Re: [PATCH 0/3] OOM detection rework v4

[Minchan Kim]

On Wed, Mar 02, 2016 at 10:50:56AM +0100, Michal Hocko wrote:
> On Wed 02-03-16 11:19:54, Joonsoo Kim wrote:
> > On Mon, Feb 29, 2016 at 10:02:13PM +0100, Michal Hocko wrote:
> [...]
> > > > +	/*
> > > > +	 * OK, so the watermak check has failed. Make sure we do all the
> > > > +	 * retries for !costly high order requests and hope that multiple
> > > > +	 * runs of compaction will generate some high order ones for us.
> > > > +	 *
> > > > +	 * XXX: ideally we should teach the compaction to try _really_ hard
> > > > +	 * if we are in the retry path - something like priority 0 for the
> > > > +	 * reclaim
> > > > +	 */
> > > > +	if (order && order <= PAGE_ALLOC_COSTLY_ORDER)
> > > > +		return true;
> > > > +
> > > >  	return false;
> > 
> > This seems not a proper fix. Checking watermark with high order has
> > another meaning that there is high order page or not. This isn't
> > what we want here.
> 
> Why not? Why should we retry the reclaim if we do not have >=order page
> available? Reclaim itself doesn't guarantee any of the freed pages will
> form the requested order. The ordering on the LRU lists is pretty much
> random wrt. pfn ordering. On the other hand if we have a page available
> which is just hidden by watermarks then it makes perfect sense to retry
> and free even order-0 pages.
> 
> > So, following fix is needed.
> 
> > 'if (order)' check isn't needed. It is used to clarify the meaning of
> > this fix. You can remove it.
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 1993894..8c80375 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3125,6 +3125,10 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
> >         if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
> >                 return false;
> >  
> > +       /* To check whether compaction is available or not */
> > +       if (order)
> > +               order = 0;
> > +
> 
> This would enforce the order 0 wmark check which is IMHO not correct as
> per above.
> 
> >         /*
> >          * Keep reclaiming pages while there is a chance this will lead
> >          * somewhere.  If none of the target zones can satisfy our allocation
> > 
> > > >  }
> > > >  
> > > > @@ -3281,11 +3293,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> > > >  		goto noretry;
> > > >  
> > > >  	/*
> > > > -	 * Costly allocations might have made a progress but this doesn't mean
> > > > -	 * their order will become available due to high fragmentation so do
> > > > -	 * not reset the no progress counter for them
> > > > +	 * High order allocations might have made a progress but this doesn't
> > > > +	 * mean their order will become available due to high fragmentation so
> > > > +	 * do not reset the no progress counter for them
> > > >  	 */
> > > > -	if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER)
> > > > +	if (did_some_progress && !order)
> > > >  		no_progress_loops = 0;
> > > >  	else
> > > >  		no_progress_loops++;
> > 
> > This unconditionally increases no_progress_loops for high order
> > allocation, so, after 16 iterations, it will fail. If compaction isn't
> > enabled in Kconfig, 16 times reclaim attempt would not be sufficient
> > to make high order page. Should we consider this case also?
> 
> How many retries would help? I do not think any number will work
> reliably. Configurations without compaction enabled are asking for
> problems by definition IMHO. Relying on order-0 reclaim for high order
> allocations simply cannot work.

I left compaction code for a long time so a super hero might make it
perfect now but I don't think the dream come true yet and I believe
any algorithm has a drawback so we end up relying on a fallback approach
in case of not working compaction correctly.

My suggestion is to reintroduce *lumpy reclaim* and kicks in only when
compaction gave up by some reasons. It would be better to rely on
random number retrial of reclaim.