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Date: Mon, 16 Apr 2018 06:09:36 -0700 From: Matthew Wilcox <willy@...radead.org> To: Vlastimil Babka <vbabka@...e.cz> Cc: Michal Hocko <mhocko@...nel.org>, Johannes Weiner <hannes@...xchg.org>, Minchan Kim <minchan@...nel.org>, Roman Gushchin <guro@...com>, linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...com Subject: Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory On Mon, Apr 16, 2018 at 02:06:21PM +0200, Vlastimil Babka wrote: > On 04/16/2018 01:41 PM, Michal Hocko wrote: > > On Fri 13-04-18 10:37:16, Johannes Weiner wrote: > >> On Fri, Apr 13, 2018 at 04:28:21PM +0200, Michal Hocko wrote: > >>> On Fri 13-04-18 16:20:00, Vlastimil Babka wrote: > >>>> We would need kmalloc-reclaimable-X variants. It could be worth it, > >>>> especially if we find more similar usages. I suspect they would be more > >>>> useful than the existing dma-kmalloc-X :) > >>> > >>> I am still not sure why __GFP_RECLAIMABLE cannot be made work as > >>> expected and account slab pages as SLAB_RECLAIMABLE > >> > >> Can you outline how this would work without separate caches? > > > > I thought that the cache would only maintain two sets of slab pages > > depending on the allocation reuquests. I am pretty sure there will be > > other details to iron out and > > For example the percpu (and other) array caches... > > > maybe it will turn out that such a large > > portion of the chache would need to duplicate the state that a > > completely new cache would be more reasonable. > > I'm afraid that's the case, yes. I'm not sure it'll be so bad, at least for SLUB ... I think everything we need to duplicate is already percpu, and if we combine GFP_DMA and GFP_RECLAIMABLE into this, we might even get more savings. Also, we only need to do this for the kmalloc slabs; currently 13 of them. So we eliminate 13 caches and in return allocate 13 * 2 * NR_CPU pointers. That'll be a win on some machines and a loss on others, but the machines where it's consuming more memory should have more memory to begin with, so I'd count it as a win. The node partial list probably wants to be trebled in size to have one list per memory type. But I think the allocation path only changes like this: @@ -2663,10 +2663,13 @@ static __always_inline void *slab_alloc_node(struct kmem _cache *s, struct kmem_cache_cpu *c; struct page *page; unsigned long tid; + unsigned int offset = 0; s = slab_pre_alloc_hook(s, gfpflags); if (!s) return NULL; if (s->flags & SLAB_KMALLOC) offset = flags_to_slab_id(gfpflags); redo: /* * Must read kmem_cache cpu data via this cpu ptr. Preemption is @@ -2679,8 +2682,8 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, * to check if it is matched or not. */ do { - tid = this_cpu_read(s->cpu_slab->tid); - c = raw_cpu_ptr(s->cpu_slab); + tid = this_cpu_read((&s->cpu_slab[offset])->tid); + c = raw_cpu_ptr(&s->cpu_slab[offset]); } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != READ_ONCE(c->tid))); > > Is this worth exploring > > at least? I mean something like this should help with the fragmentation > > already AFAIU. Accounting would be just free on top. > > Yep. It could be also CONFIG_urable so smaller systems don't need to > deal with the memory overhead of this. > > So do we put it on LSF/MM agenda? We have an agenda? :-)
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