From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: 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 12:24:43 +0100 [thread overview]
Message-ID: <20181108112443.huqkju4uwrenvtnu@pathway.suse.cz> (raw)
In-Reply-To: <20181108022138.GA2343@jagdpanzerIV>
On Thu 2018-11-08 11:21:38, Sergey Senozhatsky wrote:
> On (11/07/18 11:21), Petr Mladek wrote:
> What is the problem:
> - we have tons of CPUs, with tons of tasks running on them, with preemption,
> and interrupts, and potentially printk-s coming from various
> contexts/CPUs/tasks etc. so one 'cont' buffer is not enough.
>
> What is the proposed solution:
> - if 1 is not enough then 16 will do. And if 16 is not enough then this
> is not our problem anymore, it's a kernel misconfiguration and users'
> fault.
I believe that I mentioned this more times. 16 buffers is the first
attempt. We could improve it later in several ways:
+ add more buffers
+ combine buffers on stack, dynamically allocated and static ones.
BTW: I do not remember seeing mixed lines from anything even close to 16
CPUs. Maybe I was just lucky or my memory is leaky.
> 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.
What do you suggest instead, please?
I am afraid that we are running in cycles. The other serious
alternative was having per-process and per-context buffers
but it was rejected several times.
> Not to mention that buffered printk does not flush on panic.
> So, frankly, as of now, I don't see buffered printk as a 'cont'
> replacement.
The static array of buffers can be flushed on panic.
> 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?
> [..]
> > I opened this problem once and it got lost. So I did not want to
> > complicate it at this moment.
>
> - I don't exactly like the completely of the vprintk_buffered. If
> buffered printk is for single line, then it must be for single
> line only.
My undestanding is that the new API is similar to the current cont
buffer from this point of view:
+ buffer size is LOG_LINE_MAX
+ it is flushed when full
The only difference is that it is flushed also when there is a
complete line. Is this a problem, please?
> And I'm not buying the "we will need this for printk origin
> info injection" argument.
I was against this idea several times. The current API does
not do anything like this.
> - 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?
Best Regards,
Petr
next prev parent reply other threads:[~2018-11-08 11:24 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 [this message]
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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