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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIuaE-002ElZ-3c; Fri, 20 Jan 2023 16:49:42 +0000 Date: Fri, 20 Jan 2023 16:49:42 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Cc: Michal Hocko , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free Message-ID: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 9u8izzhwa3tday7dgjh33iqbppbsftga X-Rspam-User: X-Rspamd-Queue-Id: CCD7D100007 X-Rspamd-Server: rspam06 X-HE-Tag: 1674233423-343298 X-HE-Meta: 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 HiR9clcQ 1dwcKCp68+EmURxPZZd01rkbgtDYcuq6LiJE7ki5XNGm5LoTuIBm7fQLkZ8qG5/csHIVdi9yuDTYBU9zdv9svlKCxzYXLrxaPiIXSoCTd0Ox2R0d2GgYgzHuIiKH1jj1HWoy8qnu6mZNF4KJCs3dtmPq8ZAMRpXZLA2GjZxnaEgxpFMm23QBtv50VY1hQmT/Awsrt06qJU0vcl98WlHZ6rTSoddIPvTNd8RnPSLR+XyQBElSJKUT5m2VFWMcuTPAzJu9+PUVCZXD1f6BnqrGdOaUV49Oplchmwou9 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jan 20, 2023 at 08:45:21AM -0800, Suren Baghdasaryan wrote: > On Fri, Jan 20, 2023 at 8:20 AM Suren Baghdasaryan wrote: > > > > On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko wrote: > > > > > > On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote: > > > > On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko wrote: > > > > > > > > > > On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote: > > > > > > call_rcu() can take a long time when callback offloading is enabled. > > > > > > Its use in the vm_area_free can cause regressions in the exit path when > > > > > > multiple VMAs are being freed. To minimize that impact, place VMAs into > > > > > > a list and free them in groups using one call_rcu() call per group. > > > > > > > > > > After some more clarification I can understand how call_rcu might not be > > > > > super happy about thousands of callbacks to be invoked and I do agree > > > > > that this is not really optimal. > > > > > > > > > > On the other hand I do not like this solution much either. > > > > > VM_AREA_FREE_LIST_MAX is arbitrary and it won't really help all that > > > > > much with processes with a huge number of vmas either. It would still be > > > > > in housands of callbacks to be scheduled without a good reason. > > > > > > > > > > Instead, are there any other cases than remove_vma that need this > > > > > batching? We could easily just link all the vmas into linked list and > > > > > use a single call_rcu instead, no? This would both simplify the > > > > > implementation, remove the scaling issue as well and we do not have to > > > > > argue whether VM_AREA_FREE_LIST_MAX should be epsilon or epsilon + 1. > > > > > > > > Yes, I agree the solution is not stellar. I wanted something simple > > > > but this is probably too simple. OTOH keeping all dead vm_area_structs > > > > on the list without hooking up a shrinker (additional complexity) does > > > > not sound too appealing either. > > > > > > I suspect you have missed my idea. I do not really want to keep the list > > > around or any shrinker. It is dead simple. Collect all vmas in > > > remove_vma and then call_rcu the whole list at once after the whole list > > > (be it from exit_mmap or remove_mt). See? > > > > Yes, I understood your idea but keeping dead objects until the process > > exits even when the system is low on memory (no shrinkers attached) > > seems too wasteful. If we do this I would advocate for attaching a > > shrinker. > > Maybe even simpler, since we are hit with this VMA freeing flood > during exit_mmap (when all VMAs are destroyed), we pass a hint to > vm_area_free to batch the destruction and all other cases call > call_rcu()? I don't think there will be other cases of VMA destruction > floods. ... or have two different call_rcu functions; one for munmap() and one for exit. It'd be nice to use kmem_cache_free_bulk().