linux-kernel - Re: [PATCH] Clocksource: Avoid misjudgment of clocksource

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Message-ID: <20211209164400.GC641268@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 9 Dec 2021 08:44:00 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Gang Li <ligang.bdlg@...edance.com>
Cc:     john.stultz@...aro.org, linux-kernel@...r.kernel.org,
        sboyd@...nel.org, tglx@...utronix.de, yanghui.def@...edance.com,
        brookxu.cn@...il.com, luming.yu@...il.com,
        songmuchun@...edance.com, zhengqi.arch@...edance.com
Subject: Re: [PATCH] Clocksource: Avoid misjudgment of clocksource

On Thu, Dec 09, 2021 at 09:14:05PM +0800, Gang Li wrote:
> Hi all,
> 
> How about changing clocksource_watchdog from classical timer to hrtimer?
> hrtimer is more accurate and stable, and makes it much easier for us to deal
> with misjudgments.

To make hrtimer more accurate in the busy-system scenarios that are
under test, we would need to specify HRTIMER_MODE_HARD.  Otherwise,
hrtimer handlers execute in softirq, just like timers do.  So without
the HRTIMER_MODE_HARD, the hrtimers would be delayed just as badly as
we are seeing for the normal timers.

Unfortunately, use of HRTIMER_MODE_HARD has response-time consequences,
especially when the timer reads need to be retried.  So my guess is
that hrtimers are not a helpful path forward here.

							Thanx, Paul