linux-kernel - Re: [PATCH v2 3/4] perf/core: introduce PMU_EV_CAP_READ_ACTIVE

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Date:	Mon, 8 Aug 2016 11:12:08 -0500
From:	Nilay Vaish <nilayvaish@...il.com>
To:	David Carrillo-Cisneros <davidcc@...gle.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <ak@...ux.intel.com>,
	Kan Liang <kan.liang@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Paul Turner <pjt@...gle.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2 3/4] perf/core: introduce PMU_EV_CAP_READ_ACTIVE_PKG

On 7 August 2016 at 15:10, David Carrillo-Cisneros <davidcc@...gle.com> wrote:
> Hi Nilay,
>
>>>  static int perf_event_read(struct perf_event *event, bool group)
>>>  {
>>> -       int ret = 0;
>>> +       int ret = 0, cpu_to_read;
>>>
>>> -       /*
>>> -        * If event is enabled and currently active on a CPU, update the
>>> -        * value in the event structure:
>>> -        */
>>> -       if (event->state == PERF_EVENT_STATE_ACTIVE) {
>>> +       cpu_to_read = find_cpu_to_read(event);
>>> +
>>> +       if (cpu_to_read >= 0) {
>>>                 struct perf_read_data data = {
>>>                         .event = event,
>>>                         .group = group,
>>>                         .ret = 0,
>>>                 };
>>> -               ret = smp_call_function_single(event->oncpu,
>>> +               ret = smp_call_function_single(cpu_to_read,
>>>                                                __perf_event_read, &data,
>>> 1);
>>>                 ret = ret ? : data.ret;
>>>         } else if (event->state == PERF_EVENT_STATE_INACTIVE) {
>>>
>>
>> I would like to suggest a small change to this patch.  I think the check on
>> PERF_EVENT_STATE_ACTIVE should be retained in the perf_event_read()
>> function.  The new function should assume that the event is active.  I find
>> this more readable since the next check in function perf_event_read() is on
>> PERF_EVENT_STATE_INACTIVE.
>
> Two oncoming flags that Intel CQM/CMT will use are meant to allow read
> even if event is inactive. This makes sense in CQM/CMT because the hw
> RMID is always reserved. I am ok with keeping the check for
> STATE_ACTIVE until those flags are actually introduced, tough.


Hello David

Lets go with checking PERF_EVENT_STATE_ACTIVE in perf_event_read() for
the time being.  With the new version of the patch that you posted, I
find that checking PERF_EVENT_STATE_ACTIVE in find_cpu_to_read() makes
you introduce another if statement for checking STATE_INACTIVE.

If your CQM/CMT patches later need the code structure you have now, I
would also support it.  But as of now, I think, it is better to check
STATE_ACTIVE in perf_event_read().


Thanks
Nilay