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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Alexander Potapenko <glider@google.com>,
	Dmitriy Vyukov <dvyukov@google.com>,
	kbuild test robot <fengguang.wu@intel.com>,
	syzkaller <syzkaller@googlegroups.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] printk: inject caller information into the body of message
Date: Sat, 29 Sep 2018 20:15:14 +0900
Message-ID: <91efcff8-dc6d-b7b4-9ac8-2f3882289f95@i-love.sakura.ne.jp> (raw)
In-Reply-To: <20180929105151.GA1392@tigerII.localdomain>

On 2018/09/29 19:51, Sergey Senozhatsky wrote:
> On (09/28/18 20:01), Tetsuo Handa wrote:
>>> Yes, this makes sense. At the same time we can keep pr_line buffer
>>> in .bss
>>>
>>> 	static char buffer[1024];
>>> 	static DEFINE_PR_LINE_BUF(..., buffer);
>>>
>>> just like you have already mentioned. But that's going to require a
>>> case-by-case handling; so a big list of printk buffers is a simpler
>>> option. Fallback, tho, can be painful. On a system with 1024 CPUs can
>>> one have more than 16 concurrent cont printks? If the answer is yes,
>>> then we are looking at the same broken cont output as before.
>>
>> I'm OK with making "16" configurable (at kernel configuration and/or
>> at kernel boot like log_buf_len= kernel command line parameter).
> 
> Do we really want this? Why .bss placement doesn't work for you?
> 
> 	void oom(...)
> 	{
> 		static DEFINE_PR_LINE(KERN_ERR, pr);
> 
> 		pr_line(&pr, ....);
> 		pr_line(&pr, "\n");
> 	}
> 
> the underlying buffer will be static; the pr_line will get re-init
> (offset = 0) every time we call the function, which is OK. And we can
> pass &pr to any function oom() invokes. What am I missing?

Because there is no guarantee that memory information is dumped under the
oom_lock mutex. The oom_lock is held when calling out_of_memory(), and it
cannot be held when reporting GFP_ATOMIC memory allocation failures.

> 
>> We could even allow each "struct task_struct" to have corresponding
>> "struct printk_buffer".
> 
> Tetsuo, realistically, we can't. Sorry. No one will let us to have a printk
> buffer on per-task_struct basis. Even if someone will let us to do this,
> a miracle, a single per-task_struct buffer won't work. Because, then
> someone will discover that a very simple API
> 
> 	buffered_printk(current->printk_buffer, "......");
> 
> does not work if buffered_printk() gets interrupted by IRQ, etc. in case
> if that new context also does
> 
> 	buffered_printk(current->printk_buffer, "......");
> 
> So then we will have per-context per-task_struct printk buffer: for task,
> for exceptions, for softirq, for hardirq, for NMI, etc. This is not worth
> it.

The number of "struct task_struct" instances is volatile. But number of non
"struct task_struct" contexts is finite which can be determined at boot (or
initialization) time.

My intention is that allocate "struct printk_buffer" when "struct task_struct"
is created (i.e. upon dup_task_struct()) and release "struct printk_buffer"
when "struct task_struct" is destroyed (i.e. upon free_task_struct()), and
allocate "struct printk_buffer" for non "struct task_struct" contexts when a
CPU is onlined and release "struct printk_buffer" for non "struct task_struct"
contexts when a CPU is offlined. Then, it will be guaranteed that there is
enough "struct printk_buffer" for any callers.

> 
> Let's just have a very simple seq_buf based pr_line API. No config options,
> no command line arguments - heap, bss or stack for buffer placement. Or even
> simpler.

We cannot avoid "** %u printk messages dropped **\n" inside printk() upon out
of space. But I don't want line buffered printk() API to truncate upon out of
space for line buffered printk() API. I want buffered printk() API to flush
incomplete line even if it resulted in printed in multiple lines.
Injecting caller information can mitigate "printed in multiple lines" case.


