Re: [PATCH 3/4] mm: slub: Do not take expensive steps for SLUBs speculative high-order allocations

[David Rientjes <rientjes@google.com>]

On Tue, 17 May 2011, Mel Gorman wrote:

> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 9f8a97b..057f1e2 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1972,6 +1972,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> > >  {
> > >  	int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
> > >  	const gfp_t wait = gfp_mask & __GFP_WAIT;
> > > +	const gfp_t can_wake_kswapd = !(gfp_mask & __GFP_NO_KSWAPD);
> > >  
> > >  	/* __GFP_HIGH is assumed to be the same as ALLOC_HIGH to save a branch. */
> > >  	BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
> > > @@ -1984,7 +1985,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> > >  	 */
> > >  	alloc_flags |= (__force int) (gfp_mask & __GFP_HIGH);
> > >  
> > > -	if (!wait) {
> > > +	if (!wait && can_wake_kswapd) {
> > >  		/*
> > >  		 * Not worth trying to allocate harder for
> > >  		 * __GFP_NOMEMALLOC even if it can't schedule.
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 98c358d..c5797ab 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -1170,7 +1170,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> > >  	 * Let the initial higher-order allocation fail under memory pressure
> > >  	 * so we fall-back to the minimum order allocation.
> > >  	 */
> > > -	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY | __GFP_NO_KSWAPD) & ~__GFP_NOFAIL;
> > > +	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NO_KSWAPD) &
> > > +			~(__GFP_NOFAIL | __GFP_WAIT | __GFP_REPEAT);
> > >  
> > >  	page = alloc_slab_page(alloc_gfp, node, oo);
> > >  	if (unlikely(!page)) {
> > 
> > It's unnecessary to clear __GFP_REPEAT, these !__GFP_NOFAIL allocations 
> > will immediately fail.
> > 
> 
> We can enter enter direct compaction or direct reclaim
> at least once. If compaction is enabled and we enter
> reclaim/compaction, the presense of __GFP_REPEAT makes a difference
> in should_continue_reclaim().  With compaction disabled, the presense
> of the flag is relevant in should_alloc_retry() with it being possible
> to loop in the allocator instead of failing the SLUB allocation and
> dropping back.
> 

You've cleared __GFP_WAIT, so it cannot enter direct compaction or direct 
reclaim, so clearing __GFP_REPEAT here doesn't actually do anything.  
That's why I suggested adding a comment about why you're clearing 
__GFP_WAIT: to make it obvious that these allocations will immediately 
fail if the alloc is unsuccessful and we don't need to add __GFP_NORETRY 
or remove __GFP_REPEAT.

> Maybe you meant !__GFP_WAIT instead of !__GFP_NOFAIL which makes
> more sense.

No, I meant !__GFP_NOFAIL since the high priority allocations (if 
PF_MEMALLOC or TIF_MEMDIE) will not loop forever looking for a page 
without that bit.  That allows this !__GFP_WAIT allocation to immediately 
fail.  __GFP_NORETRY and __GFP_REPEAT are no-ops unless you have 
__GFP_WAIT.