Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

Sergey Senozhatsky
2018-01-20T07:14:09Z

On (01/19/18 13:20), Steven Rostedt wrote:
[..]
> I was thinking about this a bit more, and instead of offloading a
> recursive printk, perhaps its best to simply throttle it. Because the
> problem may not go away if a printk thread takes over, because the bug
> is really the printk infrastructure filling the printk buffer keeping
> printk from ever stopping.

right. I didn't quite got it how that would help. if we would
kick_offload every time we add new printks after call_console_drivers(),
then we can just end up in a kick_offload loop traveling across all CPUs.

[..]
> asmlinkage int vprintk_emit(int facility, int level,
> const char *dict, size_t dictlen,
> @@ -1849,6 +1918,17 @@ asmlinkage int vprintk_emit(int facility, int level,
>
> /* This stops the holder of console_sem just where we want him */
> logbuf_lock_irqsave(flags);
> +
> + if (recursion_check_test()) {
> + /* A printk happened within a printk at the same context */
> + if (this_cpu_inc_return(recursion_count) > recursion_max) {
> + atomic_inc(&recursion_overflow);
> + logbuf_unlock_irqrestore(flags);
> + printed_len = 0;
> + goto out;
> + }
> + }

didn't have time to look at this carefully, but is this possible?

printks from console_unlock()->call_console_drivers() are redirected
to printk_safe buffer. we need irq_work on that CPU to flush its
printk_safe buffer.

> EXPORT_SYMBOL(vprintk_emit);
> @@ -2343,9 +2428,14 @@ void console_unlock(void)
> seen_seq = log_next_seq;
> }
>
> - if (console_seq < log_first_seq) {
> + if (console_seq < log_first_seq || atomic_read(&recursion_overflow)) {
> + size_t missed;
> +
> + missed = atomic_xchg(&recursion_overflow, 0);
> + missed += log_first_seq - console_seq;
> +
> len = sprintf(text, "** %u printk messages dropped **\n",
> - (unsigned)(log_first_seq - console_seq));
> + (unsigned)missed);
>
> /* messages are gone, move to first one */
> console_seq = log_first_seq;

how are we going to distinguish between lockdep splats, for instance,
or WARNs from call_console_drivers() -> foo_write(), which are valuable,
and kmalloc() print outs, which might be less valuable? are we going to
lose all of them now? then we can do a much simpler thing - steal one
bit from `printk_context' and use if for a new PRINTK_NOOP_CONTEXT, which
will be set around call_console_drivers(). vprintk_func() would redirect
printks to vprintk_noop(fmt, args), which will do nothing.

-ss