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

Peter Zijlstra
2015-04-15T12:24:58Z

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?