linux-kernel - Re: [PATCH 2/4] generic-smp: remove kmalloc usage

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Message-Id: <1235034629.4612.35.camel@laptop>
Date:	Thu, 19 Feb 2009 10:10:29 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 2/4] generic-smp: remove kmalloc usage

On Thu, 2009-02-19 at 15:01 +1030, Rusty Russell wrote:
> On Thursday 19 February 2009 02:35:35 Ingo Molnar wrote:
> > 
> > * Rusty Russell <rusty@...tcorp.com.au> wrote:
> > 
> > > On Tuesday 17 February 2009 20:13:59 Ingo Molnar wrote:
> > > > We should not bend backwards trying to preserve that kmalloc() 
> > > > [and prove that it's safe and race-free] - i.e. the burden of 
> > > > proof is on the person insisting that it's needed, not on the 
> > > > person wanting to remove it.
> > > 
> > > Respectfully disagree.  The kmalloc has been there for a very long time,
> > > and doing fine AFAICT.
> > 
> > The kmalloc(GFP_ATOMIC) has been in kernel/smp.c for about half 
> > a year
> 
> Oops, yes.
> 
> So if we care about the kmalloc, why didn't we see benchmarks when we
> switched from the x86 smp_call_function_mask to the generic one?  Or did
> I just miss them (there's nothing in the git commit).
> 
> Now, I think the current patch is quite neat and may not been benchmarks to
> justify it, but it'd still be nice if it were faster, but noone seems to know.

I think the problem is that even on a lively machine these routines just
aren't called that often:

CAL:         74        104         93        116   Function call interrupts
make clean; make -j8 bzImage
CAL:         74        104         93        116   Function call interrupts

We could of course construct some artificial ubench to stress it...

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