From: David Rientjes Subject: Re: [PATCH 3/4] mm: slub: Do not take expensive steps for SLUBs speculative high-order allocations Date: Tue, 17 May 2011 12:31:52 -0700 (PDT) Message-ID: References: <1305295404-12129-1-git-send-email-mgorman@suse.de> <1305295404-12129-4-git-send-email-mgorman@suse.de> <20110517084227.GI5279@suse.de> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Andrew Morton , James Bottomley , Colin King , Raghavendra D Prabhu , Jan Kara , Chris Mason , Christoph Lameter , Pekka Enberg , Rik van Riel , Johannes Weiner , linux-fsdevel , linux-mm , linux-kernel , linux-ext4 To: Mel Gorman Return-path: In-Reply-To: <20110517084227.GI5279@suse.de> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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.