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SubjectRe: long sleep_on_page delays writing to slow storage
On Wed, Nov 09, 2011 at 07:06:46PM +0100, Andrea Arcangeli wrote:
> On Wed, Nov 09, 2011 at 05:52:01PM +0000, Mel Gorman wrote:
> > -#define alloc_pages_vma(gfp_mask, order, vma, addr, node) \
> > - alloc_pages(gfp_mask, order)
> > +#define alloc_pages_vma(gfp_mask, order, vma, addr, node, drop_mmapsem) \
> > + ({ \
> > + if (drop_mmapsem) \
> > + up_read(&vma->vm_mm->mmap_sem); \
> > + alloc_pages(gfp_mask, order); \
> > + })
>
> I wouldn't change alloc_pages_vma. I think it's better to add and have
> that called only by khugepaged:
>
> alloc_pages_vma_up_read(gfp_mask, order, vma, addr, node)
> {
> __alloc_pages_vma(gfp_mask, order, vma, addr, node, true);
> }
>
> alloc_pages_vma(gfp_mask, order, vma, addr, node)
> {
> __alloc_pages_vma(gfp_mask, order, vma, addr, node, false);
> }
>
> I wonder if a change like this would be enough?
>
> sync_migration = !(gfp_mask & __GFP_NO_KSWAPD);
>
> But even if hidden in a new function, the main downside overall is the
> fact we'll pass one more var through the stack of fast paths.
>
> Johannes I recall you reported this too and Mel suggested the above
> change, did it help in the end?

Yes, it completely fixed the latency problem.

That said, I haven't looked at the impact on the THP success rate, but
a regression there is probably less severe than half-minute-stalls in
interactive applications.


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