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Subject: Re: [PATCH 2/4] generic-smp: remove kmalloc usage
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>, Jens Axboe <jens.axboe@oracle.com>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       Oleg Nesterov <oleg@redhat.com>
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Date: Thu, 19 Feb 2009 10:10:29 +0100
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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@rustcorp.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...