Re: [RFC 3/3] mm: use watermak checks for __GFP_REPEAT high order allocations

[Michal Hocko <mhocko@suse.cz>]

On Thu 19-11-15 15:17:35, David Rientjes wrote:
> On Wed, 18 Nov 2015, Michal Hocko wrote:
[...]
> > @@ -3167,24 +3166,21 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  
> >  	/*
> >  	 * Do not retry high order allocations unless they are __GFP_REPEAT
> > -	 * and even then do not retry endlessly unless explicitly told so
> > +	 * unless explicitly told so.
> >  	 */
> > -	pages_reclaimed += did_some_progress;
> > -	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> > -		if (!(gfp_mask & __GFP_NOFAIL) &&
> > -		   (!(gfp_mask & __GFP_REPEAT) || pages_reclaimed >= (1<<order)))
> > -			goto noretry;
> > -
> > -		if (did_some_progress)
> > -			goto retry;
> > -	}
> > +	if (order > PAGE_ALLOC_COSTLY_ORDER &&
> > +			!(gfp_mask & (__GFP_REPEAT|__GFP_NOFAIL)))
> > +		goto noretry;
> 
> Who is allocating order > PAGE_ALLOC_COSTLY_ORDER with __GFP_REPEAT and 
> would be affected by this change?

E.g. hugetlb pages. I have tested this in my testing scenario 3.

> >  
> >  	/*
> >  	 * Be optimistic and consider all pages on reclaimable LRUs as usable
> >  	 * but make sure we converge to OOM if we cannot make any progress after
> >  	 * multiple consecutive failed attempts.
> > +	 * Costly __GFP_REPEAT 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 backoff for them
> >  	 */
> > -	if (did_some_progress)
> > +	if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER)
> >  		stall_backoff = 0;
> >  	else
> >  		stall_backoff = min(stall_backoff+1, MAX_STALL_BACKOFF); 
> 
> This makes sense if there are high-order users of __GFP_REPEAT since 
> only using a number of pages reclaimed by itself isn't helpful.

Yes, that was my thinking

Thanks!

-- 
Michal Hocko
SUSE Labs

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