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

[Mel Gorman <mgorman@suse.de>]

On Wed, Apr 15, 2015 at 02:24:40PM +0200, Peter Zijlstra wrote:
> 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?

No, in mine I can use sizeof to "discover" it even though the answer is
always a page.

> But even then, what's
> more likely to change, extra members in our struct or growing the
> allocation to two (or more) pages?

Either approach requires careful treatment. I can switch to your method
in V2 because to me, they're equivalent in terms of readability and
maintenance.

-- 
Mel Gorman
SUSE Labs