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Howlett" To: Matthew Wilcox Cc: Suren Baghdasaryan , Michal Hocko , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, 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: <20230120170815.yuylbs27r6xcjpq5@revolver> Mail-Followup-To: "Liam R. 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U2FsdGVkX1/NMpfAoEvWqwh7Dm/LXNN7IvXff7qh+8sJyJNzc4GQ/0VsC+feuYW1tMB89AQq2AmI330ACu/oticpc2QM4AMwQwTSsK2GHIWRU2LmzqBr+qHTfLO6Zf26/bYzynksSv4hHhlIiEDuoXBVQ/cQL1A3NcBe5uBxcRvQst9HA5ofqwQliOFm6DPZmFTky+eb+L+YsBp9F8QekQ/fIgQ6ygJEgZt/bvHSqe3PxPjkL3vjV32Z3M67MIPxuy/5JOG+KzKnDI9QFs+dL+4GvAe4eCN3IkyEkZTEfCki6bu8L+XAY2kOj5SSuN/kkK7WlTYlxPP7pPlbBiUng+sSJTGhpTKHnsC6Df9CTs9Tu9e0sZI6vGXX7tJ6s+lUb7FJ0g9r+Q6ZMlhvUocoHOycu8JsvqpnqEo/BQBu6brsco7GpT3q87DJ9veV+FySfEn3PBN1YohDbp3gUSiB1Z5nfNBjTVhre9NsidLwmD4wbrwkydAaT+v3tX73FY7o1WPf3u421fp+MXytT0JxsuLZXu5Gq8Osz3gfojRJkXqmqlX1KfWMJgiJsQn2GDg3V+Sn606xoGEClkSNo80yQOi/VyWy64KwA1TtyABoqvik+MJXeTESubaQrTPdbXA/OpT49hSdK0mt/aTjzO5icFSJRddnTlukPadFvjtI8djXpf2MY78TddGpvkduZK7rcys/y1pVpsc9ehq5gpk6RPNbqgBHK9u02nqDZoLp+YoyMNe3B6F5R6dYpeOVze/pGU3oQ6SgvoUCeQn84sEpUoke2KgWCtTpGSPyaRaVxY7oSQW74ODVTzXB8xc4Be840KK00zQUvd03f7D99XSD8OfmQMWP2KYjboutggk/LWGgnRCIKd2aQoP4Wa/AFuiVJL5ZbBHQhX3+Tt+FjCDys+WNQWynX54N5B2z+be3BsmJN6t48IVrhlCEhttyIkMjHGjKi6xbT2pXzQQp0D5 phOJgwnw 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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(). Do we even need a call_rcu on exit? At the point of freeing the VMAs we have set the MMF_OOM_SKIP bit and unmapped the vmas under the read lock. Once we have obtained the write lock again, I think it's safe to say we can just go ahead and free the VMAs directly.