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Date: Thu, 10 Jun 2010 18:29:00 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Paul Mackerras <paulus@samba.org>,
       Stephane Eranian <eranian@google.com>,
       Cyrill Gorcunov <gorcunov@gmail.com>,
       Zhang Yanmin <yanmin_zhang@linux.intel.com>,
       Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 1/5] perf: Provide a proper stop action for software
	events
Message-ID: <20100610162858.GD5255@nowhere>
References: <1276141760-11590-1-git-send-regression-fweisbec@gmail.com> <1276141760-11590-2-git-send-regression-fweisbec@gmail.com> <1276166813.2077.96.camel@twins> <1276168242.2077.134.camel@twins> <20100610161234.GA5255@nowhere> <1276186576.2077.554.camel@twins>
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On Thu, Jun 10, 2010 at 06:16:16PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-06-10 at 18:12 +0200, Frederic Weisbecker wrote:
> > On Thu, Jun 10, 2010 at 01:10:42PM +0200, Peter Zijlstra wrote:
> > > On Thu, 2010-06-10 at 12:46 +0200, Peter Zijlstra wrote:
> > > > 
> > > > Something like the below would work, the only 'problem' is that it grows
> > > > hw_perf_event.
> > > 
> > > If we do the whole PAUSEd thing right, we'd not need this I think.
> > 
> > 
> > It's not needed, and moreover software_pmu:stop/start() can be the same
> > than software:pmu:disable/enable() without the need to add another check
> > in the fast path.
> > 
> > But we need perf_event_stop/start() to work on software events. And in fact
> > now that we use the hlist_del_init, it's safe, but a bit wasteful in
> > the period reset path. That's another problem that is not critical, but
> > if you want to solve this by ripping the differences between software and
> > hardware (which I agree with), we need a ->reset_period callback.
>
>
> Why? ->start() should reprogram the hardware, so a
> ->stop()/poke-at-state/->start() cycle is much more flexible.


Imagine you have several software and hardware events running on the
same cpu. Each time you reset this period for a software event, you do
a hw_pmu_disable() / hw_pmu_enable(), which writes/read the hardware
register for each hardware events, amongst other wasteful things.