From: Steven Rostedt <rostedt@goodmis.org> To: Vlastimil Babka <vbabka@suse.cz> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Cong Wang <xiyou.wangcong@gmail.com>, Dave Hansen <dave.hansen@intel.com>, Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>, Petr Mladek <pmladek@suse.com>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, "yuwang.yuwang" <yuwang.yuwang@alibaba-inc.com> Subject: Re: [PATCH] mm: don't warn about allocations which stall for too long Date: Wed, 1 Nov 2017 13:54:09 -0400 [thread overview] Message-ID: <20171101135409.0190afb1@gandalf.local.home> (raw) In-Reply-To: <40ed01d3-1475-cd4a-0dff-f7a6ee24d5e9@suse.cz> On Wed, 1 Nov 2017 18:42:25 +0100 Vlastimil Babka <vbabka@suse.cz> wrote: > On 11/01/2017 04:33 PM, Steven Rostedt wrote: > > On Wed, 1 Nov 2017 09:30:05 +0100 > > Vlastimil Babka <vbabka@suse.cz> wrote: > > > >> > >> But still, it seems to me that the scheme only works as long as there > >> are printk()'s coming with some reasonable frequency. There's still a > >> corner case when a storm of printk()'s can come that will fill the ring > >> buffers, and while during the storm the printing will be distributed > >> between CPUs nicely, the last unfortunate CPU after the storm subsides > >> will be left with a large accumulated buffer to print, and there will be > >> no waiters to take over if there are no more printk()'s coming. What > >> then, should it detect such situation and defer the flushing? > > > > No! > > > > If such a case happened, that means the system is doing something > > really stupid. > > Hm, what about e.g. a soft lockup that triggers backtraces from all > CPU's? Yes, having softlockups is "stupid" but sometimes they do happen > and the system still recovers (just some looping operation is missing > cond_resched() and took longer than expected). It would be sad if it > didn't recover because of a printk() issue... I still think such a case would not be huge for the last printer. > > > Btw, each printk that takes over, does one message, so the last one to > > take over, shouldn't have a full buffer anyway. > > There might be multiple messages per each CPU, e.g. the softlockup > backtraces. And each one does multiple printks, still spreading the love around. > > > But still, if you have such a hypothetical situation, the system should > > just crash. The printk is still bounded by the length of the buffer. > > Although it is slow, it will finish. > > Finish, but with single CPU doing the printing, which is wrong? I don't think so. This is all hypothetical anyway. I need to implement my solution, and then lets see if this can actually happen. > > > Which is not the case with the > > current situation. And the current situation (as which this patch > > demonstrates) does happen today and is not hypothetical. > > Yep, so ideally it can be fixed without corner cases :) If there is any corner cases. I guess the test would be to trigger a soft lockup on all CPUs to print out a dump at the same time. But then again, how is a soft lockup on all CPUs not any worse than a single CPU finishing up the buffer output? -- Steve
WARNING: multiple messages have this Message-ID (diff)
From: Steven Rostedt <rostedt@goodmis.org> To: Vlastimil Babka <vbabka@suse.cz> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Cong Wang <xiyou.wangcong@gmail.com>, Dave Hansen <dave.hansen@intel.com>, Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>, Petr Mladek <pmladek@suse.com>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, "yuwang.yuwang" <yuwang.yuwang@alibaba-inc.com> Subject: Re: [PATCH] mm: don't warn about allocations which stall for too long Date: Wed, 1 Nov 2017 13:54:09 -0400 [thread overview] Message-ID: <20171101135409.0190afb1@gandalf.local.home> (raw) In-Reply-To: <40ed01d3-1475-cd4a-0dff-f7a6ee24d5e9@suse.cz> On Wed, 1 Nov 2017 18:42:25 +0100 Vlastimil Babka <vbabka@suse.cz> wrote: > On 11/01/2017 04:33 PM, Steven Rostedt wrote: > > On Wed, 1 Nov 2017 09:30:05 +0100 > > Vlastimil Babka <vbabka@suse.cz> wrote: > > > >> > >> But still, it seems to me that the scheme only works as long as there > >> are printk()'s coming with some reasonable frequency. There's still a > >> corner