From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752791AbeARCUA (ORCPT ); Wed, 17 Jan 2018 21:20:00 -0500 Received: from mail.kernel.org ([198.145.29.99]:55504 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750931AbeARCT7 (ORCPT ); Wed, 17 Jan 2018 21:19:59 -0500 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 400A02175B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=goodmis.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=rostedt@goodmis.org Date: Wed, 17 Jan 2018 21:19:53 -0500 From: Steven Rostedt To: Byungchul Park Cc: Petr Mladek , Sergey Senozhatsky , akpm@linux-foundation.org, linux-mm@kvack.org, Cong Wang , Dave Hansen , Johannes Weiner , Mel Gorman , Michal Hocko , Vlastimil Babka , Peter Zijlstra , Linus Torvalds , Jan Kara , Mathieu Desnoyers , Tetsuo Handa , rostedt@rostedt.homelinux.com, Sergey Senozhatsky , Tejun Heo , Pavel Machek , linux-kernel@vger.kernel.org, kernel-team@lge.com Subject: Re: [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes Message-ID: <20180117211953.2403d189@vmware.local.home> In-Reply-To: <4a24ce1d-a606-3add-ec30-91ce9a1a1281@lge.com> References: <20180110132418.7080-1-pmladek@suse.com> <20180110132418.7080-2-pmladek@suse.com> <20180117120446.44ewafav7epaibde@pathway.suse.cz> <4a24ce1d-a606-3add-ec30-91ce9a1a1281@lge.com> X-Mailer: Claws Mail 3.15.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 18 Jan 2018 10:53:37 +0900 Byungchul Park wrote: > Hello, > > This is a thing simulating a wait for an event e.g. > wait_for_completion() doing spinning instead of sleep, rather > than a spinlock. I mean: > > This context > ------------ > while (READ_ONCE(console_waiter)) /* Wait for the event */ > cpu_relax(); > > Another context > --------------- > WRITE_ONCE(console_waiter, false); /* Event */ I disagree. It is like a spinlock. You can say a spinlock() that is blocked is also waiting for an event. That event being the owner does a spin_unlock(). > > That's why I said this's the exact case of cross-release. Anyway > without cross-release, we usually use typical acquire/release > pairs to cover a wait for an event in the following way: > > A context > --------- > lock_map_acquire(wait); /* Or lock_map_acquire_read(wait) */ > /* Read one is better though.. */ > > /* A section, we suspect, a wait for an event might happen. */ > ... > lock_map_release(wait); > > > The place actually doing the wait > --------------------------------- > lock_map_acquire(wait); > lock_map_acquire(wait); > > wait_for_event(wait); /* Actually do the wait */ > > You can see a simple example of how to use them by searching > kernel/cpu.c with "lock_acquire" and "wait_for_completion". > > However, as I said, if you suspect that cpu_relax() includes > the wait, then it's ok to leave it. Otherwise, I think it > would be better to change it in the way I showed you above. I find your way confusing. I'm simulating a spinlock not a wait for completion. A wait for completion usually initiates something then waits for it to complete. This is trying to get into a critical area but another task is currently in it. It's simulating a spinlock as far as I can see. -- Steve