Re: [PATCH 2/2] mm: vmscan: If kswapd has been running too long, allow it to sleep

[Minchan Kim <minchan.kim@gmail.com>]

On Wed, May 18, 2011 at 6:47 PM, Mel Gorman <mgorman@suse.de> wrote:
> On Tue, May 17, 2011 at 04:22:26PM -0700, Andrew Morton wrote:
>> On Tue, 17 May 2011 10:37:04 +0400
>> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>
>> > On Mon, 2011-05-16 at 14:16 -0700, Andrew Morton wrote:
>> > > On Mon, 16 May 2011 16:06:57 +0100
>> > > Mel Gorman <mgorman@suse.de> wrote:
>> > >
>> > > > Under constant allocation pressure, kswapd can be in the situation where
>> > > > sleeping_prematurely() will always return true even if kswapd has been
>> > > > running a long time. Check if kswapd needs to be scheduled.
>> > > >
>> > > > Signed-off-by: Mel Gorman <mgorman@suse.de>
>> > > > Acked-by: Rik van Riel <riel@redhat.com>
>> > > > ---
>> > > >  mm/vmscan.c |    4 ++++
>> > > >  1 files changed, 4 insertions(+), 0 deletions(-)
>> > > >
>> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > > > index af24d1e..4d24828 100644
>> > > > --- a/mm/vmscan.c
>> > > > +++ b/mm/vmscan.c
>> > > > @@ -2251,6 +2251,10 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>> > > >         unsigned long balanced = 0;
>> > > >         bool all_zones_ok = true;
>> > > >
>> > > > +       /* If kswapd has been running too long, just sleep */
>> > > > +       if (need_resched())
>> > > > +               return false;
>> > > > +
>> > > >         /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
>> > > >         if (remaining)
>> > > >                 return true;
>> > >
>> > > I'm a bit worried by this one.
>> > >
>> > > Do we really fully understand why kswapd is continuously running like
>> > > this?  The changelog makes me think "no" ;)
>> > >
>> > > Given that the page-allocating process is madly reclaiming pages in
>> > > direct reclaim (yes?) and that kswapd is madly reclaiming pages on a
>> > > different CPU, we should pretty promptly get into a situation where
>> > > kswapd can suspend itself.  But that obviously isn't happening.  So
>> > > what *is* going on?
>> >
>> > The triggering workload is a massive untar using a file on the same
>> > filesystem, so that's a continuous stream of pages read into the cache
>> > for the input and a stream of dirty pages out for the writes.  We
>> > thought it might have been out of control shrinkers, so we already
>> > debugged that and found it wasn't.  It just seems to be an imbalance in
>> > the zones that the shrinkers can't fix which causes
>> > sleeping_prematurely() to return true almost indefinitely.
>>
>> Is the untar disk-bound?  The untar has presumably hit the writeback
>> dirty_ratio?  So its rate of page allocation is approximately equal to
>> the write speed of the disks?
>>
>
> A reasonable assumption but it gets messy.
>
>> If so, the VM is consuming 100% of a CPU to reclaim pages at a mere
>> tens-of-megabytes-per-second.  If so, there's something seriously wrong
>> here - under favorable conditions one would expect reclaim to free up
>> 100,000 pages/sec, maybe more.
>>
>> If the untar is not disk-bound and the required page reclaim rate is
>> equal to the rate at which a CPU can read, decompress and write to
>> pagecache then, err, maybe possible.  But it still smells of
>> inefficient reclaim.
>>
>
> I think it's higher than just the rate of data but couldn't guess by
> how much exactly. Reproducing this locally would have been nice but
> the following conditions are likely happening on the problem machine.
>
>   SLUB is using high-orders for its slabs, kswapd and reclaimers are
>   reclaiming at a faster rate than required for just the data. SLUB
>   is using order-2 allocs for inodes so every 18 files created by
>   untar, we need an order-2 page. For ext4_io_end, we need order-3
>   allocs and we are allocating these due to delayed block allocation.
>
>   So for example: 50 files, each less than 1 page in size needs 50
>   order-0 pages, 3 order-2 page and 2 order-3 pages
>
>   To satisfy the high order pages, we are reclaiming at least 28
>   pages. For compaction, we are migrating these so we are allocating
>   a further 28 pages and then copying putting further pressure on
>   the system. We may do this multiple times as order-0 allocations
>   could be breaking up the pages again. Without compaction, we are
