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 10:26:50 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Valentin Schneider <valentin.schneider@....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

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