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Subject: Re: [PATCH v3 10/11] xen: Update sched clock offset to avoid system
 instability in hibernation
To:     Anchal Agarwal <anchalag@amazon.com>, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
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References: <cover.1598042152.git.anchalag@amazon.com>
 <238e837b8d4e17925801c4e85de17bdfca4ddd00.1598042152.git.anchalag@amazon.com>
From:   boris.ostrovsky@oracle.com
Organization: Oracle Corporation
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Date:   Sun, 13 Sep 2020 13:52:10 -0400
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On 8/21/20 6:30 PM, Anchal Agarwal wrote:
> Save/restore xen_sched_clock_offset in syscore suspend/resume during PM
> hibernation. Commit '867cefb4cb1012: ("xen: Fix x86 sched_clock() interface
> for xen")' fixes xen guest time handling during migration. A similar issue
> is seen during PM hibernation when system runs CPU intensive workload.
> Post resume pvclock resets the value to 0 however, xen sched_clock_offset
> is never updated. System instability is seen during resume from hibernation
> when system is under heavy CPU load. Since xen_sched_clock_offset is not
> updated, system does not see the monotonic clock value and the scheduler
> would then think that heavy CPU hog tasks need more time in CPU, causing
> the system to freeze


I don't think you need to explain why non-monotonic clocks are bad.
(and, in fact, the same applies to commit message in patch 8)


>
> Signed-off-by: Anchal Agarwal <anchalag@amazon.com>
> ---
>  arch/x86/xen/suspend.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
> index b12db6966af6..a62e08a11681 100644
> --- a/arch/x86/xen/suspend.c
> +++ b/arch/x86/xen/suspend.c
> @@ -98,8 +98,9 @@ static int xen_syscore_suspend(void)
>  		return 0;
>  
>  	gnttab_suspend();
> -
>  	xen_manage_runstate_time(-1);
> +	xen_save_sched_clock_offset();
> +
>  	xrfp.domid = DOMID_SELF;
>  	xrfp.gpfn = __pa(HYPERVISOR_shared_info) >> PAGE_SHIFT;
>  
> @@ -120,6 +121,12 @@ static void xen_syscore_resume(void)
>  	xen_hvm_map_shared_info();
>  
>  	pvclock_resume();
> +
> +	/*
> +	 * Restore xen_sched_clock_offset during resume to maintain
> +	 * monotonic clock value
> +	 */


I'd drop this comment, we know what the call does.


-boris


> +	xen_restore_sched_clock_offset();
>  	xen_manage_runstate_time(0);
>  	gnttab_resume();
>  }