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Subject: Re: [PATCH 08/20] mm: Optimize fullmm TLB flushing
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Fri, 29 Jun 2012 07:58:16 +1000
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Catalin Marinas <catalin.marinas@arm.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>, "akpm@linux-foundation.org" <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Mel Gorman <mel@csn.ul.ie>, Nick Piggin <npiggin@kernel.dk>, Alex Shi <alex.shi@intel.com>, "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>, Konrad Rzeszutek Wilk <konrad@darnok.org>, David Miller <davem@davemloft.net>, Russell King <rmk@arm.linux.org.uk>, Chris Metcalf <cmetcalf@tilera.com>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Tony Luck <tony.luck@intel.com>, Paul Mundt <lethal@linux-sh.org>, Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>, Ralf Baechle <ralf@linux-mips.org>, Kyle McMartin <kyle@mcmartin.ca>, James Bottomley <jejb@parisc-linux.org>, Chris Zankel <chris@zankel.net>

On Fri, 2012-06-29 at 07:57 +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2012-06-28 at 18:52 +0200, Peter Zijlstra wrote:
> > No I think you're right (as always).. also an IPI will not force
> > schedule the thread that might be running on the receiving cpu, also
> > we'd have to wait for any such schedule to complete in order to
> > guarantee the mm isn't lazily used anymore.
> > 
> > Bugger.. 
> 
> You can still do it if the mm count is 1 no ? Ie, current is the last
> holder of a reference to the mm struct... which will probably be the
> common case for short lived programs.

Also I just remembered... x86 flushes in SMP via IPIs right ? So maybe
you can invent a "detach and flush" variant of it ? 

Cheers,
Ben.


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