Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events

[James Clark]

On 30/09/2021 19:47, Stephane Eranian wrote:
> On Thu, Sep 23, 2021 at 9:02 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>>
>> Hi Leo,
>>
>> On Thu, Sep 23, 2021 at 7:23 AM Leo Yan <leo.yan@xxxxxxxxxx> wrote:
>>>
>>> Hi Namhyung,
>>>
>>> On Thu, Sep 16, 2021 at 02:01:21PM -0700, Namhyung Kim wrote:
>>>
>>> [...]
>>>
>>>>> Before we had discussion for enabling PID/TID for SPE samples; in the patch
>>>>> set [1], patches 07, 08 set sample's pid/tid based on the Arm SPE context
>>>>> packets.  To enable hardware tracing context ID, you also needs to enable
>>>>> kernel config CONFIG_PID_IN_CONTEXTIDR.
>>>>
>>>> Thanks for sharing this.
>>>>
>>>> Yeah I also look at the context info but having a dependency on a kconfig
>>>> looks limiting its functionality.  Also the kconfig says it has some overhead
>>>> in the critical path (even if perf is not running, right?) - but not sure how
>>>> much it can add.
>>>
>>> Yes, after enabled config PID_IN_CONTEXTIDR, the kernel will always
>>> write PID into the system register CONTEXTIDR during process context
>>> switching.  Please see the flow:
>>>
>>>   __switch_to() (arch/arm64/kernel/process.c)
>>>     `-> contextidr_thread_switch(next)
>>
>> Thanks for the info.  I assume it's a light-weight operation.
>>
>>
> I'd like to understand why it was believed that having SPE record to
> PID could be too expensive
> vs. what I am seeing with all the tracking of context switches and the
> volume of data this generates.
> 

I think the justification about it being expensive is that when PID_IN_CONTEXTIDR
is set, there is an extra few instructions to write that value on every context
switch, whether SPE is enabled or not. So it has a system wide impact.