  reply index

Thread overview: 94+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <201804232233.CIC65675.OJSOMFQOFFHVtL@I-love.SAKURA.ne.jp>
     [not found] ` <CACT4Y+boyw_Qy=y-iTnsKZrtTgF0Hk3nHN_xtqUdX4etgiYDQw@mail.gmail.com>
2018-04-24  1:33   ` printk feature for syzbot? Sergey Senozhatsky
2018-04-24 14:40     ` Steven Rostedt
2018-04-26 10:06     ` Petr Mladek
2018-05-10  4:22       ` Sergey Senozhatsky
2018-05-10 11:30         ` Petr Mladek
2018-05-10 12:11           ` Sergey Senozhatsky
2018-05-10 14:22             ` Steven Rostedt
2018-05-10 14:50         ` Tetsuo Handa
2018-05-11  1:45           ` Sergey Senozhatsky
     [not found]             ` <201805110238.w4B2cIGH079602@www262.sakura.ne.jp>
2018-05-11  6:21               ` Sergey Senozhatsky
2018-05-11  9:17                 ` Dmitry Vyukov
2018-05-11  9:50                   ` Sergey Senozhatsky
2018-05-11 11:58                     ` [PATCH] printk: inject caller information into the body of message Tetsuo Handa
2018-05-17 11:21                       ` Sergey Senozhatsky
2018-05-17 11:52                         ` Sergey Senozhatsky
2018-05-18 12:15                         ` Petr Mladek
2018-05-18 12:25                           ` Dmitry Vyukov
2018-05-18 12:54                             ` Petr Mladek
2018-05-18 13:08                               ` Dmitry Vyukov
2018-05-24  2:21                                 ` Sergey Senozhatsky
2018-05-23 10:19                           ` Tetsuo Handa
2018-05-24  2:14                           ` Sergey Senozhatsky
2018-05-26  6:36                             ` Dmitry Vyukov
2018-06-20  5:44                               ` Dmitry Vyukov
2018-06-20  8:31                                 ` Sergey Senozhatsky
2018-06-20  8:45                                   ` Dmitry Vyukov
2018-06-20  9:06                                     ` Sergey Senozhatsky
2018-06-20  9:18                                       ` Sergey Senozhatsky
2018-06-20  9:31                                         ` Dmitry Vyukov
2018-06-20 11:07                                           ` Sergey Senozhatsky
2018-06-20 11:32                                             ` Dmitry Vyukov
2018-06-20 13:06                                               ` Sergey Senozhatsky
2018-06-22 13:06                                                 ` Tetsuo Handa
2018-06-25  1:41                                                   ` Sergey Senozhatsky
2018-06-25  9:36                                                     ` Dmitry Vyukov
2018-06-27 10:29                                                       ` Tetsuo Handa
2018-09-10 11:20                                                 ` Alexander Potapenko
2018-09-12  6:53                                                   ` Sergey Senozhatsky
2018-09-12 16:05                                                     ` Steven Rostedt
2018-09-13  7:12                                                       ` Sergey Senozhatsky
2018-09-13 12:26                                                         ` Petr Mladek
2018-09-13 14:28                                                           ` Sergey Senozhatsky
2018-09-14  1:22                                                             ` Steven Rostedt
2018-09-14  2:15                                                               ` Sergey Senozhatsky
2018-09-14  6:57                                                             ` Sergey Senozhatsky
2018-09-14 10:37                                                               ` Tetsuo Handa
2018-09-14 11:50                                                                 ` Sergey Senozhatsky
2018-09-14 12:03                                                                   ` Tetsuo Handa
2018-09-14 12:22                                                                     ` Sergey Senozhatsky
2018-09-19 11:02                                                                       ` Tetsuo Handa
2018-09-24  8:11                                                                         ` Tetsuo Handa
2018-09-27 16:10                                                                           ` Tetsuo Handa
2018-09-28  9:02                                                                             ` Sergey Senozhatsky
2018-09-28  9:09                                                                           ` Sergey Senozhatsky
2018-09-28 11:01                                                                             ` Tetsuo Handa
2018-09-29 10:51                                                                               ` Sergey Senozhatsky
2018-09-29 11:15                                                                                 ` Tetsuo Handa [this message]
2018-10-01  2:37                                                                                   ` Sergey Senozhatsky
2018-10-01  2:58                                                                                     ` Sergey Senozhatsky
2018-10-01 11:21                                                                                     ` Tetsuo Handa
2018-10-02  6:38                                                                                       ` Sergey Senozhatsky
2018-10-08 10:31                                                                                         ` Tetsuo Handa
2018-10-08 16:03                                                                                           ` Petr Mladek
2018-10-08 20:48                                                                                             ` Tetsuo Handa
2018-10-09 14:52                                                                                               ` Petr Mladek
2018-10-09 21:19                                                                                                 ` Tetsuo Handa
2018-10-10 10:14                                                                                                   ` Tetsuo Handa
2018-10-11 10:20                                                                                                     ` Tetsuo Handa
2018-10-11 13:47                                                                                                       ` Steven Rostedt
2018-10-08 15:43                                                                                         ` Petr Mladek
2018-09-28  8:56                                                                         ` Sergey Senozhatsky
2018-09-28 11:21                                                                           ` Tetsuo Handa
2018-09-29 11:13                                                                             ` Sergey Senozhatsky
2018-09-29 11:39                                                                               ` Tetsuo Handa
2018-10-01  5:52                                                                               ` Sergey Senozhatsky
2018-10-01  8:37                                                                                 ` Sergey Senozhatsky
2018-10-01 18:06                                                                               ` Steven Rostedt
2018-09-14  1:12                                                         ` Steven Rostedt
2018-09-14  1:55                                                           ` Sergey Senozhatsky
2018-06-21  8:29                                               ` Sergey Senozhatsky
2018-06-20  9:30                                       ` Dmitry Vyukov
2018-06-20 11:19                                         ` Sergey Senozhatsky
2018-06-20 11:25                                           ` Dmitry Vyukov
2018-06-20 11:37                                         ` Fengguang Wu
2018-06-20 12:31                                           ` Dmitry Vyukov
2018-06-20 12:41                                             ` Fengguang Wu
2018-06-20 12:45                                               ` Dmitry Vyukov
2018-06-20 12:48                                                 ` Fengguang Wu
2018-05-11 13:37                     ` printk feature for syzbot? Steven Rostedt
2018-05-15  5:20                       ` Sergey Senozhatsky
2018-05-15 14:39                         ` Steven Rostedt
2018-05-11 11:02                 ` [PATCH] printk: fix possible reuse of va_list variable Tetsuo Handa
2018-05-11 11:27                   ` Sergey Senozhatsky
2018-05-17 11:57                   ` Petr Mladek

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