linux-kernel - RE: [PATCH 2/2] perf,core: use parent avg sample period as child initial period

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Date:   Sun, 29 Jan 2017 15:24:31 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     'Peter Zijlstra' <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: RE: [PATCH 2/2] perf,core: use parent avg sample period as child
 initial period

> 
> On Wed, Jan 18, 2017 at 08:21:02AM -0500, kan.liang@...el.com wrote:
> > From: Kan Liang <kan.liang@...el.com>
> >
> > perf brings additional overhead when monitoring the task which
> > frequently generates child task.
> >
> > When inheriting a event from parent task to child task, the
> > sample_period of original parent event (parent_event->parent) will be
> > assigned to child event as its initial period, which is usually the
> > default sample_period 1.
> 
> 
> Why is that mostly 1? I would expect the parent event's sample_period to
> ramp up as well.

The conclusion is based on the observation in the specific test case
mentioned in the description. The sample_period is for the original parent
event (parent_event->parent), not the direct parent.

I did several tests these days, it can be 100% reproduced by this particular
test case in multiplexing.
I think the reason is that the original parent event is scheduled out shortly
and never gets a chance to trigger interrupt in the first round. So its
sample_period doesn't get updated. Then the test case repeatedly
generates child tasks and child events. The new child events will be
scheduled in/out continually. The original parent event never gets a
change to be scheduled again. So the original parent event's
sample_period is kept1.

Thanks,
Kan