From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751236AbeAPBqs (ORCPT + 1 other); Mon, 15 Jan 2018 20:46:48 -0500 Received: from mail-pf0-f170.google.com ([209.85.192.170]:46739 "EHLO mail-pf0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750826AbeAPBqq (ORCPT ); Mon, 15 Jan 2018 20:46:46 -0500 X-Google-Smtp-Source: ACJfBouoQC14pHq9XVUUJu3zcZE6K/GE05cqnH7hUcW15yYQBGQ27zZzyx1P4cknWr4BqK2PCZOIdw== Date: Tue, 16 Jan 2018 10:46:41 +0900 From: Sergey Senozhatsky To: Steven Rostedt Cc: Sergey Senozhatsky , Sergey Senozhatsky , Petr Mladek , Tejun Heo , 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@home.goodmis.org, Byungchul Park , Pavel Machek , linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup Message-ID: <20180116014641.GA6607@jagdpanzerIV> References: <20180111045817.GA494@jagdpanzerIV> <20180111093435.GA24497@linux.suse> <20180111103845.GB477@jagdpanzerIV> <20180111112908.50de440a@vmware.local.home> <20180112025612.GB6419@jagdpanzerIV> <20180111222140.7fd89d52@gandalf.local.home> <20180112100544.GA441@jagdpanzerIV> <20180112072123.33bb567d@gandalf.local.home> <20180113072834.GA1701@tigerII.localdomain> <20180115070637.1915ac20@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180115070637.1915ac20@gandalf.local.home> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On (01/15/18 07:06), Steven Rostedt wrote: > > > Yep, but I'm still not convinced you are seeing an issue with a single > > > printk. > > > > what do you mean by this? > > I'm not sure your issues happen because a single printk is locked up, > but you have many printks in one area. hm, need to think about it. > > > An OOM does not do everything in one printk, it calls hundreds. > > > Having hundreds of printks is an issue, especially in critical sections. > > > > unless your console_sem owner is preempted. as long as it is preempted > > it doesn't really matter how many times we call printk from which CPUs > > and from which sections, but what matters - who is going to print that all > > out when console_sem is running again and how much time will it take. > > that's what I'm saying. > > OK, if this is an issue, then we could do: > > preempt_disable(); > if (console_trylock_spinning()) > console_unlock(); > preempt_enable(); > > Which would prevent any printks from being preempted, but allow for > other console_lock owners to be so. yes, non-preemptible printk->console_unlock() is good for a number of reasons. [..] > > > > vprintk_emit() > > > > { > > > > > > > > console_trylock_spinning(void) > > > > { > > > > printk_safe_enter_irqsave(flags); > > > > while (READ_ONCE(console_waiter)) // spins as long as call_console_drivers() on other CPU > > > > cpu_relax(); > > > > printk_safe_exit_irqrestore(flags); > > > > ---> } > > > > | // preemptible up until printk_safe_enter_irqsave() in console_unlock() > > > > > > Again, this means the waiter is not in a critical section. Why do we > > > care? > > > > which is not what I was talking about. the point was that you said > > And would be fixed with the preempt_disable() I added above. yes. and it's, basically, very close to a revert of the commit I mentioned. [..] > > that is not true. we can have preemption "during" hand off. hand off, > > thus, is a "delayed approach", by definition. so if you consider the > > possibility of "if the machine were to crash in the transfer, we lost > > all that data" and if you consider this to be important [otherwise you > > wouldn't bring that up, would you] then the reality is that your patch > > has the same problem as printk_kthread. > > With the preempt_disable() there really isn't a delay. I agree, we > shouldn't let printk preempt (unless we have CONFIG_PREEMPT_RT enabled, > but that's another story). yes. > > so very schematically, for hand-off it's something like > > > > if (... console_trylock_spinning()) // grabbed the ownership > > > > << ... preempted ... >> > > > > console_unlock(); > > Which I think we should stop, with the preempt_disable(). yes. > > for printk_kthread it's something like > > > > wake_up_process(printk_kthread); > > up(console_sem); > > > > > > in the later case we at least have console_sem unlocked. so any other CPU > > that might do printk() can grab the lock and emit the logbuf messages. but > > in case on hand-off, we have console_sem locked, so no printk() will be > > able to emit the messages, we need that specific task to become running. > > > > > > hence the following: > > > > [..] > > > > reverting 6b97a20d3a7909daa06625d4440c2c52d7bf08d7 may be the right > > > > thing after all. > > > > this was cryptic and misleading. sorry. > > some clarifications. > > > > what I meant was that with 6b97a20d3a7909daa06625d4440c2c52d7bf08d7 > > I think I badly broke printk() [some of paths]. I know what I tried > > I think adding the preempt_disable() would fix printk() but let non > printk console_unlock() still preempt. yes. might be a bit risky, but can try. and yes, we still have console_lock() call sites, which can sleep under console_sem, so scheduler still can mess up with us, but that's a different story. agreed. > > to fix (and you don't have to explain to me what a lock up is) with > > that patch, but I don't think the patch ended up to be a clear win. > > a very simple explanation would be: > > > > instead of having a direct nonpreemptible path > > > > logbuf -> for(;;) call_console_drivers -> happy user > > > > we now have > > > > logbuf -> for(;;) { call_console_drivers, scheduler ... ???} -> happy user > > > > which is a big change. with a non-zero potential for regressions. > > and it didn't take long to find out that not all "happy users" were > > exactly happy with the new scheme of things. glance through Tetsuo's > > emails [see links in my another email], Tetsuo reported that printk can > > stall for minutes now. basically, the worse the system state is the lower > > printk throughput can be [down to zero chars in the worst case]. that's > > why I think that my patch was a mistake. and that's why in my out-of-tree > > patches I'm moving towards the non-preemptible path from logbuf through > > console to a happy user [just like it used to be]. but, obviously, I can't > > just restore preempt_disable()/preempt_enable() in vprintk_emit(). that's > > why I bound console_unlock() to watchdog threshold and move towards the > > batched non-preemptible print outs (enabling preemption and up()-ing the > > console_sem at the end of each print out batch). this is not super good, > > preemption is still here, but at least not after every line console_unlock() > > prints. up() console_sem also increases chances that, for instance, systemd > > or any other task that is sleeping in TASK_UNINTERRUPTIBLE on console_sem > > now has a chance to be woken up sooner (not only after we flush all pending > > logbuf messages and finally up() the console_sem). > > I rather try simpler approaches first (like adding the preempt_disable() > on top of my patch) than an elaborate scheme of printk_kthreads. ok, agreed. -ss From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 039226B0038 for ; Mon, 15 Jan 2018 20:46:48 -0500 (EST) Received: by mail-pf0-f199.google.com with SMTP id e185so10759402pfg.23 for ; Mon, 15 Jan 2018 17:46:47 -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 x20sor220296pfh.99.2018.01.15.17.46.46 for (Google Transport Security); Mon, 15 Jan 2018 17:46:46 -0800 (PST) Date: Tue, 16 Jan 2018 10:46:41 +0900 From: Sergey Senozhatsky Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup Message-ID: <20180116014641.GA6607@jagdpanzerIV> References: <20180111045817.GA494@jagdpanzerIV> <20180111093435.GA24497@linux.suse> <20180111103845.GB477@jagdpanzerIV> <20180111112908.50de440a@vmware.local.home> <20180112025612.GB6419@jagdpanzerIV> <20180111222140.7fd89d52@gandalf.local.home> <20180112100544.GA441@jagdpanzerIV> <20180112072123.33bb567d@gandalf.local.home> <20180113072834.GA1701@tigerII.localdomain> <20180115070637.1915ac20@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180115070637.1915ac20@gandalf.local.home> Sender: owner-linux-mm@kvack.org List-ID: To: Steven Rostedt Cc: Sergey Senozhatsky , Sergey Senozhatsky , Petr Mladek , Tejun Heo , 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@home.goodmis.org, Byungchul Park , Pavel Machek , linux-kernel@vger.kernel.org On (01/15/18 07:06), Steven Rostedt wrote: > > > Yep, but I'm still not convinced you are seeing an issue with a single > > > printk. > > > > what do you mean by this? > > I'm not sure your issues happen because a single printk is locked up, > but you have many printks in one area. hm, need to think about it. > > > An OOM does not do everything in one printk, it calls hundreds. > > > Having hundreds of printks is an issue, especially in critical sections. > > > > unless your console_sem owner is preempted. as long as it is preempted > > it doesn't really matter how many times we call printk from which CPUs > > and from which sections, but what matters - who is going to print that all > > out when console_sem is running again and how much time will it take. > > that's what I'm saying. > > OK, if this is an issue, then we could do: > > preempt_disable(); > if (console_trylock_spinning()) > console_unlock(); > preempt_enable(); > > Which would prevent any printks from being preempted, but allow for > other console_lock owners to be so. yes, non-preemptible printk->console_unlock() is good