Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep

[Colin Ian King <colin.king@canonical.com>]

On Tue, 2011-05-17 at 11:38 +0100, Mel Gorman wrote:
> On Tue, May 17, 2011 at 08:50:44AM +0900, Minchan Kim wrote:
> > On Mon, May 16, 2011 at 7:27 PM, Mel Gorman <mgorman@suse.de> wrote:
> > > On Mon, May 16, 2011 at 05:58:59PM +0900, Minchan Kim wrote:
> > >> On Mon, May 16, 2011 at 5:45 PM, Mel Gorman <mgorman@suse.de> wrote:
> > >> > On Mon, May 16, 2011 at 02:04:00PM +0900, Minchan Kim wrote:
> > >> >> On Mon, May 16, 2011 at 1:21 PM, James Bottomley
> > >> >> <James.Bottomley@hansenpartnership.com> wrote:
> > >> >> > On Sun, 2011-05-15 at 19:27 +0900, KOSAKI Motohiro wrote:
> > >> >> >> (2011/05/13 23:03), Mel Gorman 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>
> > >> >> >> > ---
> > >> >> >> >   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;
> > >> >> >> > +
> > >> >> >>
> > >> >> >> Hmm... I don't like this patch so much. because this code does
> > >> >> >>
> > >> >> >> - don't sleep if kswapd got context switch at shrink_inactive_list
> > >> >> >
> > >> >> > This isn't entirely true:  need_resched() will be false, so we'll follow
> > >> >> > the normal path for determining whether to sleep or not, in effect
> > >> >> > leaving the current behaviour unchanged.
> > >> >> >
> > >> >> >> - sleep if kswapd didn't
> > >> >> >
> > >> >> > This also isn't entirely true: whether need_resched() is true at this
> > >> >> > point depends on a whole lot more that whether we did a context switch
> > >> >> > in shrink_inactive. It mostly depends on how long we've been running
> > >> >> > without giving up the CPU.  Generally that will mean we've been round
> > >> >> > the shrinker loop hundreds to thousands of times without sleeping.
> > >> >> >
> > >> >> >> It seems to be semi random behavior.
> > >> >> >
> > >> >> > Well, we have to do something.  Chris Mason first suspected the hang was
> > >> >> > a kswapd rescheduling problem a while ago.  We tried putting
> > >> >> > cond_rescheds() in several places in the vmscan code, but to no avail.
> > >> >>
> > >> >> Is it a result of  test with patch of Hannes(ie, !pgdat_balanced)?
> > >> >>
> > >> >> If it isn't, it would be nop regardless of putting cond_reshed at vmscan.c.
> > >> >> Because, although we complete zone balancing, kswapd doesn't sleep as
> > >> >> pgdat_balance returns wrong result. And at last VM calls
> > >> >> balance_pgdat. In this case, balance_pgdat returns without any work as
> > >> >> kswap couldn't find zones which have not enough free pages and goto
> > >> >> out. kswapd could repeat this work infinitely. So you don't have a
> > >> >> chance to call cond_resched.
> > >> >>
> > >> >> But if your test was with Hanne's patch, I am very curious how come
> > >> >> kswapd consumes CPU a lot.
> > >> >>
> > >> >> > The need_resched() in sleeping_prematurely() seems to be about the best
> > >> >> > option.  The other option might be just to put a cond_resched() in
> > >> >> > kswapd_try_to_sleep(), but that will really have about the same effect.
> > >> >>
> > >> >> I don't oppose it but before that, I think we have to know why kswapd
> > >> >> consumes CPU a lot although we applied Hannes' patch.
> > >> >>
> > >> >
> > >> > Because it's still possible for processes to allocate pages at the same
> > >> > rate kswapd is freeing them leading to a situation where kswapd does not
> > >> > consider the zone balanced for prolonged periods of time.
> > >>
> > >> We have cond_resched in shrink_page_list, shrink_slab and balance_pgdat.
> > >> So I think kswapd can be scheduled out although it's scheduled in
> > >> after a short time as task scheduled also need page reclaim. Although
> > >> all task in system need reclaim, kswapd cpu 99% consumption is a
> > >> natural result, I think.
> > >> Do I miss something?
> > >>
> > >
> > > Lets see;
> > >
> > > shrink_page_list() only applies if inactive pages were isolated
> > >        which in turn may not happen if all_unreclaimable is set in
> > >        shrink_zones(). If for whatver reason, all_unreclaimable is
> > >        set on all zones, we can miss calling cond_resched().
> > >
> > > shrink_slab only applies if we are reclaiming slab pages. If the first
> > >        shrinker returns -1, we do not call cond_resched(). If that
> > >        first shrinker is dcache and __GFP_FS is not set, direct
