Re: [PATCH v3 5/6] fs/resctrl: Do not invoke smp_processor_id() in preemptible context

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Reinette,

On 6/8/2026 11:10 PM, Reinette Chatre wrote:
> Hi Chenyu,
> 
> On 6/8/26 4:26 AM, Chen, Yu C wrote:
>> On 6/8/2026 4:36 PM, Thomas Gleixner wrote:
>>> On Sat, Jun 06 2026 at 10:38, Chen Yu wrote:
>>>> From: Tony Luck <tony.luck@intel.com>
>>>>
>>>> __l3_mon_event_count() and __l3_mon_event_count_sum() call
>>>> smp_processor_id() to obtain the current CPU. However, some
>>>> monitor events can be read from any CPU in task context via
>>>> mon_event_count(); in that case the calling context is
>>>> preemptible and smp_processor_id() triggers a debug warning.
>>>
>>> Is this new with this MMIO stuff or is this an existing issue? If the
>>> latter then this patch should be in front of the series and get a fixes
>>> tag. If not the change log should explain it.
>>>
>>
>> It is an existing issue. I'll adjust the sequence and provide a fixes tag.
>> Thanks!
> 
> 
> Could you please elaborate how this is an existing issue?
> 
> At this point in implementation mon_evt::any_cpu is and can only be false for
> all events associated with resource RDT_RESOURCE_L3. This is highlighted by
> the existing splat that will be triggered by the
> 		WARN_ON_ONCE(rr->evt->any_cpu);
> that is removed by this patch.
> 
> It is this series that makes it possible to read L3 cache occupancy from any CPU.
> This is only done in the following patch (6/6) that makes mon_evt::any_cpu for
> L3 cache occupancy be conditional on MMIO support enabled by this series.
> 
> This patch (5/6) replaces the above mentioned splat with a subtler check since it
> is about to become possible for L3 cache occupancy events to be read from any CPU.
> 
> I thus do not see this as an existing issue needing fixing but instead a
> necessary part of this series that is specifically a preparatory patch for the
> final MMIO based event enabling done in patch 6/6.
> 

You are right. For RDT_RESOURCE_L3 monitor data reading, before MMIO 
monitor read is
introduced, any_cpu is false, and MSRs are always read on the CPU of 
that domain in the
IPI context. Let me refine the commit log for [PATCH 5/6].

thanks,
Chenyu