From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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: Thu, 8 Nov 2018 21:30:49 +0900 [thread overview]
Message-ID: <20181108123049.GA30440@jagdpanzerIV> (raw)
In-Reply-To: <20181108112443.huqkju4uwrenvtnu@pathway.suse.cz>
On (11/08/18 12:24), Petr Mladek wrote:
> I believe that I mentioned this more times. 16 buffers is the first
> attempt. We could improve it later in several ways
Sure. Like I said - maybe it is a normal development pattern; I really
don't know.
> > Let's have one more look at what we will fix and what we will break.
> >
> > 'cont' has premature flushes.
> >
> > Why is it good.
> > It preserves the correct order of events.
> >
> > pr_cont("calling foo->init()....");
> > foo->init()
> > printk("Can't allocate buffer\n"); // premature flush
> > pr_cont("...blah\h");
> >
> > Will end up in the logbuf as:
> > [12345.123] calling foo->init()....
> > [12345.124] Can't allocate buffer
> > [12345.125] ...blah
> >
> > Where buffered printk will endup as:
> > [12345.123] Can't allocate buffer
> > [12345.124] calling foo->init().......blah
>
> We will always have this problem with API using explicit buffers.
Exactly.
> What do you suggest instead, please?
So this is why "let's not remove 'cont'" thing. Buffered printk
is not 100% identical to 'cont'. And 'cont' does a good job sometimes,
Because 'cont' looks like a buffered printk, but it behaves like a
normal printk when things go bad. Buffered printk is always buffered;
and not even aware of the fact that things go bad around.
> > If our problem is OOM and lockdep print outs, then let's address only
> > those two; let's not "fix" the rest of the kernel, especially the early
> > boot, - we can break more things than we can mend.
>
> Do you have any alternative proposal how to handle OOM and lockdep, please?
You misunderstood me. I'm not against the buffered printk in lockdep and
OOM. Albeit I must admit that lockdep patch looks quite big. I don't like
the idea of "we will remove 'cont' and replace it with buffered printk
through out the kernel".
[..]
> > - It seems that buffered printk attempts to solve too many problems.
> > I'd prefer it to address just one.
>
> This API tries to handle continuous lines more reliably.
> Do I miss anything, please?
This part:
+ /* Flush already completed lines if any. */
+ for (pos = ptr->len - 1; pos >= 0; pos--) {
+ if (ptr->buf[pos] != '\n')
+ continue;
+ ptr->buf[pos++] = '\0';
+ printk("%s\n", ptr->buf);
+ ptr->len -= pos;
+ memmove(ptr->buf, ptr->buf + pos, ptr->len);
+ /* This '\0' will be overwritten by next vsnprintf() above. */
+ ptr->buf[ptr->len] = '\0';
+ break;
+ }
+ return r;
If I'm not mistaken, this is for the futute "printk injection" work.
Right now printk("foo\nbar\n") will end up to be a single logbuf
entry, with \n in the middle and at the end. So it will look like
two lines on the serial console:
[123.123] foo
bar
Tetsuo does this \n lookup (on every vprintk_buffered) and split lines
(via memove) for "printk injection", so the output will be
[123.123] foo
[123.124] bar
Which makes it simpler to "inject" printk origin into every printed
line.
Without it we can just do
len = vsprintf();
if (len && text[len - 1] == '\n' || overflow)
flush();
Can't we?
-ss
next prev parent reply other threads:[~2018-11-08 12:30 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
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 [this message]
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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