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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Tetsuo Handa' <penguin-kernel@i-love.sakura.ne.jp>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>,
	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" <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 11:01:05 +0000	[thread overview]
Message-ID: <8354d714f6b6489d9003d6e04ee10618@AcuMS.aculab.com> (raw)
In-Reply-To: <42f33aae-a1d1-197f-a1d5-8c5ec88e88d1@i-love.sakura.ne.jp>

From: Tetsuo Handa
> Sent: 07 November 2018 10:53
> 
> On 2018/11/06 23:35, Sergey Senozhatsky wrote:
> >> 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.
> 
> Existing 'cont' handling will stay for a while. After majority of
> pr_cont()/KERN_CONT users are converted, 'cont' support will be removed
> (e.g. KERN_CONT becomes "").

A though:

Why not make the printf lock slightly 'sticky'?
- If the output line is incomplete save the cpuid.
- If there is a saved cpuid that doesn't match the current cpu then spin for a bit.

Any callers of printk() have to assume they will spin on the buffer for the
longest printk formatting (and symbol lookup might take a while) so a short
additional delay won't matter.

Then two calls to printk() for the same line won't (usually) get split and
none of the callers need any changes.

	David

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