for a number of reasons. [..] > > > > vprintk_emit() > > > > { > > > > > > > > console_trylock_spinning(void) > > > > { > > > > printk_safe_enter_irqsave(flags); > > > > while (READ_ONCE(console_waiter)) // spins as long as call_console_drivers() on other CPU > > > > cpu_relax(); > > > > printk_safe_exit_irqrestore(flags); > > > > ---> } > > > > | // preemptible up until printk_safe_enter_irqsave() in console_unlock() > > > > > > Again, this means the waiter is not in a critical section. Why do we > > > care? > > > > which is not what I was talking about. the point was that you said > > And would be fixed with the preempt_disable() I added above. yes. and it's, basically, very close to a revert of the commit I mentioned. [..] > > that is not true. we can have preemption "during" hand off. hand off, > > thus, is a "delayed approach", by definition. so if you consider the > > possibility of "if the machine were to crash in the transfer, we lost > > all that data" and if you consider this to be important [otherwise you > > wouldn't bring that up, would you] then the reality is that your patch > > has the same problem as printk_kthread. > > With the preempt_disable() there really isn't a delay. I agree, we > shouldn't let printk preempt (unless we have CONFIG_PREEMPT_RT enabled, > but that's another story). yes. > > so very schematically, for hand-off it's something like > > > > if (... console_trylock_spinning()) // grabbed the ownership > > > > << ... preempted ... >> > > > > console_unlock(); > > Which I think we should stop, with the preempt_disable(). yes. > > for printk_kthread it's something like > > > > wake_up_process(printk_kthread); > > up(console_sem); > > > > > > in the later case we at least have console_sem unlocked. so any other CPU > > that might do printk() can grab the lock and emit the logbuf messages. but > > in case on hand-off, we have console_sem locked, so no printk() will be > > able to emit the messages, we need that specific task to become running. > > > > > > hence the following: > > > > [..] > > > > reverting 6b97a20d3a7909daa06625d4440c2c52d7bf08d7 may be the right > > > > thing after all. > > > > this was cryptic and misleading. sorry. > > some clarifications. > > > > what I meant was that with 6b97a20d3a7909daa06625d4440c2c52d7bf08d7 > > I think I badly broke printk() [some of paths]. I know what I tried > > I think adding the preempt_disable() would fix printk() but let non > printk console_unlock() still preempt. yes. might be a bit risky, but can try. and yes, we still have console_lock() call sites, which can sleep under console_sem, so scheduler still can mess up with us, but that's a different story. agreed. > > to fix (and you don't have to explain to me what a lock up is) with > > that patch, but I don't think the patch ended up to be a clear win. > > a very simple explanation would be: > > > > instead of having a direct nonpreemptible path > > > > logbuf -> for(;;) call_console_drivers -> happy user > > > > we now have > > > > logbuf -> for(;;) { call_console_drivers, scheduler ... ???} -> happy user > > > > which is a big change. with a non-zero potential for regressions. > > and it didn't take long to find out that not all "happy users" were > > exactly happy with the new scheme of things. glance through Tetsuo's > > emails [see links in my another email], Tetsuo reported that printk can > > stall for minutes now. basically, the worse the system state is the lower > > printk throughput can be [down to zero chars in the worst case]. that's > > why I think that my patch was a mistake. and that's why in my out-of-tree > > patches I'm moving towards the non-preemptible path from logbuf through > > console to a happy user [just like it used to be]. but, obviously, I can't > > just restore preempt_disable()/preempt_enable() in vprintk_emit(). that's > > why I bound console_unlock() to watchdog threshold and move towards the > > batched non-preemptible print outs (enabling preemption and up()-ing the > > console_sem at the end of each print out batch). this is not super good, > > preemption is still here, but at least not after every line console_unlock() > > prints. up() console_sem also increases chances that, for instance, systemd > > or any other task that is sleeping in TASK_UNINTERRUPTIBLE on console_sem > > now has a chance to be woken up sooner (not only after we flush all pending > > logbuf messages and finally up() the console_sem). > > I rather try simpler approaches first (like adding the preempt_disable() > on top of my patch) than an elaborate scheme of printk_kthreads. ok, agreed. -ss -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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