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

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Message-Id: <1234568885.4831.14.camel@laptop>
Date:	Sat, 14 Feb 2009 00:48:05 +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 Sat, 2009-02-14 at 07:46 +1030, Rusty Russell wrote:
> On Thursday 12 February 2009 22:39:14 Peter Zijlstra wrote:
> > So it put in unconditionally, how about this?
> > 
> > 
> > --
> > Subject: generic-smp: remove single ipi fallback for smp_call_function_many()
> > 
> > In preparation of removing the kmalloc() calls from the generic-ipi code
> > get rid of the single ipi fallback for smp_call_function_many().
> > 
> > Because we cannot get around carrying the cpumask in the data -- imagine
> > 2 such calls with different but overlapping masks -- put in a full mask.
> 
> OK, if you really want this, please just change it to:
> 	unsigned long cpumask_bits[BITS_TO_LONGS(CONFIG_NR_CPUS)];
> 
> The 'struct cpumask' will be undefined soon when CONFIG_CPUMASK_OFFSTACK=y,
> which will prevent assignment and declaration on stack.
> 
> 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.

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