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Date:   Thu, 7 May 2020 22:43:43 -0700
From:   Ian Rogers <>
To:     Andi Kleen <>
Cc:     Peter Zijlstra <>,
        Ingo Molnar <>,
        Arnaldo Carvalho de Melo <>,
        Mark Rutland <>,
        Alexander Shishkin <>,
        Jiri Olsa <>,
        Namhyung Kim <>,
        Alexei Starovoitov <>,
        Daniel Borkmann <>,
        Martin KaFai Lau <>,
        Song Liu <>, Yonghong Song <>,
        Andrii Nakryiko <>,
        John Fastabend <>,
        KP Singh <>,
        Kajol Jain <>,
        John Garry <>,
        Jin Yao <>,
        Kan Liang <>,
        Cong Wang <>,
        Kim Phillips <>,
        LKML <>,
        Networking <>, bpf <>,
        linux-perf-users <>,
        Stephane Eranian <>
Subject: Re: [RFC PATCH 0/7] Share events between metrics

On Thu, May 7, 2020 at 2:47 PM Andi Kleen <> 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:
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

> >
> > 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!

> -Andi

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