Messages in this thread | | | Date | Fri, 18 Feb 2022 10:08:17 +0100 | From | Petr Mladek <> | Subject | main loop: was: Re: [PATCH printk v1 10/13] printk: add kthread console printers |
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On Mon 2022-02-07 20:49:20, John Ogness wrote: > Create a kthread for each console to perform console printing. During > normal operation (@system_state == SYSTEM_RUNNING), the kthread > printers are responsible for all printing on their respective > consoles. > > During non-normal operation, console printing is done as it has been: > within the context of the printk caller or within irq work triggered > by the printk caller. > > Console printers synchronize against each other and against console > lockers by taking the console lock for each message that is printed. > --- > include/linux/console.h | 2 + > kernel/printk/printk.c | 159 +++++++++++++++++++++++++++++++++++++++- > 2 files changed, 159 insertions(+), 2 deletions(-) > > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -3345,6 +3390,116 @@ bool pr_flush(int timeout_ms, bool reset_on_progress) > } > EXPORT_SYMBOL(pr_flush); > > +static bool printer_should_wake(struct console *con, u64 seq) > +{ > + short flags; > + > + if (kthread_should_stop()) > + return true; > + > + if (console_suspended) > + return false; > + > + /* > + * This is an unsafe read to con->flags, but false positives > + * are not an issue as long as they are rare.
This is not convincing. Anyway, it looks like an optimization. I suggest to do the check under console_lock in the main loop. printk kthread is a slow path anyway.
Also we should call there the entire console_is_usable(). It should do the same checks as in console_flush_all() and the direct mode.
> + */ > + flags = data_race(READ_ONCE(con->flags)); > + if (!(flags & CON_ENABLED)) > + return false; > + > + return prb_read_valid(prb, seq, NULL); > +} > + > +static int printk_kthread_func(void *data) > +{ > + struct console *con = data; > + char *dropped_text = NULL; > + char *ext_text = NULL; > + bool progress; > + bool handover; > + u64 seq = 0; > + char *text; > + int error; > + > + pr_info("%sconsole [%s%d]: printing thread started\n", > + (con->flags & CON_BOOT) ? "boot" : "", > + con->name, con->index); > + [...] > + > + for (;;) { > + error = wait_event_interruptible(log_wait, printer_should_wake(con, seq)); > + > + if (kthread_should_stop()) > + break; > + > + if (error) > + continue; > + > + do { > + console_lock(); > + if (console_suspended) { > + console_unlock(); > + break; > + } > +
We should check here also console_is_usable() to be in sync with console_flush_all().
> + /* > + * Even though the printk kthread is always preemptible, it is > + * still not allowed to call cond_resched() from within > + * console drivers. The task may become non-preemptible in the > + * console driver call chain. For example, vt_console_print() > + * takes a spinlock and then can call into fbcon_redraw(), > + * which can conditionally invoke cond_resched(). > + */ > + console_may_schedule = 0;
I do not understand the reason. How is this different from console_unlock() and console_flush_all(), please?
> + progress = console_emit_next_record(con, text, ext_text, > + dropped_text, &handover); > + if (handover) > + break; > + > + seq = con->seq; > + > + /* Unlock console without invoking direct printing. */ > + __console_unlock(); > + } while (progress); > + } > +out: > + kfree(dropped_text); > + kfree(ext_text); > + kfree(text); > + pr_info("%sconsole [%s%d]: printing thread stopped\n", > + (con->flags & CON_BOOT) ? "boot" : "", > + con->name, con->index); > + return 0; > +} > +
Best Regards, Petr
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