linux-kernel - Re: [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group

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Date:   Wed, 16 Dec 2020 17:41:30 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, bp@...en8.de,
        tony.luck@...el.com, kuo-lang.tseng@...el.com, shakeelb@...gle.com,
        mingo@...hat.com, babu.moger@....com, james.morse@....com,
        hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group


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. With

  tsk->closid = x;

  barrier();

  if (task_curr(tsk))
      smp_call(...);

that explicitely cannot happen.

>
> Thank you very much
>
> Reinette