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

From: Petr Mladek
Date: Wed Dec 14 2016 - 05:51:27 EST

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,