From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Dmitriy Vyukov <dvyukov@google.com>,
Steven Rostedt <rostedt@goodmis.org>,
Alexander Potapenko <glider@google.com>,
Fengguang Wu <fengguang.wu@intel.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.
Date: Wed, 7 Nov 2018 11:21:54 +0100 [thread overview]
Message-ID: <20181107102154.pobr7yrl5il76be6@pathway.suse.cz> (raw)
In-Reply-To: <20181106143502.GA32748@tigerII.localdomain>
On Tue 2018-11-06 23:35:02, Sergey Senozhatsky wrote:
> On (11/02/18 22:31), Tetsuo Handa wrote:
> > (1) Call get_printk_buffer() and acquire "struct printk_buffer *".
> >
> > (2) Rewrite printk() calls in the following way. The "ptr" is
> > "struct printk_buffer *" obtained in step (1).
> >
> > printk(fmt, ...) => printk_buffered(ptr, fmt, ...)
> > vprintk(fmt, args) => vprintk_buffered(ptr, fmt, args)
> > pr_emerg(fmt, ...) => bpr_emerg(ptr, fmt, ...)
> > pr_alert(fmt, ...) => bpr_alert(ptr, fmt, ...)
> > pr_crit(fmt, ...) => bpr_crit(ptr, fmt, ...)
> > pr_err(fmt, ...) => bpr_err(ptr, fmt, ...)
> > pr_warning(fmt, ...) => bpr_warning(ptr, fmt, ...)
> > pr_warn(fmt, ...) => bpr_warn(ptr, fmt, ...)
> > pr_notice(fmt, ...) => bpr_notice(ptr, fmt, ...)
> > pr_info(fmt, ...) => bpr_info(ptr, fmt, ...)
> > pr_cont(fmt, ...) => bpr_cont(ptr, fmt, ...)
> >
> > (3) Release "struct printk_buffer" by calling put_printk_buffer().
>
> [..]
>
> > Since we want to remove "struct cont" eventually, we will try to remove
> > both "implicit printk() users who are expecting KERN_CONT behavior" and
> > "explicit pr_cont()/printk(KERN_CONT) users". Therefore, converting to
> > this API is recommended.
>
> - The printk-fallback sounds like a hint that the existing 'cont' handling
> better stay in the kernel. I don't see how the existing 'cont' is
> significantly worse than
> bpr_warn(NULL, ...)->printk() // no 'cont' support
> I don't see why would we want to do it, sorry. I don't see "it takes 16
> printk-buffers to make a thing go right" as a sure thing.
I see it the following way:
+ mixed cont lines are very rare but they happen
+ 16 buffers are more than 1 so it could only be better [*]
+ the printk_buffer() code is self-contained and does not
complicate the logic of the classic printk() code [**]
[*] A missing put_printk_buffer() might cause that we would get
out of buffers. But the same problem is with locks,
disabled preemption, disabled interrupts, seq_buffer,
alloc/free. Such problems happen but they are rare.
Also I do not expect that the same buffer would be shared
between many functions. Therefore it should be easy
to use it correctly.
[**] I admit that cont buffer implementation is much easier
after removing the early flush to consoles but still...
Anyway, I do not think that both implementations are worth it.
We could keep both for some transition period but we should
remove the old one later.
> A question.
>
> How bad would it actually be to:
>
> - Allocate seq_buf 512-bytes buffer (GFP_ATOMIC) just-in-time, when we
> need it.
> // How often systems cannot allocate a 512-byte buffer? //
>
> - OK, assuming that systems around the world are so badly OOM like all the
> time and even kmalloc(512) is absolutely impossible, then have a fallback
> to the existing 'cont' handling; it just looks to me better than a plain
> printk()-fallback with removed 'cont' support.
This would prevent removing the fallback to struct cont. OOM is
one important scenario where continuous lines are used.
> - Do not allocate seq_buf if we are in printk-safe or in printk-nmi mode.
> To avoid "buffering for the sake of buffering". IOW, when in printk-safe
> use printk-safe.
Sure, my plan is to add a helper function is_buffered_printk_context() or so
that would check printk_context. Then we could do the following in
vprintk_buffered()
if (is_buffered_printk_context())
vprintk_func(....);
It might be added on top of the current patchset. I opened this
problem once and it got lost. So I did not want to complicate
it at this moment.
Best Regards,
Petr
next prev parent reply other threads:[~2018-11-07 10:22 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-02 13:31 [PATCH v6 1/3] printk: Add line-buffered printk() API Tetsuo Handa
2018-11-02 13:31 ` [PATCH 2/3] mm: Use line-buffered printk() for show_free_areas() Tetsuo Handa
2018-11-07 14:07 ` Petr Mladek
2018-11-02 13:31 ` [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages Tetsuo Handa
2018-11-02 13:36 ` Peter Zijlstra
2018-11-03 2:00 ` Tetsuo Handa
2018-11-06 8:38 ` Petr Mladek
2018-11-06 9:05 ` Sergey Senozhatsky
2018-11-06 12:57 ` Petr Mladek
2018-11-06 9:56 ` Tetsuo Handa
2018-11-07 15:19 ` Petr Mladek
2018-11-08 4:45 ` Sergey Senozhatsky
2018-11-08 11:37 ` Tetsuo Handa
2018-11-09 6:12 ` Sergey Senozhatsky
2018-11-09 9:55 ` Tetsuo Handa
2018-11-09 15:43 ` Petr Mladek
2018-11-10 2:42 ` Tetsuo Handa
2018-11-23 12:46 ` Petr Mladek
2018-11-23 13:12 ` Tetsuo Handa
2018-11-23 15:56 ` Steven Rostedt
2018-11-24 0:24 ` Tetsuo Handa
2018-11-26 4:34 ` Sergey Senozhatsky
2018-11-28 13:29 ` David Laight
2018-11-29 10:09 ` Tetsuo Handa
2018-11-30 16:01 ` Petr Mladek
2018-11-10 8:52 ` Tetsuo Handa
2018-11-23 12:52 ` Petr Mladek
2018-11-09 14:08 ` Linus Torvalds
2018-11-09 14:42 ` Steven Rostedt
2018-11-08 11:53 ` Petr Mladek
2018-11-08 12:44 ` Sergey Senozhatsky
2018-11-08 14:21 ` Sergey Senozhatsky
2018-11-09 9:54 ` Tetsuo Handa
2018-11-02 14:40 ` [PATCH v6 1/3] printk: Add line-buffered printk() API Matthew Wilcox
2018-11-03 1:55 ` Tetsuo Handa
2018-11-02 18:12 ` kbuild test robot
2018-11-06 14:35 ` Sergey Senozhatsky
2018-11-07 10:21 ` Petr Mladek [this message]
2018-11-07 12:54 ` Tetsuo Handa
2018-11-08 2:21 ` Sergey Senozhatsky
2018-11-08 11:24 ` Petr Mladek
2018-11-08 11:46 ` Tetsuo Handa
2018-11-08 12:30 ` Sergey Senozhatsky
2018-11-09 14:10 ` Petr Mladek
2018-11-12 7:59 ` Sergey Senozhatsky
2018-11-12 10:42 ` Tetsuo Handa
2018-11-17 10:14 ` Tetsuo Handa
2018-11-07 10:52 ` Tetsuo Handa
2018-11-07 11:01 ` David Laight
2018-11-07 12:00 ` Petr Mladek
2018-11-07 11:45 ` Petr Mladek
2018-11-08 2:30 ` Sergey Senozhatsky
2018-11-07 13:41 ` Petr Mladek
2018-11-07 14:06 ` Petr Mladek
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