linux-kernel - Re: [patch] generic-ipi: remove kmalloc, cleanup

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Message-Id: <1234778236.4703.8.camel@laptop>
Date:	Mon, 16 Feb 2009 10:57:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...x.de>,
	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Carsten Emde <ce@...g.ch>, Clark Williams <williams@...hat.com>
Subject: Re: [patch] generic-ipi: remove kmalloc, cleanup

On Mon, 2009-02-16 at 17:46 +1030, Rusty Russell wrote:
> On Saturday 14 February 2009 10:18:05 Peter Zijlstra wrote:
> > On Sat, 2009-02-14 at 07:46 +1030, Rusty Russell wrote:
> > > I'd be fascinated to see perf numbers once you kill the kmalloc.  Because
> > > this patch will add num_possible_cpus * NR_CPUS/8 bytes to the kernel which
> > > is something we're trying to avoid unless necessary.
> > 
> > You're free to make it a pointer and do node affine allocations from an
> > init section of choice and add a hotplug handler.
> > 
> > But I'm not quite sure how perf is affected by size overhead on
> > ridiculous configs.
> 
> No, I meant "can you actually measure the perf win of this patch?".  If you
> did so, I missed it?

Over what, the always single-ipi case, or the kmalloc case?

The thing is, we're removing that kmalloc because its somewhat of a wart
on the whole thing.

> But if this patch is worthwhile, the right way to do this is make it a
> cpumask_var_t, and do the alloc_cpumask_var_node() in that init routine.

Right, and installing a hotplug handler and ... *sigh*

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