linux-kernel - Re: [PATCH 06/13] mm: Preemptible mmu

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Date:	Fri, 09 Apr 2010 10:18:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH 06/13] mm: Preemptible mmu_gather

On Fri, 2010-04-09 at 13:25 +1000, Nick Piggin wrote:
> On Thu, Apr 08, 2010 at 09:17:43PM +0200, Peter Zijlstra wrote:
> > @@ -39,30 +33,48 @@
> >  struct mmu_gather {
> >  	struct mm_struct	*mm;
> >  	unsigned int		nr;	/* set to ~0U means fast mode */
> > +	unsigned int		max;	/* nr < max */
> >  	unsigned int		need_flush;/* Really unmapped some ptes? */
> >  	unsigned int		fullmm; /* non-zero means full mm flush */
> > -	struct page *		pages[FREE_PTE_NR];
> > +#ifdef HAVE_ARCH_MMU_GATHER
> > +	struct arch_mmu_gather	arch;
> > +#endif
> > +	struct page		**pages;
> > +	struct page		*local[8];
> 
> Have you done some profiling on this? What I would like to see, if
> it's not too much complexity, is to have a small set of pages to
> handle common size frees, and then use them up first by default
> before attempting to allocate more.
> 
> Also, it would be cool to be able to chain allocations to avoid
> TLB flushes even on big frees (overridable by arch of course, in
> case they're doing some non-preeemptible work or you wish to break
> up lock hold times). But that might be just getting over engineered.

Did no profiling at all, back when I wrote this I was in a hurry to get
this working for -rt.

But yes, those things do look like something we want to look into, we
can easily add a head structure to these pages like we did for the RCU
batches.

But as it stands I think we can do those things as incrementals on top
of this, no?

What kind of workload would you recommend I use to profile this?

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