Re: [PATCH v3] perf evsel: Fix missing exclude_{host,guest} setting

[Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>]

On November 7, 2021 7:25:45 AM GMT-03:00, Ravi Bangoria <ravi.bangoria@amd.com> wrote:
>
>
>On 07-Nov-21 12:54 AM, Arnaldo Carvalho de Melo wrote:
>> Em Fri, Nov 05, 2021 at 11:00:29AM -0700, Namhyung Kim escreveu:
>>> Hello,
>>>
>>> On Wed, Nov 3, 2021 at 12:22 AM Ravi Bangoria <ravi.bangoria@amd.com> wrote:
>>>>
>>>>> The current logic for the perf missing feature has a bug that it can
>>>>> wrongly clear some modifiers like G or H.  Actually some PMUs don't
>>>>> support any filtering or exclusion while others do.  But we check it
>>>>> as a global feature.
>>>>
>>>> (Sorry to pitch in bit late)
>>>>
>>>> AMD has one more problem on a similar line. On AMD, non-precise and
>>>> precise sampling are provided by core and IBS pmu respectively. Plus,
>>>> core pmu has filtering capability but IBS does not. Perf by default
>>>> sets precise_ip=3 and exclude_guest=1 and goes on decreasing precise_ip
>>>> with exclude_guest set until perf_event_open() succeeds. This is
>>>> causing perf to always fallback to core pmu (non-precise mode) even if
>>>> it's perfectly feasible to do precise sampling. Do you guys think this
>>>> problem should also be addressed while designing solution for Namhyung's
>>>> patch or solve it seperately like below patch:
>>>>
>>>> ---><---
>>>>
>>>> From 48808299679199c39ff737a30a7f387669314fd7 Mon Sep 17 00:00:00 2001
>>>> From: Ravi Bangoria <ravi.bangoria@amd.com>
>>>> Date: Tue, 2 Nov 2021 11:01:12 +0530
>>>> Subject: [PATCH] perf/amd/ibs: Don't set exclude_guest by default
>>>>
>>>> Perf tool sets exclude_guest by default while calling perf_event_open().
>>>> Because IBS does not have filtering capability, it always gets rejected
>>>> by IBS PMU driver and thus perf falls back to non-precise sampling. Fix
>>>> it by not setting exclude_guest by default on AMD.
>>>>
>>>> Before:
>>>>   $ sudo ./perf record -C 0 -vvv true |& grep precise
>>>>     precise_ip                       3
>>>>   decreasing precise_ip by one (2)
>>>>     precise_ip                       2
>>>>   decreasing precise_ip by one (1)
>>>>     precise_ip                       1
>>>>   decreasing precise_ip by one (0)
>>>>
>>>> After:
>>>>   $ sudo ./perf record -C 0 -vvv true |& grep precise
>>>>     precise_ip                       3
>>>>   decreasing precise_ip by one (2)
>>>>     precise_ip                       2
>>>>
>>>> Reported-by: Kim Phillips <kim.phillips@amd.com>
>>>> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
>>>
>>> It'd be nice if it can cover explicit -e cycles:pp as well.  Anyway,
>> 
>> Ravi, please consider Namhyung's request, a patch on top as I'm adding
>> this already.
>
>For explicit :pp modifier, evsel->precise_max does not get set and thus perf
>does not try with different attr->precise_ip values while exclude_guest set.
>So no issue with explicit :pp:
>
>  $ sudo ./perf record -C 0 -e cycles:pp -vvv |& grep "precise_ip\|exclude_guest"
>    precise_ip                       2
>    exclude_guest                    1
>    precise_ip                       2
>    exclude_guest                    1
>  switching off exclude_guest, exclude_host
>    precise_ip                       2
>  ^C
>
>Also, with :P modifier, evsel->precise_max gets set but exclude_guest does
>not and thus :P also works fine:
>
>  $ sudo ./perf record -C 0 -e cycles:P -vvv |& grep "precise_ip\|exclude_guest"
>    precise_ip                       3
>  decreasing precise_ip by one (2)
>    precise_ip                       2
>  ^C

Perfect, I'll add this analysis to the cset log message.

- Arnaldo
>
>Thanks,
>Ravi