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    SubjectRe: [PATCH 1/4] Properly account for the number of page cache pages zone_reclaim() can reclaim
    On Wed, Jun 10, 2009 at 09:41:34PM +0800, Mel Gorman wrote:
    > On Wed, Jun 10, 2009 at 07:59:44PM +0800, Wu Fengguang wrote:
    > > On Wed, Jun 10, 2009 at 06:31:53PM +0800, Mel Gorman wrote:
    > > > On Wed, Jun 10, 2009 at 09:19:39AM +0800, Wu Fengguang wrote:
    > > > > On Wed, Jun 10, 2009 at 01:01:41AM +0800, Mel Gorman wrote:
    > > > > > On NUMA machines, the administrator can configure zone_reclaim_mode that
    > > > > > is a more targetted form of direct reclaim. On machines with large NUMA
    > > > > > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
    > > > > > unmapped pages will be reclaimed if the zone watermarks are not being met.
    > > > > >
    > > > > > There is a heuristic that determines if the scan is worthwhile but the
    > > > > > problem is that the heuristic is not being properly applied and is basically
    > > > > > assuming zone_reclaim_mode is 1 if it is enabled.
    > > > > >
    > > > > > Historically, once enabled it was depending on NR_FILE_PAGES which may
    > > > > > include swapcache pages that the reclaim_mode cannot deal with. Patch
    > > > > > vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch by
    > > > > > Kosaki Motohiro noted that zone_page_state(zone, NR_FILE_PAGES) included
    > > > > > pages that were not file-backed such as swapcache and made a calculation
    > > > > > based on the inactive, active and mapped files. This is far superior
    > > > > > when zone_reclaim==1 but if RECLAIM_SWAP is set, then NR_FILE_PAGES is a
    > > > > > reasonable starting figure.
    > > > > >
    > > > > > This patch alters how zone_reclaim() works out how many pages it might be
    > > > > > able to reclaim given the current reclaim_mode. If RECLAIM_SWAP is set
    > > > > > in the reclaim_mode it will either consider NR_FILE_PAGES as potential
    > > > > > candidates or else use NR_{IN}ACTIVE}_PAGES-NR_FILE_MAPPED to discount
    > > > > > swapcache and other non-file-backed pages. If RECLAIM_WRITE is not set,
    > > > > > then NR_FILE_DIRTY number of pages are not candidates. If RECLAIM_SWAP is
    > > > > > not set, then NR_FILE_MAPPED are not.
    > > > > >
    > > > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    > > > > > Acked-by: Christoph Lameter <cl@linux-foundation.org>
    > > > > > ---
    > > > > > mm/vmscan.c | 52 ++++++++++++++++++++++++++++++++++++++--------------
    > > > > > 1 files changed, 38 insertions(+), 14 deletions(-)
    > > > > >
    > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
    > > > > > index 2ddcfc8..2bfc76e 100644
    > > > > > --- a/mm/vmscan.c
    > > > > > +++ b/mm/vmscan.c
    > > > > > @@ -2333,6 +2333,41 @@ int sysctl_min_unmapped_ratio = 1;
    > > > > > */
    > > > > > int sysctl_min_slab_ratio = 5;
    > > > > >
    > > > > > +static inline unsigned long zone_unmapped_file_pages(struct zone *zone)
    > > > > > +{
    > > > > > + return zone_page_state(zone, NR_INACTIVE_FILE) +
    > > > > > + zone_page_state(zone, NR_ACTIVE_FILE) -
    > > > > > + zone_page_state(zone, NR_FILE_MAPPED);
    > > > >
    > > > > This may underflow if too many tmpfs pages are mapped.
    > > > >
    > > >
    > > > You're right. This is also a bug now in mmotm for patch
    > > > vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch which
    > > > is where I took this code out of and didn't think deeply enough about.
    > > > Well spotted.
    > > >
    > > > Should this be something like?
    > > >
    > > > static unsigned long zone_unmapped_file_pages(struct zone *zone)
    > > > {
    > > > unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);
    > > > unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE)
    > > > zone_page_state(zone, NR_ACTIVE_FILE);
    > > >
    > > > return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
    > > > }
    > > >
    > > > ?
    > > >
    > > > If that returns 0, it does mean that there are very few pages that the
    > > > current reclaim_mode is going to be able to deal with so even if the
    > > > count is not perfect, it should be good enough for what we need it for.
    > >
    > > Agreed. We opt to give up direct zone reclaim than to risk busy looping ;)
    > >
    >
    > Yep. Those busy loops doth chew up the CPU time, heat the planet and
    > wear out Ye Olde Bugzilla with the wailing of unhappy users :)
    >
    > > > > > +}
    > > > > > +
    > > > > > +/* Work out how many page cache pages we can reclaim in this reclaim_mode */
    > > > > > +static inline long zone_pagecache_reclaimable(struct zone *zone)
    > > > > > +{
    > > > > > + long nr_pagecache_reclaimable;
    > > > > > + long delta = 0;
    > > > > > +
    > > > > > + /*
    > > > > > + * If RECLAIM_SWAP is set, then all file pages are considered
    > > > > > + * potentially reclaimable. Otherwise, we have to worry about
    > > > > > + * pages like swapcache and zone_unmapped_file_pages() provides
    > > > > > + * a better estimate
    > > > > > + */
    > > > > > + if (zone_reclaim_mode & RECLAIM_SWAP)
    > > > > > + nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
    > > > > > + else
    > > > > > + nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);
    > > > > > +
    > > > > > + /* If we can't clean pages, remove dirty pages from consideration */
    > > > > > + if (!(zone_reclaim_mode & RECLAIM_WRITE))
    > > > > > + delta += zone_page_state(zone, NR_FILE_DIRTY);
    > > > > > +
    > > > > > + /* Beware of double accounting */
    > > > >
    > > > > The double accounting happens for NR_FILE_MAPPED but not
    > > > > NR_FILE_DIRTY(dirty tmpfs pages won't be accounted),
    > > >
    > > > I should have taken that out. In an interim version, delta was altered
    > > > more than once in a way that could have caused underflow.
    > > >
    > > > > so this comment
    > > > > is more suitable for zone_unmapped_file_pages(). But the double
    > > > > accounting does affects this abstraction. So a more reasonable
    > > > > sequence could be to first substract NR_FILE_DIRTY and then
    > > > > conditionally substract NR_FILE_MAPPED?
    > > >
    > > > The end result is the same I believe and I prefer having the
    > > > zone_unmapped_file_pages() doing just that and nothing else because it's
    > > > in line with what zone_lru_pages() does.
    > >
    > > OK.
    > >
    > > > > Or better to introduce a new counter NR_TMPFS_MAPPED to fix this mess?
    > > > >
    > > >
    > > > I considered such a counter and dismissed it but maybe it merits wider discussion.
    > > >
    > > > My problem with it is that it would affect the pagecache add/remove hot paths
    > > > and a few other sites and increase the amount of accouting we do within a
    > > > zone. It seemed unjustified to help a seldom executed slow path that only
    > > > runs on NUMA.
    > >
    > > We are not talking about NR_TMPFS_PAGES, but NR_TMPFS_MAPPED :)
    > >
    > > We only need to account it in page_add_file_rmap() and page_remove_rmap(),
    > > I don't think they are too hot paths. And the relative cost is low enough.
    > >
    > > It will look like this.
    > >
    >
    > Ok, you're right, that is much simplier than what I had in mind. I was fixated
    > on accounting for TMPFS pages. I think this patch has definite possibilities
    > and would help us with the tmpfs problem. If the tests come back "failed",
    > I'll be adding taking this logic and seeing can it be made work.

