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McKenney" 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, willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, 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: <20230119192002.GX2948950@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org 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-Rspam-User: X-Rspamd-Queue-Id: C868C4000C X-Rspamd-Server: rspam01 X-Stat-Signature: bmeictehcod17agoz731qix5tpbioa41 X-HE-Tag: 1674156005-8525 X-HE-Meta: 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 pJ2D8VWC 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 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 Thu, Jan 19, 2023 at 10:52:03AM -0800, 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. WDYT about time domain throttling to > limit draining the list to say once per second like this: > > void vm_area_free(struct vm_area_struct *vma) > { > struct mm_struct *mm = vma->vm_mm; > bool drain; > > free_anon_vma_name(vma); > > spin_lock(&mm->vma_free_list.lock); > list_add(&vma->vm_free_list, &mm->vma_free_list.head); > mm->vma_free_list.size++; > - drain = mm->vma_free_list.size > VM_AREA_FREE_LIST_MAX; > + drain = jiffies > mm->last_drain_tm + HZ; > > spin_unlock(&mm->vma_free_list.lock); > > - if (drain) > + if (drain) { > drain_free_vmas(mm); > + mm->last_drain_tm = jiffies; > + } > } > > Ultimately we want to prevent very frequent call_rcu() calls, so > throttling in the time domain seems appropriate. That's the simplest > way I can think of to address your concern about a quick spike in VMA > freeing. It does not place any restriction on the list size and we > might have excessive dead vm_area_structs if after a large spike there > are no vm_area_free() calls but I don't know if that's a real problem, > so not sure we should be addressing it at this time. WDYT? Just to double-check, we really did try the very frequent call_rcu() invocations and we really did see a problem, correct? Although it is not perfect, call_rcu() is designed to take a fair amount of abuse. So if we didn't see a real problem, the frequent call_rcu() invocations might be a bit simpler. Thanx, Paul