas,
> 
> > On Wed, Nov 17, 2021 at 10:37:51AM +0800, Feng Tang wrote:
> > Normally the tsc_sync will get checked every time system enters idle state,
> > but Thomas Gleixner mentioned there is still a caveat that a system won't
> > enter idle [1], either because it's too busy or configured purposely to not
> > enter idle. Setup a periodic timer (every 10 minitues) to make sure the
> > check is always on.
> > 
> > [1]. https://lore.kernel.org/lkml/875z286xtk.fsf@nanos.tec.linutronix.de/
> > Signed-off-by: Feng Tang <feng.tang@...el.com>
> 
> I can see this timer interrupting my system's nohz_full CPUs. It'd be nice to
> be able to avoid the noise. A solution is using 'tsc=reliable', but IIUC this
> is not what the flag was created for. Any ideas/suggestions?
 
This patch is about correctness. And yes, as you said, the 'tsc=reliable'
works. Another thought is to leverage the 'housekeeping_mask' to exclude
the isolated nohz_full CPUs, but the 'caveat' what the patch try to solve
remains.

Also before this patchset, the clocksource watchdog timer is fired every
500ms, which should interrupt nohz_full CUPS more? 

Thanks,
Feng

> Regards,
> Nicolas