Re: [RFC/PATCHSET 0/5] perf ftrace: Implement function latency histogram (v1)

[Namhyung Kim <namhyung@kernel.org>]

On Wed, Dec 1, 2021 at 3:59 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>
> Em Tue, Nov 30, 2021 at 04:36:49PM -0800, Stephane Eranian escreveu:
> > On Tue, Nov 30, 2021 at 2:58 PM Namhyung Kim <namhyung@kernel.org> wrote:
> > > On Tue, Nov 30, 2021 at 6:37 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>
> > > > Em Mon, Nov 29, 2021 at 03:18:25PM -0800, Namhyung Kim escreveu:
> > > > > I've implemented 'latency' subcommand in the perf ftrace command to
> > > > > show a histogram of function latency.
>
> > > > > To handle new subcommands, the existing functionality is moved to
> > > > > 'trace' subcommand while preserving backward compatibility of not
> > > > > having a subcommand at all (defaults to 'trace').
>
> > > > > The latency subcommand accepts a target (kernel, for now) function
> > > > > with -T option and shows a histogram like below:
>
> > > > Humm, wouldn't be interesting to shorten this by having a new 'perf
> > > > flat' (function latency) tool, on the same level as 'perf ftrace' and
> > > > leave 'perf ftrace' to just being a convenient perf interface to what
> > > > ftrace provides?
>
> > > That would be fine.  I also think 'perf ftrace latency' is
> > > bit too long.  But if we would add a new feature
> > > like argdist (in BCC) later, I thought it'd be nice being
> > > a subcommand in the perf ftrace together.
>
> > > But it's up to you.  I'll make a change if you prefer
> > > 'flat' (or how about 'fnlat' instead?).
>
> fnlat would be ok, flat was just funny to avoid suggesting it :-)

:)

>
> > I am not too fond of the flat option because as we had more bpf tools
> > like function latency, then we keep extending the list of commands
> > each with a small span which is different
> > from what we have right now.
>
> I think we should focus on the tool end result, not on how it is
> implemented, i.e. in this specific "function latency" tool ftrace is
> used with BPF, but we could perhaps have used some other mechanism.

Agreed, but I think function latency belongs to function tracing
conceptually.  So I added it as a subcommand in perf ftrace
not just because of the implementation.

>
> I think all these tools should have as much as possible a common set of
> options, like the targets (cpu, cgroup, pid, tid, etc) so that one can
> go from different views for those targets by just changing the name of
> the tool.

Currently, perf ftrace shares the target options with both subcommands.
Please see common_options in cmd_ftrace().

Thanks,
Namhyung


>
> We have things like:
>
> $ perf sched
>
>  Usage: perf sched [<options>] {record|latency|map|replay|script|timehist}
>
>     -D, --dump-raw-trace  dump raw trace in ASCII
>     -f, --force           don't complain, do it
>     -i, --input <file>    input file name
>     -v, --verbose         be more verbose (show symbol address, etc)
>
> With different 3rd level subcommads, but in the 'perf sched' case is the
> component being observed, not the mechanism being used to obtain/present
> the observation data.
>
> - Arnaldo