linux-kernel - Re: [patch] mm: reduce pagetable-freeing latencies

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Message-Id: <1185358093.5439.325.camel@localhost.localdomain>
Date:	Wed, 25 Jul 2007 20:08:13 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch] mm: reduce pagetable-freeing latencies

On Wed, 2007-07-25 at 11:46 +0200, Andi Kleen wrote:
> > You could perhaps use C99 variable length arrays to avoid the stack
> > waste when not needed, however Andi once told me that generates rather
> > dubious code.
> 
> It generates frame pointers, but that's not that bad.  I'm not 
> aware of any other bad side effects. Ok the compiler will limit
> your goto usage, but that's more a good thing.
> 
> But since you always have to strictly limit the array in kernel code anyways
> you could as well just allocate the fixed limit.

Plan is fixed array or 4 or maybe 8 entries (pointers), that shouldn't
be -too- bad. The code path I'm a bit worried about is
unmap_mapping_ranges() which goes into zapping page tables from deep
within filesystems.

At worst, I can reduce the fixed array to 1 entry. That means that if
the batch can't manage to get a page to use for the page list, it will
end up doing the flush for each page :-) But that should rarely happen,
in fact, I would expect it to be able to get a page the next time around
because it just freed one...

Ben.

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