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[71.184.117.43]) by smtp.gmail.com with ESMTPSA id q34sm637000qtk.32.2020.08.20.19.29.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 20 Aug 2020 19:29:59 -0700 (PDT) Date: Thu, 20 Aug 2020 22:29:57 -0400 From: Qian Cai To: akpm@linux-foundation.org, catalin.marinas@arm.com, oleg@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH] mm/kmemleak: rely on rcu for task stack scanning Message-ID: <20200821022955.GD4622@lca.pw> References: <20200820203902.11308-1-dave@stgolabs.net> <20200821002554.GB4622@lca.pw> <20200821012750.qxiklfhuaryajvhn@linux-p48b> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200821012750.qxiklfhuaryajvhn@linux-p48b> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 20, 2020 at 06:27:50PM -0700, Davidlohr Bueso wrote: > On Thu, 20 Aug 2020, Qian Cai wrote: > > > On Thu, Aug 20, 2020 at 01:39:02PM -0700, Davidlohr Bueso wrote: > > > kmemleak_scan() currently relies on the big tasklist_lock > > > hammer to stabilize iterating through the tasklist. Instead, > > > this patch proposes simply using rcu along with the rcu-safe > > > for_each_process_thread flavor (without changing scan semantics), > > > which doesn't make use of next_thread/p->thread_group and thus > > > cannot race with exit. Furthermore, any races with fork() > > > and not seeing the new child should be benign as it's not > > > running yet and can also be detected by the next scan. > > > > It is not entirely clear to me what problem the patch is trying to solve. If > > this is about performance, we will probably need some number. > > So in this case avoiding the tasklist_lock could prove beneficial for performance > considering the scan operation is done periodically. I have seen improvements > of 30%-ish when doing similar replacements on very pathological microbenchmarks > (ie stressing get/setpriority(2)). > > However my main motivation is that it's one less user of the global lock, > something that Linus has long time wanted to see gone eventually (if ever) > even if the traditional fairness issues has been dealt with now with qrwlocks. > Of course this is a very long ways ahead. This patch also kills another user > of the deprecated tsk->thread_group. This makes thing clearer. Reviewed-by: Qian Cai