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

[Juergen Gross]

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@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()).

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