Re: [PATCH v5 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting

[Wanpeng Li]

2016-06-08 18:14 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>
>
> On 08/06/2016 05:05, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> This patch adds guest steal-time support to full dynticks CPU
>> time accounting. After the following commit:
>>
>> ff9a9b4c4334 ("sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity")
>>
>> ... time sampling became jiffy based, even if it's still listened
>> to ring boundaries, so steal_account_process_tick() is reused
>> to account how many 'ticks' are stolen-time, after the last accumulation.
>>
>> Suggested-by: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>> v4 -> v5:
>>  * apply same logic to account_idle_time, so change get_vtime_delta instead
>> v3 -> v4:
>>  * fix grammar errors, thanks Ingo
>>  * cleanup fragile codes, thanks Ingo
>> v2 -> v3:
>>  * convert steal time jiffies to cputime
>> v1 -> v2:
>>  * fix divide zero bug, thanks Rik
>>
>>  kernel/sched/cputime.c | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> index 75f98c5..b62f9f8 100644
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -257,7 +257,7 @@ void account_idle_time(cputime_t cputime)
>>               cpustat[CPUTIME_IDLE] += (__force u64) cputime;
>>  }
>>
>> -static __always_inline bool steal_account_process_tick(void)
>> +static __always_inline unsigned long steal_account_process_tick(void)
>>  {
>>  #ifdef CONFIG_PARAVIRT
>>       if (static_key_false(&paravirt_steal_enabled)) {
>> @@ -279,7 +279,7 @@ static __always_inline bool steal_account_process_tick(void)
>>               return steal_jiffies;
>>       }
>>  #endif
>> -     return false;
>> +     return 0;
>>  }
>>
>>  /*
>> @@ -681,12 +681,17 @@ static cputime_t vtime_delta(struct task_struct *tsk)
>>  static cputime_t get_vtime_delta(struct task_struct *tsk)
>>  {
>>       unsigned long now = READ_ONCE(jiffies);
>> -     unsigned long delta = now - tsk->vtime_snap;
>> +     cputime_t delta_time, steal_time;
>>
>> +     steal_time = jiffies_to_cputime(steal_account_process_tick());
>> +     delta_time = jiffies_to_cputime(now - tsk->vtime_snap);
>>       WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);
>>       tsk->vtime_snap = now;
>>
>> -     return jiffies_to_cputime(delta);
>> +     if (steal_time < delta_time)
>> +             delta_time -= steal_time;
>> +
>> +     return delta_time;
>
> I think this is wrong.  If you get more steal time than delta time
> (which as Rik noticed can happen due to partial jiffies), you will end
> up accounting things twice, once in steal_account_process_tick and once
> here.  In other words you'll get the exact bug you're trying to fix.
>
> The right thing is to add a max_jiffies argument to
> steal_account_process_tick.  steal_account_process_tick will not attempt
> to remove more than max_jiffies.  Here you pass delta_jiffies (i.e. now
> - tsk->vtime_snap) to steal_account_process_tick, existing callers can
> pass ULONG_MAX.  You can then
>
>         return jiffies_to_cputime(delta_jiffies - steal_jiffies);
>
> in get_vtime_delta and not worry about underflow.

I see, I will do it in next version.

Regards,
Wanpeng Li