linux-kernel - Re: [PATCH 6/9] perf report: Support instruction latency

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Date:   Fri, 5 Feb 2021 09:51:24 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, eranian@...gle.com,
        namhyung@...nel.org, jolsa@...hat.com, ak@...ux.intel.com,
        yao.jin@...ux.intel.com, maddy@...ux.vnet.ibm.com
Subject: Re: [PATCH 6/9] perf report: Support instruction latency



On 2/5/2021 7:55 AM, Athira Rajeev wrote:
>>> Because in other archs, the var2_w of ‘perf_sample_weight’ could be used to capture something else than the Local INSTR Latency.
>>> Can we have some weak function to populate the header string ?
>> I agree that the var2_w has different meanings among architectures. We should not force it to data->ins_lat.
>>
>> The patch as below should fix it. Does it work for you?
> My point about weak function was actually for the arch specific header string. But I guess we should not force it to data->ins_lat

Yes, I don't think PowerPC should force var2_w to data->ins_lat. I think 
you can create your own field.

> as you mentioned. I checked the below patch defining an ‘arch_perf_parse_sample_weight' for powerpc and it works.
> 
> But one observation is that, for cases with kernel having support for PERF_SAMPLE_WEIGHT_STRUCT but missing arch specific support for  ‘arch_perf_parse_sample_weight', it will report ‘Local Weight’ wrongly since weak function takes it as 64 bit. Not sure if that is a valid case to consider though.

Currently, the PERF_SAMPLE_WEIGHT_STRUCT is only enabled on X86 by default.
https://lore.kernel.org/lkml/1612296553-21962-6-git-send-email-kan.liang@linux.intel.com/

For PowerPC, the PERF_SAMPLE_WEIGHT is still the default setting. There 
is no way to set PERF_SAMPLE_WEIGHT_STRUCT via perf tool.
I don't think the above case will happen.

Thanks,
Kan