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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 93D60C05027 for ; Fri, 20 Jan 2023 17:34:13 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4Nz6780kG9z3fJm for ; Sat, 21 Jan 2023 04:34:12 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=lxP195rX; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=none (no SPF record) smtp.mailfrom=infradead.org (client-ip=2001:8b0:10b:1236::1; helo=casper.infradead.org; envelope-from=willy@infradead.org; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=lxP195rX; dkim-atps=neutral Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4Nz66648zcz2yxB for ; Sat, 21 Jan 2023 04:33:18 +1100 (AEDT) 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 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-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: michel@lespinasse.org, joelaf@google.com, songliubraving@fb.com, Michal Hocko , leewalsh@google.com, david@redhat.com, peterz@infradead.org, bigeasy@linutronix.de, peterx@redhat.com, dhowells@redhat.com, linux-mm@kvack.org, edumazet@google.com, jglisse@google.com, punit.agrawal@bytedance.com, arjunroy@google.com, dave@stgolabs.net, minchan@google.com, x86@kernel.org, hughd@google.com, gurua@google.com, laurent.dufour@fr.ibm.com, linux-arm-kernel@lists.infradead.org, rientjes@google.com, axelrasmussen@google.com, kernel-team@android.com, soheil@google.com, paulmck@kernel.org, jannh@google.com, "Liam R. Howlett" , shakeelb@google.com, luto@kernel.org, gthelen@google.com, ldufour@linux.ibm.com, vbabka@suse.cz, posk@google.com, lstoakes@gmail.com, peterjung1337@gmail.com, linuxppc-dev@lists.ozlabs.org, kent.overstreet@linux.dev, hughlynch@google.com, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, tatashin@google. com, mgorman@techsingularity.net Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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?