linux-kernel - Re: [PATCH RFC v2 0/4] perf: IRQ-bound performance events

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Message-ID: <20140105175949.GC27909@tassilo.jf.intel.com>
Date:	Sun, 5 Jan 2014 09:59:49 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <ak@...ux.jf.intel.com>
Subject: Re: [PATCH RFC v2 0/4] perf: IRQ-bound performance events

On Sat, Jan 04, 2014 at 07:22:32PM +0100, Alexander Gordeev wrote:
> Hello,
> 
> This is version 2 of RFC "perf: IRQ-bound performance events". That is an
> introduction of IRQ-bound performance events - ones that only count in a
> context of a hardware interrupt handler. Ingo suggested to extend this
> functionality to softirq and threaded handlers as well:

Did you measure the overhead in workloads that do a lot of interrupts?
I assume two WRMSR could be a significant part of the cost of small interrupts.

For counting at least it would be likely a lot cheaper to just RDPMC
and subtract manually.

The cache miss example below is certainly misleading, as cache misses
by interrupts are often a "debt", that is they are forced on whoever
is interrupted. I don't think that is a good use of this.

I guess it can be useful for cycles.

-Andi
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