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

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Date:   Thu, 17 Dec 2020 10:39:43 +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 16/12/20 18:26, Reinette Chatre wrote:
> Hi Valentin,
>> 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.
>

IMO the 'task_curr()' check only makes sense if it happens *after* the
write, the logic being: 'closid/rmid has been written to, does the task now
need interrupting?'

In the above 'else' clause, task_curr() would need to be re-evaluated after
the write: the task wasn't current *before* the write, but nothing
guarantees this still holds *after* the write.

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