On Sat, Sep 04, 2010 at 11:59:45AM +0800, Andrew Morton wrote:
> On Sat, 4 Sep 2010 11:23:11 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>
> > > Still, given the improvements in performance from this patchset,
> > > I'd say inclusion is a no-braniner....
> >
> > In your case it's not really high memory pressure, but maybe too many
> > concurrent direct reclaimers, so that when one reclaimed some free
> > pages, others kick in and "steal" the free pages. So we need to kill
> > the second cond_resched() call (which effectively gives other tasks a
> > good chance to steal this task's vmscan fruits), and only do
> > drain_all_pages() when nothing was reclaimed (instead of allocated).
>
> Well... cond_resched() will only resched when this task has been
> marked for preemption. If that's happening at such a high frequency
> then Something Is Up with the scheduler, and the reported context
> switch rate will be high.
Yes it may not necessarily schedule away. But if ever this happens,
the task will likely run into drain_all_pages() when re-gain CPU.
Because the drain_all_pages() cost is very high, it don't need too
many reschedules to create the IPI storm..
> > Dave, will you give a try of this patch? It's based on Mel's.
> >
> >
> > --- linux-next.orig/mm/page_alloc.c 2010-09-04 11:08:03.000000000 +0800
> > +++ linux-next/mm/page_alloc.c 2010-09-04 11:16:33.000000000 +0800
> > @@ -1850,6 +1850,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_m
> >
> > cond_resched();
> >
> > +retry:
> > /* We now go into synchronous reclaim */
> > cpuset_memory_pressure_bump();
> > p->flags |= PF_MEMALLOC;
> > @@ -1863,26 +1864,23 @@ __alloc_pages_direct_reclaim(gfp_t gfp_m
> > lockdep_clear_current_reclaim_state();
> > p->flags &= ~PF_MEMALLOC;
> >
> > - cond_resched();
> > -
> > - if (unlikely(!(*did_some_progress)))
> > + if (unlikely(!(*did_some_progress))) {
> > + if (!drained) {
> > + drain_all_pages();
> > + drained = true;
> > + goto retry;
> > + }
> > return NULL;
> > + }
> >
> > -retry:
> > page = get_page_from_freelist(gfp_mask, nodemask, order,
> > zonelist, high_zoneidx,
> > alloc_flags, preferred_zone,
> > migratetype);
> >
> > - /*
> > - * If an allocation failed after direct reclaim, it could be because
> > - * pages are pinned on the per-cpu lists. Drain them and try again
> > - */
> > - if (!page && !drained) {
> > - drain_all_pages();
> > - drained = true;
> > + /* someone steal our vmscan fruits? */
> > + if (!page && *did_some_progress)
> > goto retry;
> > - }
>
> Perhaps the fruit-stealing event is worth adding to the
> userspace-exposed vm stats somewhere. But not in /proc - somewhere
> more temporary, in debugfs.
There are no existing debugfs interfaces for vm stats, and I need to
go out right now.. So I did the following quick (and temporary) hack
to allow Dave to collect the information. Will revisit the proper
interface to use later :)
Thanks,
Fengguang
---
include/linux/mmzone.h | 1 +
mm/page_alloc.c | 4 +++-
mm/vmstat.c | 1 +
3 files changed, 5 insertions(+), 1 deletion(-)
--- linux-next.orig/include/linux/mmzone.h 2010-09-04 12:30:26.000000000 +0800
+++ linux-next/include/linux/mmzone.h 2010-09-04 12:30:36.000000000 +0800
@@ -104,6 +104,7 @@ enum zone_stat_item {
NR_ISOLATED_ANON, /* Temporary isolated pages from anon lru */
NR_ISOLATED_FILE, /* Temporary isolated pages from file lru */
NR_SHMEM, /* shmem pages (included tmpfs/GEM pages) */
+ NR_RECLAIM_STEAL,
#ifdef CONFIG_NUMA
NUMA_HIT, /* allocated in intended node */
NUMA_MISS, /* allocated in non intended node */
--- linux-next.orig/mm/page_alloc.c 2010-09-04 12:28:09.000000000 +0800
+++ linux-next/mm/page_alloc.c 2010-09-04 12:33:39.000000000 +0800
@@ -1879,8 +1879,10 @@ retry:
migratetype);
/* someone steal our vmscan fruits? */
- if (!page && *did_some_progress)
+ if (!page && *did_some_progress) {
+ inc_zone_state(preferred_zone, NR_RECLAIM_STEAL);
goto retry;
+ }
return page;
}
--- linux-next.orig/mm/vmstat.c 2010-09-04 12:31:30.000000000 +0800
+++ linux-next/mm/vmstat.c 2010-09-04 12:31:42.000000000 +0800
@@ -732,6 +732,7 @@ static const char * const vmstat_text[]
"nr_isolated_anon",
"nr_isolated_file",
"nr_shmem",
+ "nr_reclaim_steal",
#ifdef CONFIG_NUMA
"numa_hit",
"numa_miss",
--
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