Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

Pavel Machek
2017-07-05T15:59:49Z

On Mon 2017-07-03 20:11:30, Sergey Senozhatsky wrote:
> On (06/30/17 22:38), Sergey Senozhatsky wrote:
> > On (06/30/17 15:16), Petr Mladek wrote:
> > > Anyway, the handshake during offloading might be pretty
> > > problematic. To be honest, I do not have much experience
> > > with it. I have shared some my fears in the other mail[1].
> > > Jan Kara spent a lot of time on this and probably could
> > > say more.
> > >
> > > Maybe, we could try to look into the throotling path. Slowing down
> > > massive printk() callers looks necessary when things gets
> > > out of control.
> >
> > throttling, in some form, is already there. I think.
> >
> > there is a printk_delay() function. which we can silently activate
> > when things don't look cool anymore. and printk_delay() is already
> > getting called on every vprintk_emit() entry. the question is -- how
> > big should be our delay value, and... when do we need to activate
> > printk_delay()?
> >
> > when the distance between console_seq and log_next_seq... suggests
> > that we will drop (overwrite) un-flushed messages sooner than console_seq
> > reaches log_next_seq? so log_next_seq is closer to log_first_seq than
> > console_seq to log_next_seq.
>
> something like below, may be. a sketch, just to demonstrate the
> idea. but, once polished, can go to printk out of series.
>
> ===8<===8<===8<===
>
> Throttle printk() callers when we detect that consoles are
> far behind the logbuf: we printed to the consoles 4 times
> less messages than we still have to print.
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
> kernel/printk/printk.c | 38 ++++++++++++++++++++++++++++++++------
> 1 file changed, 32 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index f24d3789faa0..fd546bd95207 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1758,17 +1758,43 @@ static void call_console_drivers(const char *ext_text, size_t ext_len,
> }
> }
>
> +#define PRINTK_FLOOD_DEFAULT_DELAY 10
> +
> int printk_delay_msec __read_mostly;

= 10;

> + if (console_seen < 4 * console_to_see) {
> + if (printk_delay_msec)
> + __printk_delay(printk_delay_msec);
> + else
> + __printk_delay(PRINTK_FLOOD_DEFAULT_DELAY);

And get rid of if () here? We want to enable delays of 0...
Pavel
--
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