> > >        reclaimers will not shrink at all. However, if there are
> > >        enough of them running or if one of the other shrinkers
> > >        is running for a very long time, kswapd could be starved
> > >        acquiring the shrinker_rwsem and never reaching the
> > >        cond_resched().
> > 
> > Don't we have to move cond_resched?
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 292582c..633e761 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -231,8 +231,10 @@ unsigned long shrink_slab(struct shrink_control *shrink,
> >         if (scanned == 0)
> >                 scanned = SWAP_CLUSTER_MAX;
> > 
> > -       if (!down_read_trylock(&shrinker_rwsem))
> > -               return 1;       /* Assume we'll be able to shrink next time */
> > +       if (!down_read_trylock(&shrinker_rwsem)) {
> > +               ret = 1;
> > +               goto out; /* Assume we'll be able to shrink next time */
> > +       }
> > 
> >         list_for_each_entry(shrinker, &shrinker_list, list) {
> >                 unsigned long long delta;
> > @@ -280,12 +282,14 @@ unsigned long shrink_slab(struct shrink_control *shrink,
> >                         count_vm_events(SLABS_SCANNED, this_scan);
> >                         total_scan -= this_scan;
> > 
> > -                       cond_resched();
> >                 }
> > 
> >                 shrinker->nr += total_scan;
> > +               cond_resched();
> >         }
> >         up_read(&shrinker_rwsem);
> > +out:
> > +       cond_resched();
> >         return ret;
> >  }
> > 
> 
> This makes some sense for the exit path but if one or more of the
> shrinkers takes a very long time without sleeping (extremely long
> list searches for example) then kswapd will not call cond_resched()
> between shrinkers and still consume a lot of CPU.
> 
> > >
> > > balance_pgdat() only calls cond_resched if the zones are not
> > >        balanced. For a high-order allocation that is balanced, it
> > >        checks order-0 again. During that window, order-0 might have
> > >        become unbalanced so it loops again for order-0 and returns
> > >        that was reclaiming for order-0 to kswapd(). It can then find
> > >        that a caller has rewoken kswapd for a high-order and re-enters
> > >        balance_pgdat() without ever have called cond_resched().
> > 
> > If kswapd reclaims order-o followed by high order, it would have a
> > chance to call cond_resched in shrink_page_list. But if all zones are
> > all_unreclaimable is set, balance_pgdat could return any work. Okay.
> > It does make sense.
> > By your scenario, someone wakes up kswapd with higher order, again.
> > So re-enters balance_pgdat without ever have called cond_resched.
> > But if someone wakes up higher order again, we can't have a chance to
> > call kswapd_try_to_sleep. So your patch effect would be nop, too.
> > 
> > It would be better to put cond_resched after balance_pgdat?
> > 
> 
> Which will leave kswapd runnable instead of going to sleep but
> guarantees a scheduling point. Lets see if the problem is that
> cond_resched is being missed although if this was the case then patch
> 4 would truly be a no-op but Colin has already reported that patch 1 on
> its own didn't fix his problem. If the problem is sandybridge-specific
> where kswapd remains runnable and consuming large amounts of CPU in
> turbo mode then we know that there are other cond_resched() decisions
> that will need to be revisited.
> 
> Colin or James, would you be willing to test with patch 1 from this
> series and Minchan's patch below? Thanks.

This works OK fine.  Ran 250 test cycles for about 2 hours.

> 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 292582c..61c45d0 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2753,6 +2753,7 @@ static int kswapd(void *p)
> >                 if (!ret) {
> >                         trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
> >                         order = balance_pgdat(pgdat, order, &classzone_idx);
> > +                       cond_resched();
> >                 }
> >         }
> >         return 0;
> > 
> > >
> > > While it appears unlikely, there are bad conditions which can result
> > > in cond_resched() being avoided.
> > 
> > >
> > > --
> > > Mel Gorman
> > > SUSE Labs
> > >
> > 
> > 
> > 
> > -- 
> > Kind regards,
> > Minchan Kim
>