linux-kernel - Re: [PATCH] use per cpu data for single cpu ipi calls

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Message-ID: <20090129134224.GH24391@elte.hu>
Date:	Thu, 29 Jan 2009 14:42:24 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, rusty@...tcorp.com.au,
	npiggin@...e.de, torvalds@...ux-foundation.org, tglx@...utronix.de,
	arjan@...radead.org
Subject: Re: [PATCH] use per cpu data for single cpu ipi calls


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2009-01-29 at 12:13 +0100, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > > Can generic_smp_call_function_single_interrupt() ever see 
> > > > CSD_FLAG_ALLOC set now?  If not, that kfree can go away.
> > > 
> > > Like said above, removing that kmalloc will hurt people.
> > 
> > the whole promise of generic-IPI seems to dwindle and we'll get back to 
> > roughly where we started out from.
> 
> I'd not go that far, the per-cpu csd is a nice fallback, but removing 
> that kmalloc is just silly.
> 
> And I see no reason why arch specific could do any better (aside from 
> sparc which has fancy ipis).

well if the hw can embedd at least 64 bit of dataload in the IPI itself, 
we could get rid of a lot of complications. But since we cannot (on x86 
and most other architectures), we've got to demultiplex from generic data 
which brings up serialization issues and adds overhead .

	Ingo
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