Re: [Xen-devel] [RFC PATCH V2 11/11] x86: tsc: avoid system instability in hibernation

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, Jan 07, 2020 at 11:45:26PM +0000, Anchal Agarwal wrote:
> From: Eduardo Valentin <eduval@xxxxxxxxxx>
> 
> System instability are seen during resume from hibernation when system
> is under heavy CPU load. This is due to the lack of update of sched
> clock data, and the scheduler would then think that heavy CPU hog
> tasks need more time in CPU, causing the system to freeze
> during the unfreezing of tasks. For example, threaded irqs,
> and kernel processes servicing network interface may be delayed
> for several tens of seconds, causing the system to be unreachable.

> The fix for this situation is to mark the sched clock as unstable
> as early as possible in the resume path, leaving it unstable
> for the duration of the resume process. This will force the
> scheduler to attempt to align the sched clock across CPUs using
> the delta with time of day, updating sched clock data. In a post
> hibernation event, we can then mark the sched clock as stable
> again, avoiding unnecessary syncs with time of day on systems
> in which TSC is reliable.

This makes no frigging sense what so bloody ever. If the clock is
stable, we don't care about sched_clock_data. When it is stable you get
a linear function of the TSC without complicated bits on.

When it is unstable, only then do we care about the sched_clock_data.

> Reviewed-by: Erik Quanstrom <quanstro@xxxxxxxxxx>
> Reviewed-by: Frank van der Linden <fllinden@xxxxxxxxxx>
> Reviewed-by: Balbir Singh <sblbir@xxxxxxxxxx>
> Reviewed-by: Munehisa Kamata <kamatam@xxxxxxxxxx>
> Tested-by: Anchal Agarwal <anchalag@xxxxxxxxxx>
> Signed-off-by: Eduardo Valentin <eduval@xxxxxxxxxx>
> ---

NAK, the code very much relies on never getting marked stable again
after it gets set to unstable.

> diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> index 1152259a4ca0..374d40e5b1a2 100644
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -116,7 +116,7 @@ static void __scd_stamp(struct sched_clock_data *scd)
>       scd->tick_raw = sched_clock();
>  }
>  
> -static void __set_sched_clock_stable(void)
> +void set_sched_clock_stable(void)
>  {
>       struct sched_clock_data *scd;
>  
> @@ -236,7 +236,7 @@ static int __init sched_clock_init_late(void)
>       smp_mb(); /* matches {set,clear}_sched_clock_stable() */
>  
>       if (__sched_clock_stable_early)
> -             __set_sched_clock_stable();
> +             set_sched_clock_stable();
>  
>       return 0;
>  }
> -- 
> 2.15.3.AMZN
> 

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