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

[Mel Gorman <mgorman@suse.de>]

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().

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().

While it appears unlikely, there are bad conditions which can result
in cond_resched() being avoided.

-- 
Mel Gorman
SUSE Labs

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