case when a storm of printk()'s can come that will fill the ring > >> buffers, and while during the storm the printing will be distributed > >> between CPUs nicely, the last unfortunate CPU after the storm subsides > >> will be left with a large accumulated buffer to print, and there will be > >> no waiters to take over if there are no more printk()'s coming. What > >> then, should it detect such situation and defer the flushing? > > > > No! > > > > If such a case happened, that means the system is doing something > > really stupid. > > Hm, what about e.g. a soft lockup that triggers backtraces from all > CPU's? Yes, having softlockups is "stupid" but sometimes they do happen > and the system still recovers (just some looping operation is missing > cond_resched() and took longer than expected). It would be sad if it > didn't recover because of a printk() issue... I still think such a case would not be huge for the last printer. > > > Btw, each printk that takes over, does one message, so the last one to > > take over, shouldn't have a full buffer anyway. > > There might be multiple messages per each CPU, e.g. the softlockup > backtraces. And each one does multiple printks, still spreading the love around. > > > But still, if you have such a hypothetical situation, the system should > > just crash. The printk is still bounded by the length of the buffer. > > Although it is slow, it will finish. > > Finish, but with single CPU doing the printing, which is wrong? I don't think so. This is all hypothetical anyway. I need to implement my solution, and then lets see if this can actually happen. > > > Which is not the case with the > > current situation. And the current situation (as which this patch > > demonstrates) does happen today and is not hypothetical. > > Yep, so ideally it can be fixed without corner cases :) If there is any corner cases. I guess the test would be to trigger a soft lockup on all CPUs to print out a dump at the same time. But then again, how is a soft lockup on all CPUs not any worse than a single CPU finishing up the buffer output? -- Steve -- 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: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-11-01 17:54 UTC|newest] Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-10-26 11:28 [PATCH] mm: don't warn about allocations which stall for too long Tetsuo Handa 2017-10-26 11:28 ` Tetsuo Handa 2017-10-26 11:41 ` Michal Hocko 2017-10-26 11:41 ` Michal Hocko 2017-11-08 10:30 ` peter enderborg 2017-11-09 8:52 ` Michal Hocko 2017-11-09 8:52 ` Michal Hocko 2017-11-09 9:34 ` peter enderborg 2017-11-09 10:09 ` Michal Hocko 2017-11-09 10:09 ` Michal Hocko 2017-11-09 10:19 ` Tetsuo Handa 2017-11-09 10:19 ` Tetsuo Handa 2017-10-26 14:37 ` Johannes Weiner 2017-10-26 14:37 ` Johannes Weiner 2017-10-31 19:32 ` Steven Rostedt 2017-11-01 8:30 ` Vlastimil Babka 2017-11-01 8:30 ` Vlastimil Babka 2017-11-01 13:38 ` Petr Mladek 2017-11-01 13:38 ` Petr Mladek 2017-11-01 15:36 ` Steven Rostedt 2017-11-01 15:36 ` Steven Rostedt 2017-11-02 11:46 ` Petr Mladek 2017-11-02 11:46 ` Petr Mladek 2017-11-02 14:49 ` Steven Rostedt 2017-11-02 14:49 ` Steven Rostedt 2017-11-01 15:33 ` Steven Rostedt 2017-11-01 15:33 ` Steven Rostedt 2017-11-01 17:42 ` Vlastimil Babka 2017-11-01 17:42 ` Vlastimil Babka 2017-11-01 17:54 ` Steven Rostedt [this message] 2017-11-01 17:54 ` Steven Rostedt 2017-11-02 8:53 ` Sergey Senozhatsky 2017-11-02 8:53 ` Sergey Senozhatsky 2017-11-02 9:14 ` Sergey Senozhatsky 2017-11-02 9:14 ` Sergey Senozhatsky 2017-11-02 14:55 ` Steven Rostedt 2017-11-02 14:55 ` Steven Rostedt 2017-11-02 12:55 ` Michal Hocko 2017-11-02 12:55 ` Michal Hocko 2017-11-02 15:56 ` Steven Rostedt 2017-11-02 15:56 ` Steven Rostedt 2017-11-02 17:06 ` [PATCH v2] printk: Add console owner and waiter logic to load balance console writes Steven Rostedt 2017-11-02 17:06 ` Steven Rostedt 2017-11-02 17:10 ` Steven Rostedt 2017-11-02 17:10 ` Steven Rostedt 2017-11-02 17:38 ` Steven Rostedt 2017-11-02 17:38 ` Steven Rostedt 2017-11-03 10:19 ` Jan Kara 2017-11-03 10:19 ` Jan Kara 2017-11-03 11:18 ` Steven Rostedt 2017-11-03 11:18 ` Steven Rostedt
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