linux-kernel - Re: [PATCH 1/4 tip/sched/core] sched: rename preempt_notifier to sched

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Message-ID: <20091127045209.GA13914@elte.hu>
Date:	Fri, 27 Nov 2009 05:52:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/4 tip/sched/core] sched: rename preempt_notifier to
 sched_notifier and always enable it


* Tejun Heo <tj@...nel.org> wrote:

> Hello, Peter, Ingo.
> 
> 11/26/2009 09:40 PM, Peter Zijlstra wrote:
> > CALLBACK_EVENT() would be my preferred name, and shouldn't live anywhere
> > near the regular tracing bits, the tracing bits could simply add another
> > callback in it when enabled.
> 
> I haven't looked at the mm code but if the scheduler callback 
> requirement isn't gonna explode big time soon and we know which 
> functions are the candidate callbacks at build time, I think this can 
> be done pretty efficiently with an ulong enable mask per task and 
> fixed function dispatch such that no callback case just goes through 
> one likely() conditional test at the tracing point and callback cases 
> are dispatched using conditional direct jump.

Yes - and that's what the tracepoints infrastructure is about.

Btw., longer term it will be faster than a mask check and a 
default-untaken conditional: there's ongoign work to offer runtime 
instruction patching features for tracing callbacks. There's the jump 
patching optimization and also the immediate values patching 
optimization.

We've got old-style notifiers for regular callbacks, we've got new-style 
tracepoints which are callbacks and event source descriptors - and what 
i'm asking for is to have _one_ callback mechanism, and to use that in 
the scheduler. 5 callbacks using 3 different facilities is excessive - 
i'd like to see just two callbacks using one facility.

	Ingo
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