>   only reclaiming but can get stalled for significant periods of
>   time if dirty or writeback pages are encountered in the contiguous
>   blocks and can reclaim too many pages quite easily.
>
> So the rate of allocation required to write out data is higher than
> just the data rate. The reclaim rate could be just fine but the number
> of pages we need to reclaim to allocate slab objects can be screwy.
>
>> > > Secondly, taking an up-to-100ms sleep in response to a need_resched()
>> > > seems pretty savage and I suspect it risks undesirable side-effects.  A
>> > > plain old cond_resched() would be more cautious.  But presumably
>> > > kswapd() is already running cond_resched() pretty frequently, so why
>> > > didn't that work?
>> >
>> > So the specific problem with cond_resched() is that kswapd is still
>> > runnable, so even if there's other work the system can be getting on
>> > with, it quickly comes back to looping madly in kswapd.  If we return
>> > false from sleeping_prematurely(), we stop kswapd until its woken up to
>> > do more work.  This manifests, even on non sandybridge systems that
>> > don't hang as a lot of time burned in kswapd.
>> >
>> > I think the sandybridge bug I see on the laptop is that cond_resched()
>> > is somehow ineffective:  kswapd is usually hogging one CPU and there are
>> > runnable processes but they seem to cluster on other CPUs, leaving
>> > kswapd to spin at close to 100% system time.
>> >
>> > When the problem was first described, we tried sprinkling more
>> > cond_rescheds() in the shrinker loop and it didn't work.
>>
>> Seems to me that kswapd for some reason is doing too much work.  Or,
>> more specifically is doing its work very inefficiently.  Making kswapd
>> take arbitrary naps when it's misbehaving didn't fix that misbehaviour!
>>
>
> It is likely to be doing work inefficiently in one of two ways
>
>  1. We are reclaiming far more pages than required by the data
>     for slab objects
>
>  2. The rate we are reclaiming is fast enough that dirty pages are
>     reaching the end of the LRU quickly
>
> The latter part is also important. I doubt we are getting stalled in
> writepage as this is new data being written to disk to blocks aren't
> allocated yet but kswapd is encountering the dirty_ratio of pages
> on the LRU and churning them through the LRU and reclaims the clean
> pages in between.
>
> In effect, this "sorts" the LRU lists so the dirty pages get grouped
> together. At worst on a 2G system such as James', we have 104857
> (20% of memory in pages) pages together on the LRU, all dirty and
> all being skipped over by kswapd and direct reclaimers. This is at
> least 3276 takings of the zone LRU lock assuming we isolate pages in
> groups of SWAP_CLUSTER_MAX which a lot of list walking and CPU usage
> for no pages reclaimed.
>
> In this case, kswapd might as well take a brief nap as it can't clean
> the pages so the flusher threads can get some work done.
>
>> It would be interesting to watch kswapd's page reclaim inefficiency
>> when this is happening: /proc/vmstat:pgscan_kswapd_* versus
>> /proc/vmstat:kswapd_steal.  If that ration is high then kswapd is
>> scanning many pages and not reclaiming them.
>>
>> But given the prominence of shrink_slab in the traces, perhaps that
>> isn't happening.
>>
>
> As we are aggressively shrinking slab, we can reach the stage where
> we scan the requested number of objects and reclaim none of them
> potentially setting zone->all_unreclaimable to 1 if a lot of scanning
> has also taken place recently without pages being freed. Once this
> happens, kswapd isn't even trying to reclaim pages and is instead stuck
> in shrink_slab until a page is freed clearing zone->all_unreclaimable
> and zone->pages-scanned.

Why does it stuck in shrink_slab?
If the zone is trouble to reclaim(ie, all_unreclaimable is set),
kswapd will poll the zone only in case of DEF_PRIORITY(ie, small
window) for when the problem goes away. In high priority (0..11), the
zone will be skipped and we can't get a chance to call
shrink_[zone|slab].


>
> The ratio during that window would not change but slabs_scanned would
> continue to increase.
>
> --
> Mel Gorman
> SUSE Labs
>



-- 
Kind regards,
Minchan Kim

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