Re: [PATCH v4 00/11] perf sched: Introduce stats tool

[Ravi Bangoria <ravi.bangoria@amd.com>]

Hi Ian,

>>> Next is CPU scheduling statistics. These are simple diffs of
>>> /proc/schedstat CPU lines along with description. The report also
>>> prints % relative to base stat.
> 
> I wonder if this is similar to user_time and system_time:
> ```
> $ perf list
> ...
> tool:
> ...
>  system_time
>       [System/kernel time in nanoseconds. Unit: tool]
> ...
>  user_time
>       [User (non-kernel) time in nanoseconds. Unit: tool]
> ...
> ```
> These events are implemented by reading /proc/stat and /proc/pid/stat:
> https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/tool_pmu.c?h=perf-tools-next#n267
> 
> As they are events then they can appear in perf stat output and also
> within metrics.

Create synthesized events for each field of /proc/schedstat?

Your idea is interesting and, I suppose, will work best when we care
about individual counters. However, for the "perf sched stats" tool,
I see atleast two challenges:

1. One of the design goal of "perf sched stats" was to keep the
   overhead low. Currently, it reads /proc/schedstat once at the
   beginning and once at the end. Switching to per-counter events
   would require opening, reading and closing a large number of
   events which would incur significant overhead.

2. Taking a snapshot in one go allows us to correlate counts easily.
   Using synthetic events would force us to read each counter
   individually, making cross-counter correlation impossible.

Thanks,
Ravi