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

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Date:   Wed, 14 Dec 2016 11:51:20 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.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
Subject: Re: [RFC][PATCHv5 5/7] printk: report lost messages in printk
 safe/nmi contexts

On Tue 2016-12-13 10:52:37, Sergey Senozhatsky wrote:
> 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.

IMHO, the size is a negligible difference. If we are often getting
on the edge of the buffers, we have problems anyway.


> * 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).

Interesting idea! I think that there is a space for more optimization.
For example, we will not need to schedule the irq work if we are
flushing  the per-CPU buffers from irq work and we know that
we will flush consoles or wake up the kthread right after that.
Also I though about using a global "printk_pending" variable
and queue the irqwork only when the given event was not already set.

I would leave all this optimization for a later patchset.


> > 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).

I agree.

Best Regards,
Petr