Re: [patch] mm: skip rebalance of hopeless zones

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 9 Dec 2010 10:55:24 +0900 Minchan Kim <minchan.kim@gmail.com> wrote:

> >> > leaves them to direct reclaim.
> >>
> >> Hi!
> >>
> >> We are experiencing a similar issue, though with a 757 MB Normal zone,
> >> where kswapd tries to rebalance Normal after an order-3 allocation while
> >> page cache allocations (order-0) keep splitting it back up again. __It can
> >> run the whole day like this (SSD storage) without sleeping.
> >
> > People at google have told me they've seen the same thing. __A fork is
> > taking 15 minutes when someone else is doing a dd, because the fork
> > enters direct-reclaim trying for an order-one page. __It successfully
> > frees some order-one pages but before it gets back to allocate one, dd
> > has gone and stolen them, or split them apart.
> >
> > This problem would have got worse when slub came along doing its stupid
> > unnecessary high-order allocations.
> >
> > Billions of years ago a direct-reclaimer had a one-deep cache in the
> > task_struct into which it freed the page to prevent it from getting
> > stolen.
> >
> > Later, we took that out because pages were being freed into the
> > per-cpu-pages magazine, which is effectively task-local anyway. __But
> > per-cpu-pages are only for order-0 pages. __See slub stupidity, above.
> >
> > I expect that this is happening so repeatably because the
> > direct-reclaimer is dong a sleep somewhere after freeing the pages it
> > needs - if it wasn't doing that then surely the window wouldn't be wide
> > enough for it to happen so often. __But I didn't look.
> >
> > Suitable fixes might be
> >
> > a) don't go to sleep after the successful direct-reclaim.
> 
> It can't make sure success since direct reclaim needs sleep with !GFP_AOMIC.

It doesn't necessarily need to sleep *after* successfully freeing
pages.  If it needs to sleep then do it before or during the freeing.

> >
> > b) reinstate the one-deep task-local free page cache.
> 
> I like b) so how about this?
> Just for the concept.
> 
> @@ -1880,7 +1881,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask,
> unsigned int order,
>         reclaim_state.reclaimed_slab = 0;
>         p->reclaim_state = &reclaim_state;
> 
> -       *did_some_progress = try_to_free_pages(zonelist, order,
> gfp_mask, nodemask);
> +       *did_some_progress = try_to_free_pages(zonelist, order,
> gfp_mask, nodemask, &ret_pages);
> 
>         p->reclaim_state = NULL;
>         lockdep_clear_current_reclaim_state();
> @@ -1892,10 +1893,11 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask,
> unsigned int order,
>                 return NULL;
> 
>  retry:
> -       page = get_page_from_freelist(gfp_mask, nodemask, order,
> -                                       zonelist, high_zoneidx,
> -                                       alloc_flags, preferred_zone,
> -                                       migratetype);
> +       if(!list_empty(&ret_pages)) {
> +               page = lru_to_page(ret_pages);
> +               list_del(&page->lru);
> +               free_page_list(&ret_pages);
> +       }

Maybe.  Or just pass a page*.

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