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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: <20230119195503.GY2948950@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> <20230119192002.GX2948950@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: tftkefzr6kwmy8ej1d7cpwon56uu6cbk X-Rspam-User: X-Rspamd-Queue-Id: 81C2540008 X-Rspamd-Server: rspam06 X-HE-Tag: 1674158106-716658 X-HE-Meta: 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 q3/r4Ytl 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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 11:47:36AM -0800, Suren Baghdasaryan wrote: > On Thu, Jan 19, 2023 at 11:20 AM Paul E. McKenney wrote: > > > > 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? > > Correct. More specifically with CONFIG_RCU_NOCB_CPU=y we saw > regressions when a process exits and all its VMAs get destroyed, > causing a flood of call_rcu()'s. Thank you for the reminder, real problem needs solution. ;-) Thanx, Paul > > 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.