    OK, thank you.

    > What about ramfs pages though? They have similar problems to tmpfs but are
    > not swap-backed, right?

    We don't care ramfs pages because they are unevictable :)

    Thanks,
    Fengguang

    > > ---
    > > include/linux/mmzone.h | 1 +
    > > mm/rmap.c | 4 ++++
    > > 2 files changed, 5 insertions(+)
    > >
    > > --- linux.orig/include/linux/mmzone.h
    > > +++ linux/include/linux/mmzone.h
    > > @@ -99,6 +99,7 @@ enum zone_stat_item {
    > > NR_VMSCAN_WRITE,
    > > /* Second 128 byte cacheline */
    > > NR_WRITEBACK_TEMP, /* Writeback using temporary buffers */
    > > + NR_TMPFS_MAPPED,
    > > #ifdef CONFIG_NUMA
    > > NUMA_HIT, /* allocated in intended node */
    > > NUMA_MISS, /* allocated in non intended node */
    > > --- linux.orig/mm/rmap.c
    > > +++ linux/mm/rmap.c
    > > @@ -844,6 +844,8 @@ void page_add_file_rmap(struct page *pag
    > > {
    > > if (atomic_inc_and_test(&page->_mapcount)) {
    > > __inc_zone_page_state(page, NR_FILE_MAPPED);
    > > + if (PageSwapBacked(page))
    > > + __inc_zone_page_state(page, NR_TMPFS_MAPPED);
    > > mem_cgroup_update_mapped_file_stat(page, 1);
    > > }
    > > }
    > > @@ -894,6 +896,8 @@ void page_remove_rmap(struct page *page)
    > > mem_cgroup_uncharge_page(page);
    > > __dec_zone_page_state(page,
    > > PageAnon(page) ? NR_ANON_PAGES : NR_FILE_MAPPED);
    > > + if (!PageAnon(page) && PageSwapBacked(page))
    > > + __dec_zone_page_state(page, NR_TMPFS_MAPPED);
    > > mem_cgroup_update_mapped_file_stat(page, -1);
    > > /*
    > > * It would be tidy to reset the PageAnon mapping here,
    > >
    >
    > --
    > Mel Gorman
    > Part-time Phd Student Linux Technology Center
    > University of Limerick IBM Dublin Software Lab


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