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

Sergey Senozhatsky
2017-07-04T05:26:05Z

On (07/03/17 15:34), Steven Rostedt wrote:
> > +#define PRINTK_FLOOD_DEFAULT_DELAY 10
> > +
> > int printk_delay_msec __read_mostly;
> >
> > +static inline void __printk_delay(int m)
> > +{
> > + while (m--) {
> > + mdelay(1);
> > + touch_nmi_watchdog();
> > + }
> > +}
> > +
> > static inline void printk_delay(void)
> > {
> > - if (unlikely(printk_delay_msec)) {
> > - int m = printk_delay_msec;
> > + unsigned long flags;
> > + u64 console_seen = 0, console_to_see;
> >
> > - while (m--) {
> > - mdelay(1);
> > - touch_nmi_watchdog();
> > - }
> > + if (printk_delay_msec) {
> > + __printk_delay(printk_delay_msec);
> > + return;
> > + }
> > +
>
> This had better be an option, and not default.

yes.

> And what happens if the printk caller happens to preempt the one
> doing the writes to consoles?

in short - we just burn CPU cycles. that case is broken.

that's mostly the reason behind PRINTK_FLOOD_DEFAULT_DELAY being quite
small.

one can simply do

console_lock();
printk();
printk();
....
printk();
console_unlock();

and trigger a useless throttling. a needed one in general case,
but useless in the given circumstances.

not sure if we can properly throttle printk in all of the cases.
we know that console_sem is locked, but we don't know what for.
is CPU that owns the console_sem is now in console_unlock() or
somewhere in fbcon, or anywhere else. we probably need not to
throttle printk() if we know that console_sem is already locked
by this_cpu and we simply call printk either from IRQ that
preempted console_unlock() on this_cpu or recursive printk from
console_unlock()... and so on.

-ss