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

KOSAKI Motohiro
2009-11-14T09:34:25Z

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.

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.

[see include/linux/vmstat.h]

umm...
Why don't we have nr-allocation-failure vmstat? 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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