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From: Valentin Schneider <valentin.schneider@arm.com>
To: Reinette Chatre <reinette.chatre@intel.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 17:41:30 +0000	[thread overview]
Message-ID: <jhj1rfptzqt.mognet@arm.com> (raw)
In-Reply-To: <e250875b-1c86-660c-b9f0-4060842939bf@intel.com>


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

  reply	other threads:[~2020-12-16 17:42 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 [this message]
2020-12-16 18:26         ` Reinette Chatre
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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