Re: [Xen-devel] [PATCH v3 1/1] xen/time: do not decrease steal time after live migration on xen

[Dongli Zhang]

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@xxxxxxxxxx>
>>>>
>>>> ---
>>>> 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()).

I would choose option 4 if it is fine for reviewers.

Thank you very much!

Dongli Zhang

> 
> 
> Juergen
> 
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