linux-kernel - Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics

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Message-ID: <09770caf-e717-5b11-d11f-f30cb027bb05@linux.intel.com>
Date:   Wed, 29 May 2019 10:42:29 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics



On 5/29/2019 3:57 AM, Peter Zijlstra wrote:
> On Tue, May 28, 2019 at 02:24:56PM -0400, Liang, Kan wrote:
>>
>>
>> On 5/28/2019 9:48 AM, Peter Zijlstra wrote:
>>> On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@...ux.intel.com wrote:
>>>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>>>> index b980b9e95d2a..0d7081434d1d 100644
>>>> --- a/include/linux/perf_event.h
>>>> +++ b/include/linux/perf_event.h
>>>> @@ -133,6 +133,11 @@ struct hw_perf_event {
>>>>    			struct hw_perf_event_extra extra_reg;
>>>>    			struct hw_perf_event_extra branch_reg;
>>>> +
>>>> +			u64		saved_metric;
>>>> +			u64		saved_slots;
>>>> +			u64		last_slots;
>>>> +			u64		last_metric;
>>>
>>> This is really sad, and I'm thinking much of that really isn't needed
>>> anyway, due to how you're not using some of the other fields.
>>
>> If we don't cache the value, we have to update all metrics events when
>> reading any metrics event. I think that could bring high overhead.
> 
> Since you don't support sampling, or even 'normal' functionality on this
> FIXED3/SLOTS thing, you'll not use prev_count, sample_period,
> last_period, period_left, interrupts_seq, interrupts, freq_time_stamp
> and freq_count_stamp.
> 
> So why do you then need to grow the data structure with 4 more nonsense
> fields?
> 
> Also, I'm not sure what you need those last things for, you reset the
> value to 0 every time you read them, there is no delta to track.
> 

The 'last things' are only for per-thread Topdown RDPMC.
We have to save/restore slots and metrics value in context switch.
It's used to calculate the delta between thread shced in and current 
read/sched out.


I will split the patch into several patches, and try to make things clear.

Thanks,
Kan