linux-kernel - Re: [PATCH 3/4] mm: Gather more PFNs before sending a TLB to flush unmapped pages

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Message-ID: <20150415122440.GV5029@twins.programming.kicks-ass.net>
Date:	Wed, 15 Apr 2015 14:24:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] mm: Gather more PFNs before sending a TLB to flush
 unmapped pages

On Wed, Apr 15, 2015 at 01:15:53PM +0100, Mel Gorman wrote:
> On Wed, Apr 15, 2015 at 01:42:20PM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 15, 2015 at 11:42:55AM +0100, Mel Gorman wrote:
> > > +/*
> > > + * Use a page to store as many PFNs as possible for batch unmapping. Adjusting
> > > + * this trades memory usage for number of IPIs sent
> > > + */
> > > +#define BATCH_TLBFLUSH_SIZE \
> > > +	((PAGE_SIZE - sizeof(struct cpumask) - sizeof(unsigned long)) / sizeof(unsigned long))
> > >  
> > >  /* Track pages that require TLB flushes */
> > >  struct unmap_batch {
> > > +	/* Update BATCH_TLBFLUSH_SIZE when adjusting this structure */
> > >  	struct cpumask cpumask;
> > >  	unsigned long nr_pages;
> > >  	unsigned long pfns[BATCH_TLBFLUSH_SIZE];
> > 
> > The alternative is something like:
> > 
> > struct unmap_batch {
> > 	struct cpumask cpumask;
> > 	unsigned long nr_pages;
> > 	unsigned long pfnsp[0];
> > };
> > 
> > #define BATCH_TLBFLUSH_SIZE ((PAGE_SIZE - sizeof(struct unmap_batch)) / sizeof(unsigned long))
> > 
> > and unconditionally allocate 1 page. This saves you from having to worry
> > about the layout of struct unmap_batch.
> 
> True but then I need to calculate the size of the real array so it's
> similar in terms of readability. The plus would be that if the structure
> changes then the size calculation is not changed but then the allocation
> site and the size calculation must be kept in sync. I did not see a clear
> win of one approach over the other so flipped a coin.

I'm not seeing your argument, in both your an mine variant the
allocation is hard assumed to be 1 page, right? But even then, what's
more likely to change, extra members in our struct or growing the
allocation to two (or more) pages?
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