All of lore.kernel.org
 help / color / mirror / Atom feed
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

  reply	other threads:[~2018-11-08 11:24 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-02 13:31 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
2018-11-02 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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20181108112443.huqkju4uwrenvtnu@pathway.suse.cz \
    --to=pmladek@suse.com \
    --cc=akpm@linux-foundation.org \
    --cc=dvyukov@google.com \
    --cc=fengguang.wu@intel.com \
    --cc=glider@google.com \
    --cc=jpoimboe@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mingo@redhat.com \
    --cc=penguin-kernel@i-love.sakura.ne.jp \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=sergey.senozhatsky.work@gmail.com \
    --cc=sergey.senozhatsky@gmail.com \
    --cc=torvalds@linux-foundation.org \
    --cc=will.deacon@arm.com \
    --subject='Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.