linux-kernel - Re: [RFC][PATCHv5 5/7] printk: report lost messages in printk safe/nmi contexts

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Date:   Tue, 13 Dec 2016 10:52:37 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv5 5/7] printk: report lost messages in printk
 safe/nmi contexts

On (12/12/16 16:58), Petr Mladek wrote:
> On Thu 2016-12-01 22:55:44, Sergey Senozhatsky wrote:
[..]
> But not really because we report lost messages from both buffers
> and from all CPUs here.
[..]
> The perfect solution would be to remember the number of lost messages
> in struct printk_safe_seq_buf. Then we might bump the value directly
> in printk_safe_log_store() instead of returning the ugly -ENOSPC.

ok, I can take a look. this won't grow the per-CPU buffers bigger,
but will shrink the actual message buffer size by sizeof(atomic),
not that dramatic.

* unrelated, can be done later (if ever) *

speaking of tha actual message buffer size, we, may be, can move
`struct irq_work' out of printk_safe_seq_buf. there is already
a printk-related per-CPU irq_work in place - wake_up_klogd_work.
so we may be can use it, instead of defining a bunch of new irq_works.
this will increase the printk-safe/nmi per-CPU message buffer size
by sizeof(irq_work).

> Also we could use an universal message (no "NMI" or "printk-safe")
> because it could be printed right after flushing the messages
> that fit the buffer.

this "context" part probably can be dropped. both printk-safe and
printk-nmi per-CPU buffer sizes are controlled by a single .config
option anyway; user can't increase the printk-safe buffer size
without increasing the printk-nmi buffer size (in case if printk-safe
buffer is too small).

> This solution is good enough and still better than the previous one, so
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>

thanks.

	-ss