Re: Deadlock possibly caused by too_many_isolated.

[Wu Fengguang]

On Wed, Sep 15, 2010 at 11:17:35AM +0800, Neil Brown wrote:
> On Wed, 15 Sep 2010 11:06:40 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> 
> > On Wed, Sep 15, 2010 at 10:54:54AM +0800, Wu Fengguang wrote:
> > > On Wed, Sep 15, 2010 at 10:37:35AM +0800, Wu Fengguang wrote:
> > > > On Wed, Sep 15, 2010 at 10:23:34AM +0800, Neil Brown wrote:
> > > > > On Tue, 14 Sep 2010 20:30:18 -0400
> > > > > Rik van Riel <riel@xxxxxxxxxx> wrote:
> > > > > 
> > > > > > On 09/14/2010 07:11 PM, Neil Brown wrote:
> > > > > > 
> > > > > > > Index: linux-2.6.32-SLE11-SP1/mm/vmscan.c
> > > > > > > ===================================================================
> > > > > > > --- linux-2.6.32-SLE11-SP1.orig/mm/vmscan.c	2010-09-15 08:37:32.000000000 +1000
> > > > > > > +++ linux-2.6.32-SLE11-SP1/mm/vmscan.c	2010-09-15 08:38:57.000000000 +1000
> > > > > > > @@ -1106,6 +1106,11 @@ static unsigned long shrink_inactive_lis
> > > > > > >   		/* We are about to die and free our memory. Return now. */
> > > > > > >   		if (fatal_signal_pending(current))
> > > > > > >   			return SWAP_CLUSTER_MAX;
> > > > > > > +		if (!(sc->gfp_mask&  __GFP_IO))
> > > > > > > +			/* Not allowed to do IO, so mustn't wait
> > > > > > > +			 * on processes that might try to
> > > > > > > +			 */
> > > > > > > +			return SWAP_CLUSTER_MAX;
> > > > > > >   	}
> > > > > > >
> > > > > > >   	/*
> > > > > > 
> > > > > > Close.  We must also be sure that processes without __GFP_FS
> > > > > > set in their gfp_mask do not wait on processes that do have
> > > > > > __GFP_FS set.
> > > > > > 
> > > > > > Considering how many times we've run into a bug like this,
> > > > > > I'm kicking myself for not having thought of it :(
> > > > > > 
> > > > > 
> > > > > So maybe this?  I've added the test for __GFP_FS, and moved the test before
> > > > > the congestion_wait on the basis that we really want to get back up the stack
> > > > > and try the mempool ASAP.
> > > > 
> > > > The patch may well fail the !__GFP_IO page allocation and then
> > > > quickly exhaust the mempool.
> > > > 
> > > > Another approach may to let too_many_isolated() use much higher
> > > > thresholds for !__GFP_IO/FS and lower ones for __GFP_IO/FS. ie. to
> > > > allow at least nr2 NOIO/FS tasks to be blocked independent of the
> > > > IO/FS ones.  Since NOIO vmscans typically completes fast, it will then
> > > > very hard to accumulate enough NOIO processes to be actually blocked.
> > > > 
> > > > 
> > > >                   IO/FS tasks                NOIO/FS tasks           full
> > > >                   block here                 block here              LRU size
> > > > |-----------------|--------------------------|-----------------------|
> > > > |      nr1        |           nr2            |
> > > 
> > > How about this fix? We may need very high threshold for NOIO/NOFS to
> > > prevent possible regressions.
> > 
> > Plus __GFP_WAIT..
> > 
> > ---
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 225a759..6a896eb 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1135,6 +1135,7 @@ static int too_many_isolated(struct zone *zone, int file,
> >  		struct scan_control *sc)
> >  {
> >  	unsigned long inactive, isolated;
> > +	int ratio;
> >  
> >  	if (current_is_kswapd())
> >  		return 0;
> > @@ -1150,7 +1151,15 @@ static int too_many_isolated(struct zone *zone, int file,
> >  		isolated = zone_page_state(zone, NR_ISOLATED_ANON);
> >  	}
> >  
> > -	return isolated > inactive;
> > +	ratio = 1;
> > +	if (!(sc->gfp_mask & (__GFP_FS)))
> > +		ratio <<= 1;
> > +	if (!(sc->gfp_mask & (__GFP_IO)))
> > +		ratio <<= 1;
> > +	if (!(sc->gfp_mask & (__GFP_WAIT)))
> > +		ratio <<= 1;
> > +
> > +	return isolated > inactive * ratio;
> >  }
> >  
> >  /*
> 
> 
> Are you suggesting this instead of my patch, or as well as my patch?

Your patch surely breaks the deadlock, however might reintroduce the
old problem too_many_isolated() tried to address..

> Because while I think it sounds like a good idea I don't think it actually
> removes the chance of a deadlock, just makes it a lot less likely.
> So I think your patch combined with my patch would be a good total solution.

Deadlock means IO/FS tasks (blocked on FS lock) blocking the NOIO/FS
tasks? I think raising the threshold for NOIO/FS would be sufficient
to break the deadlock: The NOIO/FS tasks will be blocked simply
because there are so many NOIO/FS tasks competing with each other.
They do not inherently depend on the release of FS locks to proceed.

The too_many_isolated() was introduced initially to prevent OOM for
some fork-bomb workload, where no IO is involved (so no FS locks). If
removing the congestion wait for NOIO/FS tasks, the OOM may raise
again for the fork-bomb workload.

So I'd suggest to use sufficient high threshold for NOIO/FS, but still
limit the number of concurrent NOIO/FS allocations.

Thanks,
Fengguang
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