From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f70.google.com (mail-pg0-f70.google.com [74.125.83.70]) by kanga.kvack.org (Postfix) with ESMTP id 006A0440460 for ; Wed, 8 Nov 2017 23:45:53 -0500 (EST) Received: by mail-pg0-f70.google.com with SMTP id r18so4973702pgu.9 for ; Wed, 08 Nov 2017 20:45:53 -0800 (PST) Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id p4sor1565306pgc.6.2017.11.08.20.45.52 for (Google Transport Security); Wed, 08 Nov 2017 20:45:52 -0800 (PST) Date: Thu, 9 Nov 2017 13:45:48 +0900 From: Sergey Senozhatsky Subject: Re: [PATCH v3] printk: Add console owner and waiter logic to load balance console writes Message-ID: <20171109044548.GC775@jagdpanzerIV> References: <20171102134515.6eef16de@gandalf.local.home> <201711062106.ADI34320.JFtOFFHOOQVLSM@I-love.SAKURA.ne.jp> <20171107014015.GA1822@jagdpanzerIV> <20171108051955.GA468@jagdpanzerIV> <20171108092951.4d677bca@gandalf.local.home> <20171109005635.GA775@jagdpanzerIV> <20171108222905.426fc73a@vmware.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171108222905.426fc73a@vmware.local.home> Sender: owner-linux-mm@kvack.org List-ID: To: Steven Rostedt Cc: Sergey Senozhatsky , Tejun Heo , Tetsuo Handa , linux-kernel@vger.kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org, xiyou.wangcong@gmail.com, dave.hansen@intel.com, hannes@cmpxchg.org, mgorman@suse.de, mhocko@kernel.org, pmladek@suse.com, sergey.senozhatsky@gmail.com, vbabka@suse.cz On (11/08/17 22:29), Steven Rostedt wrote: > > On (11/08/17 09:29), Steven Rostedt wrote: > > > On Wed, 8 Nov 2017 14:19:55 +0900 > > > Sergey Senozhatsky wrote: > > > > > > > the change goes further. I did express some of my concerns during the KS, > > > > I'll just bring them to the list. > > > > > > > > > > > > we now always shift printing from a save - scheduleable - context to > > > > a potentially unsafe one - atomic. by example: > > > > > > And vice versa. We are now likely to go from a unscheduleable context > > > to a schedule one, where before, that didn't exist. > > > > the existence of "and vice versa" is kinda alarming, isn't it? it's sort > > of "yes, we can break some things, but we also can improve some things." > > Not really. Because the heuristic is that what calls printk will do the > printk. so what we are looking at a) we take over printing. can be from safe context to unsafe context [well, bad karma]. can be from unsafe context to a safe one. or from safe context to another safe context... or from one unsafe context to another unsafe context [bad karma again]. we really never know, no one does. lots of uncertainties - "may be X, may be Y, may be Z". a bigger picture: we still can have the same lockup scenarios as we do have today. and we also bring busy loop with us, so the new console_sem owner [regardless its current context] CPU must wait until the current console_sem finishes its call_console_drivers(). I mentioned it in my another email, you seemed to jump over that part. was it irrelevant or wrong? vs. b) we offload to printk_kthread [safe context]. why (a) is better than (b)? -ss -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org