linux-kernel - Re: register_timer_hook use in arch/sh/oprofile

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Message-ID: <20090624132447.GD6224@elte.hu>
Date:	Wed, 24 Jun 2009 15:24:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mundt <lethal@...ux-sh.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: register_timer_hook use in arch/sh/oprofile

* Paul Mundt <lethal@...ux-sh.org> wrote:

> > The main complication that such counters bring is that in their 
> > 'own metric' they do interrupt periods in an 'irregular' way 
> > (because they interrupt in the nanosec metric - being hrtimers) 
> > - but both the tools can deal with uneven periods just fine and 
> > the auto-freq code can auto-balance based on this just fine too.
> 
> How we have mostly handled these counters in the past have been 
> for long-term workload analysis. A given user wants to measure 
> certain events across the lifetime of their workload to get an 
> idea of where the hot spots are, and then send us numbers later. 
> 64-bit hardware counters give them quite a bit of time to 
> accumulate results, and in these cases precision is not so 
> important as continuous non-interactive monitoring.

Note that if i understand the constraints correctly, SH's counters 
will be _exactly_ as precise as x86 counters or PowerPC counters.

Even if SH uses a hrtimer to drive the sampling of them. [*]

The reason is that perfcounters are designed to be 'sampling 
frequency/precision invariant'. We deal with 
[delta_count,delta_time] pairs in the histograms, etc. and nothing 
assumes that periods are static.

[ And long-term analysis ('perf stat' type of runs) dont need IRQs 
  anyway - perfcounters reads outs the counts and summarizes them 
  across the measured workload. ]

Furthermore, -F (auto-freq) counters are by design variable-period 
and self-adjusting, even on x86 and PowerPC.

[ Btw., we plan to switch over the tools to use a 10 KHz auto-freq 
  sampling by default - as that gives us meaningful samples all the 
  time, regardless of how rare the underlying hardware event is. ]

	Ingo

[*] The only tradeoff is that you wont have NMI sampling. But if you 
    have _some_ sort of NMI source in SH (regardless of whether it's 
    the PMU that drives it), you can still expose that and drive 
    your perfcounter sampling via that.
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