linux-kernel - Re: [RFC][PATCH] perf: Add a few generic stalled-cycles events

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Date:	Tue, 16 Oct 2012 10:58:56 +0530
From:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:	Arun Sharma <asharma@...com>
CC:	Robert Richter <robert.richter@....com>, peterz@...radead.org,
	Anton Blanchard <anton@....ibm.com>,
	linux-kernel@...r.kernel.org, eranian@...gle.com, acme@...hat.com,
	linuxppc-dev@...abs.org, paulus@...ba.org, mpjohn@...ibm.com,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	mingo@...nel.org
Subject: Re: [RFC][PATCH] perf: Add a few generic stalled-cycles events

On 10/15/2012 10:53 PM, Arun Sharma wrote:
> On 10/15/12 8:55 AM, Robert Richter wrote:
> 
> [..]
>> Perf tool works then out-of-the-box with:
>>
>>   $ perf record -e cpu/stalled-cycles-fixed-point/ ...
>>
>> The event string can easily be reused by other architectures as a
>> quasi standard.
> 
> I like Robert's proposal better. It's hard to model all the stall events
> (eg: instruction decoder related stalls on x86) in a hardware
> independent way.
> 
> Another area to think about: software engineers are generally busy and
> have a limited amount of time to devote to hardware event based
> optimizations. The most common question I hear is: what is the expected
> perf gain if I fix this? It's hard to answer that with just the stall
> events.
> 

Hardware event based optimization is a very important aspect of real world application
tuning. CPI stack analysis is a good reason why perf should have stall events as generic
ones. But I am not clear on situations where we consider adding these new generic events
into linux/perf_event.h and the situations where we should go with the sys fs interface.
Could you please elaborate on this ?

Regards
Anshuman

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