From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id 0A5F16B5230 for ; Thu, 30 Aug 2018 12:41:02 -0400 (EDT) Received: by mail-ed1-f70.google.com with SMTP id h40-v6so3903120edb.2 for ; Thu, 30 Aug 2018 09:41:01 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id e20-v6si2066604edj.214.2018.08.30.09.41.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 30 Aug 2018 09:41:00 -0700 (PDT) Date: Thu, 30 Aug 2018 18:40:57 +0200 From: Michal Hocko Subject: Re: [PATCH] mm, thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings Message-ID: <20180830164057.GK2656@dhcp22.suse.cz> References: <82CA00EB-BF8E-4137-953B-8BC4B74B99AF@cs.rutgers.edu> <20180829154744.GC10223@dhcp22.suse.cz> <39BE14E6-D0FB-428A-B062-8B5AEDC06E61@cs.rutgers.edu> <20180829162528.GD10223@dhcp22.suse.cz> <20180829192451.GG10223@dhcp22.suse.cz> <20180830070021.GB2656@dhcp22.suse.cz> <4AFDF557-46E3-4C62-8A43-C28E8F2A54CF@cs.rutgers.edu> <20180830134549.GI2656@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Zi Yan Cc: Andrea Arcangeli , Andrew Morton , linux-mm@kvack.org, Alex Williamson , David Rientjes , Vlastimil Babka , Stefan Priebe - Profihost AG On Thu 30-08-18 10:02:23, Zi Yan wrote: > On 30 Aug 2018, at 9:45, Michal Hocko wrote: > > > On Thu 30-08-18 09:22:21, Zi Yan wrote: > >> On 30 Aug 2018, at 3:00, Michal Hocko wrote: > >> > >>> On Wed 29-08-18 18:54:23, Zi Yan wrote: > >>> [...] > >>>> I tested it against Linusa??s tree with a??memhog -r3 130ga?? in a two-socket machine with 128GB memory on > >>>> each node and got the results below. I expect this test should fill one node, then fall back to the other. > >>>> > >>>> 1. madvise(MADV_HUGEPAGE) + defrag = {always, madvise, defer+madvise}: > >>>> no swap, THPs are allocated in the fallback node. > >>>> 2. madvise(MADV_HUGEPAGE) + defrag = defer: pages got swapped to the > >>>> disk instead of being allocated in the fallback node. > >>>> 3. no madvise, THP is on by default + defrag = {always, defer, > >>>> defer+madvise}: pages got swapped to the disk instead of being > >>>> allocated in the fallback node. > >>>> 4. no madvise, THP is on by default + defrag = madvise: no swap, base > >>>> pages are allocated in the fallback node. > >>>> > >>>> The result 2 and 3 seems unexpected, since pages should be allocated in the fallback node. > >>>> > >>>> The reason, as Andrea mentioned in his email, is that the combination > >>>> of __THIS_NODE and __GFP_DIRECT_RECLAIM (plus __GFP_KSWAPD_RECLAIM > >>>> from this experiment). > >>> > >>> But we do not set __GFP_THISNODE along with __GFP_DIRECT_RECLAIM AFAICS. > >>> We do for __GFP_KSWAPD_RECLAIM though and I guess that it is expected to > >>> see kswapd do the reclaim to balance the node. If the node is full of > >>> anonymous pages then there is no other way than swap out. > >> > >> GFP_TRANSHUGE implies __GFP_DIRECT_RECLAIM. When no madvise is given, THP is on > >> + defrag=always, gfp_mask has __GFP_THISNODE and __GFP_DIRECT_RECLAIM, so swapping > >> can be triggered. > > > > Yes, but the setup tells that you are willing to pay price to get a THP. > > defered=always uses that special __GFP_NORETRY (unless it is madvised > > mapping) that should back off if the compaction failed recently. How > > much that reduces the reclaim is not really clear to me right now to be > > honest. > > > >> The key issue here is that a??memhog -r3 130ga?? uses the default memory policy (MPOL_DEFAULT), > >> which should allow page allocation fallback to other nodes, but as shown in > >> result 3, swapping is triggered instead of page allocation fallback. > > > > Well, I guess this really depends. Fallback to a different node might be > > seen as a bad thing and worse than the reclaim on the local node. > > > >>>> __THIS_NODE uses ZONELIST_NOFALLBACK, which > >>>> removes the fallback possibility and __GFP_*_RECLAIM triggers page > >>>> reclaim in the first page allocation node when fallback nodes are > >>>> removed by ZONELIST_NOFALLBACK. > >>> > >>> Yes but the point is that the allocations which use __GFP_THISNODE are > >>> optimistic so they shouldn't fallback to remote NUMA nodes. > >> > >> This can be achieved by using MPOL_BIND memory policy which restricts > >> nodemask in struct alloc_context for user space memory allocations. > > > > Yes, but that requires and explicit NUMA handling. And we are trying to > > handle those cases which do not really give a damn and just want to use > > THP if it is available or try harder when they ask by using madvise. > > > >>>> IMHO, __THIS_NODE should not be used for user memory allocation at > >>>> all, since it fights against most of memory policies. But kernel > >>>> memory allocation would need it as a kernel MPOL_BIND memory policy. > >>> > >>> __GFP_THISNODE is indeed an ugliness. I would really love to get rid of > >>> it here. But the problem is that optimistic THP allocations should > >>> prefer a local node because a remote node might easily offset the > >>> advantage of the THP. I do not have a great idea how to achieve that > >>> without __GFP_THISNODE though. > >> > >> MPOL_PREFERRED memory policy can be used to achieve this optimistic > >> THP allocation for user space. Even with the default memory policy, > >> local memory node will be used first until it is full. It seems to > >> me that __GFP_THISNODE is not necessary if a proper memory policy is > >> used. > >> > >> Let me know if I miss anything. Thanks. > > > > You are missing that we are trying to define a sensible model for those > > who do not really care about mempolicies. THP shouldn't cause more harm > > than good for those. > > > > I wish we could come up with a remotely sane and comprehensible model. > > That means that you know how hard the allocator tries to get a THP for > > you depending on the defrag configuration, your memory policy and your > > madvise setting. The easiest one I can think of is to > > - always follow mempolicy when specified because you asked for it > > explicitly > > - stay node local and low latency for the light THP defrag mode (defrag, > > madvise without hint and none) because THP is a nice to have > > - if the defrag mode is always then you are willing to pay the latency > > price but off-node might be still a no-no. > > - allow fallback for madvised mappings because you really want THP. If > > you care about specific numa placement then combine with the > > mempolicy. > > > > As you can see I do not really mention anything about the direct reclaim > > because that is just an implementation detail of the page allocator and > > compaction interaction. > > > > Maybe you can formulate a saner matrix with all the available modes that > > we have. > > > > Anyway, I guess we can agree that (almost) unconditional __GFP_THISNODE > > is clearly wrong and we should address that first. Either Andrea's > > option 2) patch or mine which does the similar thing except at the > > proper layer (I believe). We can continue discussing other odd cases on > > top I guess. Unless somebody has much brighter idea, of course. > > Thanks for your explanation. It makes sense to me. I am fine with your patch. > You can add my Tested-by: Zi Yan , since > my test result 1 shows that the problem mentioned in your changelog is solved. Thanks for your and Stefan's testing. I will wait for some more feedback. I will be offline next few days and if there are no major objections I will repost with both tested-bys early next week. -- Michal Hocko SUSE Labs