Re: [PATCH 4/5] vmscan: Have kswapd sleep for a short interval and double check it should be asleep

[Mel Gorman <mel@csn.ul.ie>]

On Sat, Nov 14, 2009 at 06:34:23PM +0900, KOSAKI Motohiro wrote:
> 2009/11/14 Mel Gorman <mel@csn.ul.ie>:
> > On Sat, Nov 14, 2009 at 03:00:57AM +0900, KOSAKI Motohiro wrote:
> >> > On Fri, Nov 13, 2009 at 07:43:09PM +0900, KOSAKI Motohiro wrote:
> >> > > > After kswapd balances all zones in a pgdat, it goes to sleep. In the event
> >> > > > of no IO congestion, kswapd can go to sleep very shortly after the high
> >> > > > watermark was reached. If there are a constant stream of allocations from
> >> > > > parallel processes, it can mean that kswapd went to sleep too quickly and
> >> > > > the high watermark is not being maintained for sufficient length time.
> >> > > >
> >> > > > This patch makes kswapd go to sleep as a two-stage process. It first
> >> > > > tries to sleep for HZ/10. If it is woken up by another process or the
> >> > > > high watermark is no longer met, it's considered a premature sleep and
> >> > > > kswapd continues work. Otherwise it goes fully to sleep.
> >> > > >
> >> > > > This adds more counters to distinguish between fast and slow breaches of
> >> > > > watermarks. A "fast" premature sleep is one where the low watermark was
> >> > > > hit in a very short time after kswapd going to sleep. A "slow" premature
> >> > > > sleep indicates that the high watermark was breached after a very short
> >> > > > interval.
> >> > > >
> >> > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> >> > >
> >> > > Why do you submit this patch to mainline? this is debugging patch
> >> > > no more and no less.
> >> > >
> >> >
> >> > Do you mean the stats part? The stats are included until such time as the page
> >> > allocator failure reports stop or are significantly reduced. In the event a
> >> > report is received, the value of the counters help determine if kswapd was
> >> > struggling or not. They should be removed once this mess is ironed out.
> >> >
> >> > If there is a preference, I can split out the stats part and send it to
> >> > people with page allocator failure reports for retesting.
> >>
> >> I'm sorry my last mail didn't have enough explanation.
> >> This stats help to solve this issue. I agreed. but after solving this issue,
> >> I don't imagine administrator how to use this stats. if KSWAPD_PREMATURE_FAST or
> >> KSWAPD_PREMATURE_SLOW significantly increased, what should admin do?
> >
> > One possible workaround would be to raise min_free_kbytes while a fix is
> > being worked on.
> 
> Please correct me, if I said wrong thing.
> 

You didn't.

> if I was admin, I don't watch this stats because kswapd frequently
> wakeup doesn't mean any trouble. instead I watch number of allocation
> failure.
> 

The stats are not tracking when kswapd wakes up. It helps track how
quickly the high or low watermarks are going under once kswapd tries to
go back to sleep.

> [see include/linux/vmstat.h]
> 
> umm...
> Why don't we have nr-allocation-failure vmstat?

There isn't a stat for allocation failures, but there are tracepoints that
can be used to find out that information if necessary. For the cases I'm
looking at the moment though, GFP_NOWARN is not a factor so just looking
through the kernel log has been sufficient.

