[SRU][X][PATCH 1/1] clocksource: Defer override invalidation unless clock is unstable

[William Breathitt Gray]

On Mon, Sep 07, 2020 at 04:15:28PM +1200, Matthew Ruffell wrote:
> From: Kyle Walker <kwalker at redhat.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1894591
> 
> Clocksources don't get the VALID_FOR_HRES flag until they have been
> checked by a watchdog. However, when using an override, the
> clocksource_select logic will clear the override value if the
> clocksource is not marked VALID_FOR_HRES during that inititial check.
> When using the boot arguments clocksource=<foo>, this selection can
> run before the watchdog, and can cause the override to be incorrectly
> cleared.
> 
> To address this condition, the override_name is only invalidated for
> unstable clocksources. Otherwise, the override is left intact until after
> the watchdog has validated the clocksource as stable/unstable.
> 
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: Ingo Molnar <mingo at kernel.org>
> Cc: Richard Cochran <richardcochran at gmail.com>
> Cc: Prarit Bhargava <prarit at redhat.com>
> Cc: Martin Schwidefsky <schwidefsky at de.ibm.com>
> Signed-off-by: Kyle Walker <kwalker at redhat.com>
> Signed-off-by: John Stultz <john.stultz at linaro.org>
> (cherry picked from commit 36374583f9084cdab4b5dcf5521a3ce55bebb9fa)
> Signed-off-by: Matthew Ruffell <matthew.ruffell at canonical.com>
> ---
>  kernel/time/clocksource.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index a20368e1a720..ac4c84361be0 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -607,9 +607,18 @@ static void __clocksource_select(bool skipcur)
>  		 */
>  		if (!(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) && oneshot) {
>  			/* Override clocksource cannot be used. */
> -			pr_warn("Override clocksource %s is not HRT compatible - cannot switch while in HRT/NOHZ mode\n",
> -				cs->name);
> -			override_name[0] = 0;
> +			if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
> +				pr_warn("Override clocksource %s is unstable and not HRT compatible - cannot switch while in HRT/NOHZ mode\n",
> +					cs->name);
> +				override_name[0] = 0;
> +			} else {
> +				/*
> +				 * The override cannot be currently verified.
> +				 * Deferring to let the watchdog check.
> +				 */
> +				pr_info("Override clocksource %s is not currently HRT compatible - deferring\n",
> +					cs->name);
> +			}
>  		} else
>  			/* Override clocksource can be used. */
>  			best = cs;
> -- 
> 2.25.1

Acked-by: William Breathitt Gray <william.gray at canonical.com>
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