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

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Date:	Mon, 8 Aug 2016 22:04:23 -0700
From:	David Carrillo-Cisneros <davidcc@...gle.com>
To:	Nilay Vaish <nilayvaish@...il.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

Hi Nilay,

Sounds good, I will post an updated version.

Thanks,
David

On Mon, Aug 8, 2016 at 9:12 AM, Nilay Vaish <nilayvaish@...il.com> wrote:
> 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