> I don't remember it.
> GFP_NOWARN failure doesn't make syslog error logging. but frequently
> GFP_NOWARN failure imply the system is under memory pressure or under
> heavy fragmentation. It is good opportunity to change min_free_kbytes.
> 
> 
> 
> >> Or, Can LKML folk make any advise to admin?
> >>
> >
> > Work with them to fix the bug :/
> >
> >> if kernel doesn't have any bug, kswapd wakeup rate is not so worth information imho.
> >> following your additional code itself looks good to me. but...
> >>
> >>
> >> > ==== CUT HERE ====
> >> > vmscan: Have kswapd sleep for a short interval and double check it should be asleep fix 1
> >> >
> >> > This patch is a fix and a claritifacation to the patch "vmscan: Have
> >> > kswapd sleep for a short interval and double check it should be asleep".
> >> > The fix is for kswapd to only check zones in the node it is responsible
> >> > for. The clarification is to rename two counters to better explain what is
> >> > being counted.
> >> >
> >> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> >> > ---
> >> >  include/linux/vmstat.h |    2 +-
> >> >  mm/vmscan.c            |   20 +++++++++++++-------
> >> >  mm/vmstat.c            |    4 ++--
> >> >  3 files changed, 16 insertions(+), 10 deletions(-)
> >> >
> >> > diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> >> > index 7d66695..0591a48 100644
> >> > --- a/include/linux/vmstat.h
> >> > +++ b/include/linux/vmstat.h
> >> > @@ -40,7 +40,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
> >> >             PGSCAN_ZONE_RECLAIM_FAILED,
> >> >  #endif
> >> >             PGINODESTEAL, SLABS_SCANNED, KSWAPD_STEAL, KSWAPD_INODESTEAL,
> >> > -           KSWAPD_PREMATURE_FAST, KSWAPD_PREMATURE_SLOW,
> >> > +           KSWAPD_LOW_WMARK_HIT_QUICKLY, KSWAPD_HIGH_WMARK_HIT_QUICKLY,
> >> >             KSWAPD_NO_CONGESTION_WAIT,
> >> >             PAGEOUTRUN, ALLOCSTALL, PGROTATED,
> >> >  #ifdef CONFIG_HUGETLB_PAGE
> >> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> > index 70967e1..5557555 100644
> >> > --- a/mm/vmscan.c
> >> > +++ b/mm/vmscan.c
> >> > @@ -1905,19 +1905,25 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> >> >  #endif
> >> >
> >> >  /* is kswapd sleeping prematurely? */
> >> > -static int sleeping_prematurely(int order, long remaining)
> >> > +static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> >> >  {
> >> > -   struct zone *zone;
> >> > +   int i;
> >> >
> >> >     /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
> >> >     if (remaining)
> >> >             return 1;
> >> >
> >> >     /* If after HZ/10, a zone is below the high mark, it's premature */
> >> > -   for_each_populated_zone(zone)
> >> > +   for (i = 0; i < pgdat->nr_zones; i++) {
> >> > +           struct zone *zone = pgdat->node_zones + i;
> >> > +
> >> > +           if (!populated_zone(zone))
> >> > +                   continue;
> >> > +
> >> >             if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
> >> >                                                             0, 0))
> >> >                     return 1;
> >> > +   }
> >> >
> >> >     return 0;
> >> >  }
> >> > @@ -2221,7 +2227,7 @@ static int kswapd(void *p)
> >> >                             long remaining = 0;
> >> >
> >> >                             /* Try to sleep for a short interval */
> >> > -                           if (!sleeping_prematurely(order, remaining)) {
> >> > +                           if (!sleeping_prematurely(pgdat, order, remaining)) {
> >> >                                     remaining = schedule_timeout(HZ/10);
> >> >                                     finish_wait(&pgdat->kswapd_wait, &wait);
> >> >                                     prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
> >> > @@ -2232,13 +2238,13 @@ static int kswapd(void *p)
> >> >                              * premature sleep. If not, then go fully
> >> >                              * to sleep until explicitly woken up
> >> >                              */
> >> > -                           if (!sleeping_prematurely(order, remaining))
> >> > +                           if (!sleeping_prematurely(pgdat, order, remaining))
> >> >                                     schedule();
> >> >                             else {
> >> >                                     if (remaining)
> >> > -                                           count_vm_event(KSWAPD_PREMATURE_FAST);
> >> > +                                           count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
> >> >                                     else
> >> > -                                           count_vm_event(KSWAPD_PREMATURE_SLOW);
> >> > +                                           count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
> >> >                             }
> >> >                     }
> >> >
> >> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> >> > index bc09547..6cc8dc6 100644
> >> > --- a/mm/vmstat.c
> >> > +++ b/mm/vmstat.c
> >> > @@ -683,8 +683,8 @@ static const char * const vmstat_text[] = {
> >> >     "slabs_scanned",
> >> >     "kswapd_steal",
> >> >     "kswapd_inodesteal",
> >> > -   "kswapd_slept_prematurely_fast",
> >> > -   "kswapd_slept_prematurely_slow",
> >> > +   "kswapd_low_wmark_hit_quickly",
> >> > +   "kswapd_high_wmark_hit_quickly",
> >> >     "kswapd_no_congestion_wait",
> >> >     "pageoutrun",
> >> >     "allocstall",
> >>
> >>
> >>
> >

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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