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

[Cody P Schafer <cody@linux.vnet.ibm.com>]

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.