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Subject: Re: [PATCH v6 1/1] xen/time: do not decrease steal time after live
 migration on xen
To:     Dongli Zhang <dongli.zhang@oracle.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
References: <1509500793-9896-1-git-send-email-dongli.zhang@oracle.com>
Cc:     jgross@suse.com, joao.m.martins@oracle.com
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <cb9986c7-2b30-5d43-946e-400e5efc41b2@oracle.com>
Date:   Wed, 1 Nov 2017 15:19:54 -0400
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On 10/31/2017 09:46 PM, Dongli Zhang wrote:
> After guest live migration on xen, steal time in /proc/stat
> (cpustat[CPUTIME_STEAL]) might decrease because steal returned by
> xen_steal_lock() might be less than this_rq()->prev_steal_time which is
> derived from previous return value of xen_steal_clock().
>
> For instance, steal time of each vcpu is 335 before live migration.
>
> cpu  198 0 368 200064 1962 0 0 1340 0 0
> cpu0 38 0 81 50063 492 0 0 335 0 0
> cpu1 65 0 97 49763 634 0 0 335 0 0
> cpu2 38 0 81 50098 462 0 0 335 0 0
> cpu3 56 0 107 50138 374 0 0 335 0 0
>
> After live migration, steal time is reduced to 312.
>
> cpu  200 0 370 200330 1971 0 0 1248 0 0
> cpu0 38 0 82 50123 500 0 0 312 0 0
> cpu1 65 0 97 49832 634 0 0 312 0 0
> cpu2 39 0 82 50167 462 0 0 312 0 0
> cpu3 56 0 107 50207 374 0 0 312 0 0
>
> Since runstate times are cumulative and cleared during xen live migration
> by xen hypervisor, the idea of this patch is to accumulate runstate times
> to global percpu variables before live migration suspend. Once guest VM is
> resumed, xen_get_runstate_snapshot_cpu() would always return the sum of new
> runstate times and previously accumulated times stored in global percpu
> variables. Comments before the call of HYPERVISOR_suspend() has been
> removed as it is inaccurate. The call can return an error code (e.g.,
> possibly -EPERM in the future).

I'd like split comment removal bit into a separate paragraph. I can do
this when committing if you don't mind.

>
> Similar and more severe issue would impact prior linux 4.8-4.10 as
> discussed by Michael Las at
> https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest,
> which would overflow steal time and lead to 100% st usage in top command
> for linux 4.8-4.10. A backport of this patch would fix that issue.
>
> References: https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
>
> ---

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>


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