Re: [PATCH] xen: always set the sched clock as unstable

From: David Vrabel
Date: Mon Apr 16 2012 - 10:59:43 EST

On 16/04/12 12:32, Jan Beulich wrote:
>>>> On 13.04.12 at 20:20, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
>> From: David Vrabel <david.vrabel@xxxxxxxxxx>
>> The sched clock was considered stable based on the capabilities of the
>> underlying hardware. This does not make sense for Xen PV guests as:
>> a) the hardware TSC is not used directly as the clock source; and b)
>> guests may migrate to hosts with different hardware capabilities.
>> It is not clear to me whether the Xen clock source is supposed to be
>> stable and whether it should be stable across migration. For a clock
>> source to be stable it must be: a) monotonic; c) synchronized across
>> CPUs; and c) constant rate.

Tim, Thomas, can you comment on the above paragraph? Is it correct?

>> There have also been reports of systems with apparently unstable
>> clocks where clearing sched_clock_stable has fixed problems with
>> migrated VMs hanging.
>> So, always set the sched clock as unstable when using the Xen clock
>> source.
>> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
>> ---
>> arch/x86/xen/time.c | 1 +
>> 1 files changed, 1 insertions(+), 0 deletions(-)
>> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
>> index 0296a95..8469b5a 100644
>> --- a/arch/x86/xen/time.c
>> +++ b/arch/x86/xen/time.c
>> @@ -473,6 +473,7 @@ static void __init xen_time_init(void)
>> do_settimeofday(&tp);
>> setup_force_cpu_cap(X86_FEATURE_TSC);
>> + sched_clock_stable = 0;
> This, unfortunately, is not sufficient afaict: If a CPU gets brought up
> post-boot, the variable may need to be cleared again. Instead you
> ought to call mark_tsc_unstable().

Yeah, mark_tsc_unstable() is the right thing to do.

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