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SubjectRe: [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group
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Hi Valentin,

On 12/16/2020 9:41 AM, Valentin Schneider wrote:
>
> On 14/12/20 18:41, Reinette Chatre wrote:
>>>> - return ret;
>>>> +
>>>> + /*
>>>> + * By now, the task's closid and rmid are set. If the task is current
>>>> + * on a CPU, the PQR_ASSOC MSR needs to be updated to make the resource
>>>> + * group go into effect. If the task is not current, the MSR will be
>>>> + * updated when the task is scheduled in.
>>>> + */
>>>> + update_task_closid_rmid(tsk);
>>>
>>> We need the above writes to be compile-ordered before the IPI is sent.
>>> There *is* a preempt_disable() down in smp_call_function_single() that
>>> gives us the required barrier(), can we deem that sufficient or would we
>>> want one before update_task_closid_rmid() for the sake of clarity?
>>>
>>
>> Apologies, it is not clear to me why the preempt_disable() would be
>> insufficient. If it is not then there may be a few other areas (where
>> resctrl calls smp_call_function_xxx()) that needs to be re-evaluated.
>
> So that's part paranoia and part nonsense from my end - the contents of
> smp_call() shouldn't matter here.
>
> If we distill the code to:
>
> tsk->closid = x;
>
> if (task_curr(tsk))
> smp_call(...);
>
> It is somewhat far fetched, but AFAICT this can be compiled as:
>
> if (task_curr(tsk))
> tsk->closid = x;
> smp_call(...);
> else
> tsk->closid = x;
>
> IOW, there could be a sequence where the closid write is ordered *after*
> the task_curr() read.

Could you please elaborate why it would be an issue is the closid write
is ordered after the task_curr() read? task_curr() does not depend on
the closid.

> With
>
> tsk->closid = x;
>
> barrier();
>
> if (task_curr(tsk))
> smp_call(...);
>
> that explicitely cannot happen.
>


Reinette

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