linux-kernel - Re: [RFC PATCH 0/7] Share events between metrics

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Message-ID: <CAP-5=fV2eNAt0LLHYXeLMR6GZi_oGZyzz8psErNkbahLQs-VLQ@mail.gmail.com>
Date:   Thu, 7 May 2020 22:43:43 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Kajol Jain <kjain@...ux.ibm.com>,
        John Garry <john.garry@...wei.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Kim Phillips <kim.phillips@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC PATCH 0/7] Share events between metrics

On Thu, May 7, 2020 at 2:47 PM Andi Kleen <ak@...ux.intel.com> wrote:
>
> > > - without this change events within a metric may get scheduled
> > >   together, after they may appear as part of a larger group and be
> > >   multiplexed at different times, lowering accuracy - however, less
> > >   multiplexing may compensate for this.
> >
> > I agree the heuristic in this patch set is naive and would welcome to
> > improve it from your toplev experience. I think this change is
> > progress on TopDownL1 - would you agree?
>
> TopdownL1 in non SMT mode should always fit. Inside a group
> deduping always makes sense.
>
> The problem is SMT mode where it doesn't fit. toplev tries
> to group each node and each level together.

Thanks Andi, I've provided some examples of TopDownL3_SMT in the cover
letter of the v3 patch set:
https://lore.kernel.org/lkml/20200508053629.210324-1-irogers@google.com/
I tested sandybridge and cascadelake and the results look similar to
the non-SMT version. Let me know if there's a different variant to
test.

> >
> > I'm wondering if what is needed are flags to control behavior. For
> > example, avoiding the use of groups altogether. For TopDownL1 I see.
>
> Yes the current situation isn't great.
>
> For Topdown your patch clearly is an improvement, I'm not sure
> it's for everything though.
>
> Probably the advanced heuristics are only useful for a few
> formulas, most are very simple. So maybe it's ok. I guess
> would need some testing over the existing formulas.

Agreed, do you have a pointer on a metric group where things would
obviously be worse? I started off with a cache miss and hit rate
metric and similar to topdown this approach is a benefit.

In v3 I've added a --metric-no-merge option to retain existing
grouping behavior, I've also added a --metric-no-group that avoids
groups for all metrics. This may be useful if the NMI watchdog can't
be disabled.

Thanks for the input!
Ian

> -Andi