Re: [Xen-devel] PV performance degraded after live migration

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/15/2017 12:02 PM, Jan Beulich wrote:
>>>> On 15.03.17 at 16:26, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 15/03/17 15:23, Olaf Hering wrote:
>>> On Wed, Mar 15, Olaf Hering wrote:
>>>
>>>> On Wed, Mar 15, Andrew Cooper wrote:
>>>>> As a crazy idea, doest this help?
>>>>> tsc->incarnation = 0
>>> This does indeed help. One system shows now the results below, which
>>> means the performance goes down during migration (to localhost) and goes
>>> back to normal after migration.
>>>
>>> What impact has such change to ->incarnation?
>> So what this means is that, after migrate, Xen sees that the advertised
>> TSC value doesn't match the current hardwares TSC, so enables rdtsc
>> interception so the values reported back can be emulated at the old
>> frequency.
>>
>> There is no easy solution to this problem.
> Especially for localhost migration it ought to be possible to deal
> with this. What I'm wondering is why the frequency comparison
> in default mode handling in tsc_set_info() is HVM-only:
>
>         if ( tsc_mode == TSC_MODE_DEFAULT && host_tsc_is_safe() &&
>              (has_hvm_container_domain(d) ?
>               (d->arch.tsc_khz == cpu_khz ||
>                hvm_get_tsc_scaling_ratio(d->arch.tsc_khz)) :
>               incarnation == 0) )
>         {
>
> Boris, you've made it be this way in 82713ec8d2 ("x86: use native
> RDTSC(P) execution when guest and host frequencies are the
> same").

Don't know why.

In fact, I looked at the history of this patch and earlier versions had

@@ -1889,10 +1890,14 @@ void tsc_set_info(struct domain *d,
         d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
         d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz;
         set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 );
-        /* use native TSC if initial host has safe TSC, has not migrated
-         * yet and tsc_khz == cpu_khz */
-        if ( host_tsc_is_safe() && incarnation == 0 &&
-                d->arch.tsc_khz == cpu_khz )
+        /*
+         * Use native TSC if initial host has safe TSC and either has not
+         * migrated yet or tsc_khz == cpu_khz (either "naturally" or via
+         * TSC scaling)
+         */
+        if ( host_tsc_is_safe() &&
+             (incarnation == 0 || d->arch.tsc_khz == cpu_khz ||
+              cpu_has_tsc_ratio) )
             d->arch.vtsc = 0;
         else 
             d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns);


However, the original code was even stricter.


-boris




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