Re: [PATCH 10/17] 2.6.17.1 perfmon2 patch for review: PMU context switch

[Chuck Ebbert <76306.1226@compuserve.com>]

In-Reply-To: <200606301541.22928.ak@suse.de>

On Fri, 30 Jun 2006 15:41:22 +0200, Andi Kleen wrote:

> > So why do we need care about context switch in cpu-wide mode?
> > It is because we support a mode where the idle thread is excluded
> > from cpu-wide monitoring. This is very useful to distinguish 
> > 'useful kernel work' from 'idle'. 
> 
> I don't quite see the point because on x86 the PMU doesn't run
> during C states anyways. So you get idle excluded automatically.

Looks like it does run:

$ pfmon -ecpu_clk_unhalted,interrupts_masked_cycles -k --system-wide -t 10
<session to end in 10 seconds>
CPU0     60351837 CPU_CLK_UNHALTED
CPU0    346548229 INTERRUPTS_MASKED_CYCLES

The CPU spent ~60 million clocks unhalted and ~350 million with interrupts
disabled.  (This is an idle 1.6GHz Turion64 machine.)

Now let's see what happens when we exclude the idle thread:

$ pfmon -ecpu_clk_unhalted,interrupts_masked_cycles -k --system-wide -t 10 --excl-idle
<session to end in 10 seconds>
CPU0    449250 CPU_CLK_UNHALTED
CPU0    161577 INTERRUPTS_MASKED_CYCLES

Looks like excluding the idle thread means interrupts that happen while idle
don't get counted either.  We took 5000 clock interrupts and I know they take
longer than that to process.

-- 
Chuck
 "You can't read a newspaper if you can't read."  --George W. Bush