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Date: Tue, 17 May 2011 09:42:27 +0100 From: Mel Gorman <mgorman@...e.de> To: David Rientjes <rientjes@...gle.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, James Bottomley <James.Bottomley@...senpartnership.com>, Colin King <colin.king@...onical.com>, Raghavendra D Prabhu <raghu.prabhu13@...il.com>, Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>, Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, Rik van Riel <riel@...hat.com>, Johannes Weiner <hannes@...xchg.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, linux-kernel <linux-kernel@...r.kernel.org>, linux-ext4 <linux-ext4@...r.kernel.org> Subject: Re: [PATCH 3/4] mm: slub: Do not take expensive steps for SLUBs speculative high-order allocations On Mon, May 16, 2011 at 02:16:46PM -0700, David Rientjes wrote: > On Fri, 13 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. Maybe you meant !__GFP_WAIT instead of !__GFP_NOFAIL which makes more sense. In that case, we clear both flags because __GFP_REPEAT && !_GFP_WAIT is a senseless combination of flags. If for whatever reason the __GFP_WAIT was re-added, the presense of __GFP_REPEAT could cause problems in reclaim that would be hard to spot again. > alloc_gfp would probably benefit from having a comment about why > __GFP_WAIT should be masked off here: that we don't want to do compaction > or direct reclaim or retry the allocation more than once (so both > __GFP_NORETRY and __GFP_REPEAT are no-ops). That would have been helpful all right. I should have caught that and explained it properly. In the event there is a new version of the patch, I'll add one. For the moment, I'm dropping this patch entirely. Christoph wants to maintain historic behaviour of SLUB to maximise the number of high-order pages it uses and at the end of the day, which option performs better depends entirely on the workload and machine configuration. -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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