linux-kernel - Re: [PATCH v3 01/11] perf, tools: Support weak groups

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Date:   Fri, 1 Sep 2017 18:57:59 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     acme@...nel.org, jolsa@...nel.org, linux-kernel@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v3 01/11] perf, tools: Support weak groups

On Thu, Aug 31, 2017 at 12:40:26PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Setting up groups can be complicated due to the
> complicated scheduling restrictions of different PMUs.
> User tools usually don't understand all these restrictions.
> Still in many cases it is useful to set up groups and
> they work most of the time. However if the group
> is set up wrong some members will not reported any values
> because they never get scheduled.
> 
> Add a concept of a 'weak group': try to set up a group,
> but if it's not schedulable fallback to not using
> a group. That gives us the best of both worlds:
> groups if they work, but still a usable fallback if they don't.
> 
> In theory it would be possible to have more complex fallback
> strategies (e.g. try to split the group in half), but
> the simple fallback of not using a group seems to work for now.
> 
> So far the weak group is only implemented for perf stat,
> not for record.
> 
> Here's an unschedulable group (on IvyBridge with SMT on)
> 
> % perf stat -e '{branches,branch-misses,l1d.replacement,l2_lines_in.all,l2_rqsts.all_code_rd}' -a sleep 1
> 
>         73,806,067      branches
>          4,848,144      branch-misses             #    6.57% of all branches
>         14,754,458      l1d.replacement
>         24,905,558      l2_lines_in.all
>    <not supported>      l2_rqsts.all_code_rd         <------- will never report anything
> 
> With the weak group:
> 
> % perf stat -e '{branches,branch-misses,l1d.replacement,l2_lines_in.all,l2_rqsts.all_code_rd}:W' -a sleep 1
> 
>        125,366,055      branches                                                      (80.02%)
>          9,208,402      branch-misses             #    7.35% of all branches          (80.01%)
>         24,560,249      l1d.replacement                                               (80.00%)
>         43,174,971      l2_lines_in.all                                               (80.05%)
>         31,891,457      l2_rqsts.all_code_rd                                          (79.92%)
> 
> The extra event scheduled with some extra multiplexing
> 
> v2: Move fallback code to separate function.
> Add comment on for_each_group_member
> Adjust to new perf_evsel__close interface
> v3:
> Fix debug print out.
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

Acked-by: Jiri Olsa <jolsa@...nel.org>

thanks,
jirka