linux-kernel - Re: [PATCH 2/3] perf: clarify comment regarding event merging

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Message-ID: <52D458E8.70203@linux.vnet.ibm.com>
Date:	Mon, 13 Jan 2014 13:21:44 -0800
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] perf: clarify comment regarding event merging

On 01/10/2014 01:36 AM, Peter Zijlstra wrote:
> On Thu, Jan 09, 2014 at 03:51:31PM -0800, Cody P Schafer wrote:
>> There are actually 2 things about software events that allow us to
>> merge them: they never fail to schedule _and_ they have transaction
>> handlers we can (and do, when they are added to !sw groups) ignore. Note
>> both of these in the comment on adding sw events to !sw groups.
>
> The latter is a direct consequence of the former. Since they can always
> be scheduled, they don't need any schedulability testing, and therefore
> the transaction stuff is useless.

Right. I guess what I was getting at were the 2 types of "schedulability":
	1. individual event schedulability (ie: "did add() return an error?")
	2. txn schedulability (ie: "did commit_txn() return an error?")

I'm in the process of adding a pmu which guarantees #1, but not #2 (it 
essentially provides access to some always-running counters which can be 
atomically copied in groups). As a result, I'm teasing apart some of the 
special casing done for sw events.

This will probably make a bit more sense with some better terminology on 
my part and some actual code. I'll update and resend later.


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