Messages in this thread | | | Date | Fri, 4 Mar 2022 14:24:30 +0100 | From | Petr Mladek <> | Subject | Re: [PATCH printk v1 08/13] printk: add pr_flush() |
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On Wed 2022-03-02 18:29:09, John Ogness wrote: > On 2022-02-17, Petr Mladek <pmladek@suse.com> wrote: > >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > >> index 02bde45c1149..1e80fd052bd5 100644 > >> --- a/kernel/printk/printk.c > >> +++ b/kernel/printk/printk.c > >> @@ -2802,8 +2804,10 @@ void console_unblank(void) > >> if (oops_in_progress) { > >> if (down_trylock_console_sem() != 0) > >> return; > >> - } else > >> + } else { > >> + pr_flush(1000, true); > > > > It would make more sense to flush the consoles after they are > > unblanked. I mean to move this to the end of the function. > > Agreed. > > > Also it is not obvious why this is not called when oops_in_progress > > is set. I guess that it is because trylock is needed in this case. > > It should be handled inside pr_flush(). > > > > I mean that pr_flush() should internally use trylock when > > @oops_in_progress is set. It will make it safe even in this > > mode. > > pr_flush() is a might_sleep() function. We agreed on this at > LPC2019.
I see.
> Creating a pr_flush() that will directly push out the messages (or > busy-wait) in non-preemptible contexts is complicated. It might be > something to attempt for the future, but I would prefer to avoid it at > this stage.
Sure. It is not needed now, definitely.
> >> console_lock(); > >> + } > >> > >> console_locked = 1; > >> console_may_schedule = 0; > >> @@ -2869,6 +2873,7 @@ struct tty_driver *console_device(int *index) > >> */ > >> void console_stop(struct console *console) > >> { > >> + pr_flush(1000, true); > > > > It would be enough to flush just the given @console. > > For v2 I will create an internal __pr_flush() to allow specifying that > only a single console is flushed. The high level pr_flush() will then > call __pr_flush() specifying all consoles.
Feel free to keep it as is when it gets to complicated. We could always optimize it later.
> > It might be possible to take over the job from the related > > kthread and flush it in this context. Well, I am not sure if > > it is a good idea. > > I agree that it might not be a good idea. Let's keep things simple for > now.
I agree.
> >> console_lock(); > >> console->flags &= ~CON_ENABLED; > >> console_unlock(); > >> @@ -2880,6 +2885,7 @@ void console_start(struct console *console) > >> console_lock(); > >> console->flags |= CON_ENABLED; > >> console_unlock(); > >> + pr_flush(1000, true); > > > > Same here. > > OK. > > >> } > >> EXPORT_SYMBOL(console_start); > >> > >> @@ -3249,6 +3255,71 @@ static int __init printk_late_init(void) > >> late_initcall(printk_late_init); > >> > >> #if defined CONFIG_PRINTK > >> +/** > >> + * pr_flush() - Wait for printing threads to catch up. > >> + * > > > > Alternative solution would be to take over the job from the kthreads > > and flush the consoles in this context. Well, I am not sure > > if it is a good idea or not. > > Since pr_flush() is might_sleep() this would be relatively simple. Just > grab the console mutex and go. My concern is that this task may have > different scheduling parameters that could negatively affect the > system. For normal operation, I really would prefer that the designated > kthreads do the work. If "waiting for the kthreads" turns out to be > problematic, then maybe we could go down this path.
The different scheduling parameters were actually my concern. I though about a high priority task waiting for normal priority kthreads.
Anyway, let's keep it simple now. Only practice will show if a more complex solution is needed.
> >> + * @timeout_ms: The maximum time (in ms) to wait. > >> + * @reset_on_progress: Reset the timeout if forward progress is seen. > >> + * > >> + * A value of 0 for @timeout_ms means no waiting will occur. A value of -1 > >> + * represents infinite waiting. > >> + * > >> + * If @reset_on_progress is true, the timeout will be reset whenever any > >> + * printer has been seen to make some forward progress. > >> + * > >> + * Context: Process context. May sleep while acquiring console lock. > >> + * Return: true if all enabled printers are caught up. > >> + */ > >> +bool pr_flush(int timeout_ms, bool reset_on_progress) > >> +{ > >> + int remaining = timeout_ms; > >> + struct console *con; > >> + u64 last_diff = 0; > >> + u64 printk_seq; > >> + u64 diff; > >> + u64 seq; > >> + > >> + might_sleep(); > >> + > >> + seq = prb_next_seq(prb); > >> + > >> + for (;;) { > >> + diff = 0; > >> + > >> + console_lock(); > >> + for_each_console(con) { > >> + if (!console_is_usable(con)) > >> + continue; > >> + printk_seq = con->seq; > >> + if (printk_seq < seq) > >> + diff += seq - printk_seq; > >> + } > >> + console_unlock(); > >> + > >> + if (diff != last_diff && reset_on_progress) > >> + remaining = timeout_ms; > >> + > >> + if (diff == 0 || remaining == 0) > >> + break; > >> + > >> + if (remaining < 0) { > >> + /* no timeout limit */ > >> + msleep(100); > >> + } else if (remaining < 100) { > >> + msleep(remaining); > >> + remaining = 0; > >> + } else { > >> + msleep(100); > >> + remaining -= 100; > >> + } > >> + > >> + last_diff = diff; > >> + } > >> + > >> + return (diff == 0); > >> +} > >> +EXPORT_SYMBOL(pr_flush); > > > > Summary: > > > > The pr_flush() API and the optional timeout look reasonable to me. > > > > Please, handle oops_in_progress in pr_flush() and make it safe in this > > mode. It will allow to move it at the end of console_unblank() where > > it makes more sense. > > I will add another oops_in_progress check at the end. pr_flush() will > not be made safe for oops_in_progress. Keep in mind that when > oops_in_progress is set, direct printing will be active, so there should > be no need for pr_flush() anyway.
Sounds good.
Best Regards, Petr
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