Re: [PATCH] mm/vmscan: Do not block forever at shrink_inactive_list().

[Dave Chinner <david@fromorbit.com>]

On Thu, Jun 05, 2014 at 09:45:26PM +0900, Tetsuo Handa wrote:
> David Rientjes wrote:
> > On Mon, 26 May 2014, Tetsuo Handa wrote:
> > 
> > > In shrink_inactive_list(), we do not insert delay at
> > > 
> > >   if (!sc->hibernation_mode && !current_is_kswapd())
> > >     wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
> > > 
> > > if sc->hibernation_mode != 0.
> > > Follow the same reason, we should not insert delay at
> > > 
> > >   while (unlikely(too_many_isolated(zone, file, sc))) {
> > >     congestion_wait(BLK_RW_ASYNC, HZ/10);
> > > 
> > >     /* We are about to die and free our memory. Return now. */
> > >     if (fatal_signal_pending(current))
> > >       return SWAP_CLUSTER_MAX;
> > >   }
> > > 
> > > if sc->hibernation_mode != 0.
> > > 
> > > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > > ---
> > >  mm/vmscan.c |    3 +++
> > >  1 files changed, 3 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 32c661d..89c42ca 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -1362,6 +1362,9 @@ static int too_many_isolated(struct zone *zone, int file,
> > >  	if (current_is_kswapd())
> > >  		return 0;
> > >  
> > > +	if (sc->hibernation_mode)
> > > +		return 0;
> > > +
> > >  	if (!global_reclaim(sc))
> > >  		return 0;
> > >  
> > 
> > This isn't the only too_many_isolated() functions that do a delay, how is 
> > the too_many_isolated() in mm/compaction.c different?
> > 
> 
> I don't know. But today I realized that this patch is not sufficient.
> 
> I'm trying to find why __alloc_pages_slowpath() cannot return for many minutes
> when a certain type of memory pressure is given on a RHEL7 environment with
> 4 CPU / 2GB RAM. Today I tried to use ftrace for examining the breakdown of
> time-consuming functions inside __alloc_pages_slowpath(). But on the first run,
> all processes are trapped into this too_many_isolated()/congestion_wait() loop
> while kswapd is not running; stalling forever because nobody can perform
> operations for making too_many_isolated() to return 0.
> 
> This means that, under rare circumstances, it is possible that all processes
> other than kswapd are trapped into too_many_isolated()/congestion_wait() loop
> while kswapd is sleeping because this loop assumes that somebody else shall
> wake up kswapd and kswapd shall perform operations for making
> too_many_isolated() to return 0. However, we cannot guarantee that kswapd is
> waken by somebody nor kswapd is not blocked by blocking operations inside
> shrinker functions (e.g. mutex_lock()).

So what you are saying is that kswapd is having problems with
getting blocked on locks held by processes in direct reclaim?

What are the stack traces that demonstrate such a dependency loop?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com