From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id E6003C05027 for ; Fri, 20 Jan 2023 17:33:16 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 87D7B6B0072; Fri, 20 Jan 2023 12:33:16 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 82D9C6B0081; Fri, 20 Jan 2023 12:33:16 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6F6F56B0082; Fri, 20 Jan 2023 12:33:16 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 5F0096B0072 for ; Fri, 20 Jan 2023 12:33:16 -0500 (EST) Received: from smtpin13.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id 11A081205B5 for ; Fri, 20 Jan 2023 17:33:16 +0000 (UTC) X-FDA: 80375873592.13.C447234 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf30.hostedemail.com (Postfix) with ESMTP id D02268000A for ; Fri, 20 Jan 2023 17:33:12 +0000 (UTC) Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=lxP195rX; spf=none (imf30.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org; dmarc=none ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1674235993; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=2jkEwjOPyig34Xw2pt9DR8sA0x9KlwAW9Pppxnavdp0=; b=DGuPI5K3cFQXds+Rz5FddV/LG2bm8wgO0ZxhnV3WyPOG8HoJjv9HOR0q68tMMthScyS/21 C6QyE46rYv+9BJCnwsg/RaN3HScN/c6yyAHacMih1pfDNUSD2Ehciz2ercCZuh6rI7unaN TztbUcCgTXZuzQvuQNyZYUrXxMQjNI8= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=lxP195rX; spf=none (imf30.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org; dmarc=none ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1674235993; a=rsa-sha256; cv=none; b=1h3XvmvoyrStywCGZfR6BbnZwYq0+WNq/veMlQF26Ua8T7U4xrzHPGQZAXFDioxOqO6+0f /eSm7Kmj0AupeYAVKW4KspWPBmAqqdnEU3qNBqbASLW3VAwLVimNVS4qFRd1dfBovhRYqY oxsahg+1lgb4KCTqHfrrZlanYXwFa9Y= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=2jkEwjOPyig34Xw2pt9DR8sA0x9KlwAW9Pppxnavdp0=; b=lxP195rXKeUf+jhr9nRw80er1N J2dgNGuhipPvd8ZmycUNsS6QhSFMWnbTFSR4+UK0chqH19q9nDIYqbgFi1isgl6IOZbxS+cQLhQFA lDn+sRScuzzCEYM2VpxSY0QdAvGnqyh8pG1eRaL5X9lFokCtki2EDBh8XyTs17oz3udjhQBwDZNqA cwcCA6sczrDp6V3seeFnEO3RtVzrDIioI2+YMX3FBSgovFmakKFMkVIgIPoGNcELTSEuxk+iAXl0I nXTkkv437nvT4sQ8Q3So4N66udy9Ji6Oe0B5GknD8lCpl46KLKb/aWSQYfUJ96FeBaC7fIK5qdcvF XMM/6C2w==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIvFm-002GbQ-8r; Fri, 20 Jan 2023 17:32:38 +0000 Date: Fri, 20 Jan 2023 17:32:38 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Cc: "Liam R. Howlett" , 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: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> <20230120170815.yuylbs27r6xcjpq5@revolver> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: D02268000A X-Stat-Signature: ormdzw9nk3bh417ps7xr853sy4gi3nph X-Rspam-User: X-HE-Tag: 1674235992-728582 X-HE-Meta: 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 TNHa5C9e MpgFWoU5rOw/1E58GlfKLh0RAPMRS4m96jH0Qx0m2+cpkU8uEjndlHEmwzf+07+7pq2NKnT+RxWb6ZzdDDkzByTbQbtT9v44WkUMh5CaLBH/ktdiS84g+R7xyIf71gc0jtTc54xIJUeX4LbxHNMmVLwi44qZtasKt2ClW/chqvDXr/Fu9ODmbxkiHUamkNt3emnIgOCQ2rbH2Nuf5kBQ6SZUH527qVFqmCsm7apeJDd833bZhdtN/gLKxpH/dAEVoZD5YdgNmKhUEW/ixkL0Kb7ndzSueeh/xT+74V1FwWWHpB01XKTgd1ZgzYw/CBMqvxTeVTx4VaDgHMOfGG1Z22b0J1LpdnBEoulzBiltL+/S+7+CvHV6bmaiV8g== 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 09:17:46AM -0800, Suren Baghdasaryan wrote: > On Fri, Jan 20, 2023 at 9:08 AM Liam R. Howlett wrote: > > > > * Matthew Wilcox [230120 11:50]: > > > 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(). > > > > 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. > > I think that would be still racy if the page fault handler found that > VMA under read-RCU protection but did not lock it yet (no locks are > held yet). If it's preempted, the VMA can be freed and destroyed from > under it without RCU grace period. The page fault handler (or whatever other reader -- ptrace, proc, etc) should have a refcount on the mm_struct, so we can't be in this path trying to free VMAs. Right?