From: Reinette Chatre <reinette.chatre@intel.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: tglx@linutronix.de, fenghua.yu@intel.com, bp@alien8.de,
tony.luck@intel.com, kuo-lang.tseng@intel.com,
shakeelb@google.com, mingo@redhat.com, babu.moger@amd.com,
james.morse@arm.com, hpa@zytor.com, x86@kernel.org,
linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group
Date: Wed, 16 Dec 2020 10:26:50 -0800 [thread overview]
Message-ID: <b75d780d-d067-12bf-b0e6-706dda200511@intel.com> (raw)
In-Reply-To: <jhj1rfptzqt.mognet@arm.com>
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
next prev parent reply other threads:[~2020-12-16 18:28 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-03 23:25 [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group Reinette Chatre
2020-12-03 23:25 ` [PATCH 1/3] x86/resctrl: Move setting task's active CPU in a mask into helpers Reinette Chatre
2020-12-07 18:29 ` Borislav Petkov
2020-12-07 21:24 ` Reinette Chatre
2020-12-08 9:49 ` Borislav Petkov
2020-12-08 16:35 ` Reinette Chatre
2020-12-09 16:47 ` James Morse
2020-12-10 0:21 ` Reinette Chatre
2020-12-03 23:25 ` [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group Reinette Chatre
2020-12-09 16:51 ` James Morse
2020-12-10 0:22 ` Reinette Chatre
2020-12-11 20:46 ` Valentin Schneider
2020-12-14 18:41 ` Reinette Chatre
2020-12-16 17:41 ` Valentin Schneider
2020-12-16 18:26 ` Reinette Chatre [this message]
2020-12-17 10:39 ` Valentin Schneider
2020-12-03 23:25 ` [PATCH 3/3] x86/resctrl: Don't move a task to the same " Reinette Chatre
2020-12-11 20:46 ` [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a " Valentin Schneider
2020-12-14 18:38 ` Reinette Chatre
2020-12-16 17:41 ` Valentin Schneider
2020-12-16 18:26 ` Reinette Chatre
2020-12-17 12:19 ` [PATCH 4/3] x86/intel_rdt: Apply READ_ONCE/WRITE_ONCE to task_struct .rmid & .closid Valentin Schneider
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