linux-kernel - Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

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Message-ID: <865018bd-6352-cb92-1b8a-9254768f0b5c@i-love.sakura.ne.jp>
Date:   Sat, 3 Nov 2018 10:55:57 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

On 2018/11/02 23:40, Matthew Wilcox wrote:
> On Fri, Nov 02, 2018 at 10:31:55PM +0900, Tetsuo Handa wrote:
>>   get_printk_buffer() tries to assign a "struct printk_buffer" from
>>   statically preallocated array. get_printk_buffer() returns NULL if
>>   all "struct printk_buffer" are in use, but the caller does not need to
>>   check for NULL.
> 
> This seems like a great way of wasting 16kB of memory.  Since you've
> already made printk_buffered() work with a NULL initial argument, what's
> the advantage over just doing kmalloc(1024, GFP_ATOMIC)?

Like "[PATCH 2/3] mm: Use line-buffered printk() for show_free_areas()."
demonstrates, kzalloc(sizeof(struct printk_buffer), GFP_ATOMIC) can fail.

And using statically preallocated buffers helps avoiding

  (1) out of buffers when memory cannot be allocated

  (2) kernel stack overflow when kernel stack is already tight (e.g.
      a memory allocation attempt from an interrupt handler which was
      invoked from deep inside call chain of a process context)

. Whether

  (A) tuning the number of statically preallocated buffers

  (B) allocating buffers on caller side (e.g. kzalloc() or in .bss section)

are useful is a future decision, for too much concurrent printk() will lockup
the system even if there are enough buffers. I think that starting with
statically preallocated buffers is (at least for now) a good choice for
minimizing risk of (1) (2) while offering practically acceptable result.