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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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.
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.

James