Re: [PATCHSET v4 next 0/3] perf stat: add user_time and system_time tool events

From: Namhyung Kim <namhyung@gmail.com>
To: Florian Fischer <florian.fischer@muhq.space>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	linux-perf-users <linux-perf-users@vger.kernel.org>,
	Ian Rogers <irogers@google.com>,
	Xing Zhengjun <zhengjun.xing@linux.intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCHSET v4 next 0/3] perf stat: add user_time and system_time tool events
Date: Mon, 25 Apr 2022 12:06:53 -0700	[thread overview]
Message-ID: <CAM9d7cgcx42xD8QxOB+JBEL85mZU_va8FMiF31SRxABH8CakLg@mail.gmail.com> (raw)
In-Reply-To: <20220423121557.z5gzbqadonmrg6ef@pasture>

On Sat, Apr 23, 2022 at 5:16 AM Florian Fischer
<florian.fischer@muhq.space> wrote:
>
> On 22.04.2022 16:52, Namhyung Kim wrote:
> > Hello,
> >
> > On Fri, Apr 22, 2022 at 3:05 PM Arnaldo Carvalho de Melo
> > <acme@kernel.org> wrote:
> > >
> > > Em Wed, Apr 20, 2022 at 12:23:51PM +0200, Florian Fischer escreveu:
> > > > This patch series adds new internal events to perf stat exposing the times spend
> > > > in user and kernel mode in nanoseconds reported by rusage.
> > > >
> > > > During some benchmarking using perf it bothered me that I could not easily
> > > > retrieve those times from perf stat when using the machine readable output.
> > > >
> > > > But perf definitely knows about those values because in the human readable output
> > > > they are present.
> > > >
> > > > Therefore I exposed the times reported by rusage via the new tool events:
> > > > user_time and system_time.
> > > >
> > > > This allows to retrieved them in machine-readable output:
> > > >
> > > > $ ./perf stat -x, -e duration_time,user_time,system_time,cache-misses -- grep -q -r duration_time tools/perf
> > > > 72134524,ns,duration_time:u,72134524,100.00,,
> > > > 65225000,ns,user_time:u,65225000,100.00,,
> > > > 6865000,ns,ssystem_time:u,6865000,100.00,,

> > Anyway it looks a little bit strange to me if we can get
> > system time in user mode only (the 'u' modifier).
>
> Sorry but I don't really understand what you mean.
> The system_time is reported to userspace via rusage filled by wait4(2).
> It will always report the value reported to the user space regardless of what
> counters perf has access to.
>
> If you run perf as user you get the same system_time (but with the ':u' suffix)
> as when you run perf as root or lower kernel.perf_event_paranoid to allow access
> to more counters.

The ':u' modifier means that the event should count only in user mode.
So I think system_time:u should be 0 by definition.
Likewise, user_time:k should be handled as such.

But as I said before, we already have the task-clock event, so it's not
clear why we need this too.  Also these tool events won't work for
other use cases like perf record.

Thanks,
Namhyung