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

[Ravi Bangoria]

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@xxxxxxx> 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@xxxxxxx>
>>> 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@xxxxxxx>
>>> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
>>
>> 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

Thanks,
Ravi