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Date: Fri, 27 Oct 2017 09:49:03 +0200
From: Juergen Gross <jgross@...e.com>
To: Dongli Zhang <dongli.zhang@...cle.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc: joao.m.martins@...cle.com
Subject: Re: [Xen-devel] [PATCH v3 1/1] xen/time: do not decrease steal time
after live migration on xen
On 27/10/17 09:40, Dongli Zhang wrote:
> Hi Juergen,
>
> On 10/27/2017 03:31 PM, Juergen Gross wrote:
>> On 27/10/17 09:16, Dongli Zhang wrote:
>>> Hi Boris,
>>>
>>> On 10/25/2017 11:12 PM, Boris Ostrovsky wrote:
>>>> On 10/25/2017 02:45 AM, 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.
>>>>>
>>>>> 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@...cle.com>
>>>>>
>>>>> ---
>>>>> Changed since v1:
>>>>> * relocate modification to xen_get_runstate_snapshot_cpu
>>>>>
>>>>> Changed since v2:
>>>>> * accumulate runstate times before live migration
>>>>>
>>>>> ---
>>>>> drivers/xen/manage.c | 1 +
>>>>> drivers/xen/time.c | 19 +++++++++++++++++++
>>>>> include/xen/xen-ops.h | 1 +
>>>>> 3 files changed, 21 insertions(+)
>>>>>
>>>>> diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
>>>>> index c425d03..9aa2955 100644
>>>>> --- a/drivers/xen/manage.c
>>>>> +++ b/drivers/xen/manage.c
>>>>> @@ -72,6 +72,7 @@ static int xen_suspend(void *data)
>>>>> }
>>>>>
>>>>> gnttab_suspend();
>>>>> + xen_accumulate_runstate_time();
>>>>> xen_arch_pre_suspend();
>>>>>
>>>>> /*
>>>>> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
>>>>> index ac5f23f..6df3f82 100644
>>>>> --- a/drivers/xen/time.c
>>>>> +++ b/drivers/xen/time.c
>>>>> @@ -19,6 +19,8 @@
>>>>> /* runstate info updated by Xen */
>>>>> static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate);
>>>>>
>>>>> +static DEFINE_PER_CPU(u64[4], old_runstate_time);
>>>>> +
>>>>> /* return an consistent snapshot of 64-bit time/counter value */
>>>>> static u64 get64(const u64 *p)
>>>>> {
>>>>> @@ -52,6 +54,7 @@ static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
>>>>> {
>>>>> u64 state_time;
>>>>> struct vcpu_runstate_info *state;
>>>>> + int i;
>>>>>
>>>>> BUG_ON(preemptible());
>>>>>
>>>>> @@ -64,6 +67,22 @@ static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
>>>>> rmb(); /* Hypervisor might update data. */
>>>>> } while (get64(&state->state_entry_time) != state_time ||
>>>>> (state_time & XEN_RUNSTATE_UPDATE));
>>>>> +
>>>>> + for (i = 0; i < 4; i++)
>>>>> + res->time[i] += per_cpu(old_runstate_time, cpu)[i];
>>>>> +}
>>>>> +
>>>>> +void xen_accumulate_runstate_time(void)
>>>>> +{
>>>>> + struct vcpu_runstate_info state;
>>>>> + int cpu;
>>>>> +
>>>>> + for_each_possible_cpu(cpu) {
>>>>> + xen_get_runstate_snapshot_cpu(&state, cpu);
>>>>> + memcpy(per_cpu(old_runstate_time, cpu),
>>>>> + state.time,
>>>>> + 4 * sizeof(u64));
>>>>
>>>> sizeof(old_runstate_time). (I think this should work for per_cpu variables)
>>>>
>>>>> + }
>>>>
>>>> Hmm.. This may not perform as intended if we are merely checkpointing
>>>> (or pausing) the guest (i.e. if HYPERVISOR_suspend() returns 1). We will
>>>> double-account for the last interval that the guest has run.
>>>>
>>>> I'd rather not have yet another per-cpu variable but I can't think of
>>>> anything else. Perhaps you or others can come up with something better.
>>>
>>> I have 3 options so far.
>>>
>>> The 1st option is to another per-cpu variable while you do not like it.
>>>
>>> The 2nd option is to borrow from what do_stolen_accounting() used to do. Compute
>>> the delta of current and previous time and do nothing if delta is less than 0.
>>> The drawback of this option is guest might wait for the new time to catch up
>>> with previous time.
>>
>> This could be a rather long time. I don't think this is the way to go.
>>
>>> The 3rd option is to check the return value of HYPERVISOR_suspend() to different
>>> if this is a migration of checkpointing. As we will double-account the runstate
>>> time for checkpointing, why not just divide it by 2? The drawback of this option
>>> is the result is not accurate as we divide the incremental (time before and
>>> after checkpointing) by 2.
>>
>> And it is wrong if you do multiple migrations.
>>
>>> Would you please let me know which option we prefer?
>>
>> Perhaps option 4:
>>
>> Allocate a buffer at suspend time for the times to add up and do the
>> correction after suspend and free the buffer again.
>
> With option 4, we need to allocate and free the buffer in xen_suspend() and we
> would not be able to encapsulate everything inside
> xen_accumulate_runstate_time(). (unless we use a array of static variables in
> xen_accumulate_runstate_time()).
Add a parameter to xen_accumulate_runstate_time() and call it before and
after suspend, e.g.:
--- a/drivers/xen/manage.c
+++ b/drivers/xen/manage.c
@@ -72,6 +72,7 @@ static int xen_suspend(void *data)
}
gnttab_suspend();
+ xen_accumulate_runstate_time(-1);
xen_arch_pre_suspend();
/*
@@ -84,6 +85,7 @@ static int xen_suspend(void *data)
: 0);
xen_arch_post_suspend(si->cancelled);
+ xen_accumulate_runstate_time(si->cancelled);
gnttab_resume();
if (!si->cancelled) {
Then the allocation and freeing of the buffer both can happen in
xen_accumulate_runstate_time() (which then might want to be called
xen_time_suspend() or such).
Juergen
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