Date:   Tue, 31 Oct 2017 07:05:19 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     jgross@...e.com, joao.m.martins@...cle.com
Subject: Re: [Xen-devel] [PATCH v5 1/1] xen/time: do not decrease steal time
 after live migration on xen



On 10/30/2017 11:13 PM, Dongli Zhang wrote:
> Hi Boris,
> 
> On 10/31/2017 08:58 AM, Boris Ostrovsky wrote:
>>
>>
>> On 10/30/2017 08:14 PM, Dongli Zhang wrote:
>>> Hi Boris,
>>>
>>> On 10/30/2017 09:34 PM, Boris Ostrovsky wrote:
>>>> On 10/30/2017 04:03 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
>>>>>
>>>>> Changed since v3:
>>>>>     * do not accumulate times in the case of guest checkpointing
>>>>>
>>>>> Changed since v4:
>>>>>     * allocate array of vcpu_runstate_info to reduce number of memory allocation
>>>>>
>>>>> ---
>>>>>    drivers/xen/manage.c         |  2 ++
>>>>>    drivers/xen/time.c           | 68
>>>>> ++++++++++++++++++++++++++++++++++++++++++--
>>>>>    include/xen/interface/vcpu.h |  2 ++
>>>>>    include/xen/xen-ops.h        |  1 +
>>>>>    4 files changed, 71 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
>>>>> index c425d03..3dc085d 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(-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);
>>>>
>>>> I am not convinced that the comment above HYPERVISOR_suspend() is
>>>> correct. The call can return an error code and so if it returns -EPERM
>>>> (which AFAICS it can't now but might in the future) then
>>>> xen_accumulate_runstate_time() will do wrong thing.
>>>
>>> I would split xen_accumulate_runstate_time() into two functions to avoid the
>>> -EPERM issue, as one is for saving and another is for accumulation, respectively.
>>>
>>> Otherwise, can you use xen_accumulate_runstate_time(2) for saving before suspend
>>> and xen_accumulate_runstate_time(si->cancelled) after resume?
>>
>>
>> I'd probably just say something like
>>
>> si->cancelled = HYPERVISOR_suspend() ? 1 : 0;
> 
> As the call of HYPERVISOR_suspend() takes 3 lines, I would make it as below and
> I think gcc would optimize it.
> 
> -   /*
> -    * This hypercall returns 1 if suspend was cancelled
> -    * or the domain was merely checkpointed, and 0 if it
> -    * is resuming in a new domain.
> -    */
>      si->cancelled = HYPERVISOR_suspend(xen_pv_domain()
>                                              ? virt_to_gfn(xen_start_info)
>                                              : 0);
> +   si->cancelled = si->cancelled ? 1 : 0;


Or xen_manage_runstate_time(si->cancelled ? 1 : 0) --- that way you 
preserve si->cancelled if anyone cares later (which at the moment nooone 
does) Either way I think is fine.


> 
>>
>> and keep xen_accumulate_runstate_time() as is (maybe rename it to
>> xen_manage_runstate_time()). And also remove the comment above the hypercall as
>> it is incorrect (but please mention the reason in the commit message)
>>
>>>
>>>>
>>>>
>>>>>        gnttab_resume();
>>>>>          if (!si->cancelled) {
>>>>> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
>>>>> index ac5f23f..cf3afb9 100644
>>>>> --- a/drivers/xen/time.c
>>>>> +++ b/drivers/xen/time.c
>>>>> @@ -19,6 +19,9 @@
>>>>>    /* runstate info updated by Xen */
>>>>>    static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate);
>>>>>    +static DEFINE_PER_CPU(u64[RUNSTATE_max], old_runstate_time);
>>>>> +static struct vcpu_runstate_info *runstate_delta;
>>>>
>>>> I'd move this inside xen_accumulate_runstate_time() since that's the
>>>
>>> If we split xen_accumulate_runstate_time() into two functions, we would leave
>>> runstate_delta as global static.
>>>
>>>> only function that uses it. And why does it need to be
>>>> vcpu_runstate_info and not u64[4]?
>>>
>>> This was suggested by Juergen to avoid the allocation and reclaim of the second
>>> dimensional array as in v4 of this patch?
>>>
>>> Or would you like to allocate sizeof(u64[4]) * num_possible_cpus() and emulate
>>> the 2d array with this 1d array and move the pointer forward sizeof(u64[4]) in
>>> each iteration?
>>
>>
>> I was thinking of
>>
>> u64 **runstate_delta = (u64 **)kmalloc(sizeof(xen_runstate.time) *
>> num_possible_cpus())
>>
>> and then you should be able to access runstate_delta[cpu][RUNSTATE_*].
> 
> Would the above code work?

Ugh... If course it will not, it's clearly wrong. So nevermind, sorry. 
Keep it the way it was originally.

-boris

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