Re: [PATCH] net/skbuff: silence warnings under memory pressure

[Qian Cai]

On Thu, 2019-09-05 at 20:32 +0900, Sergey Senozhatsky wrote:
> On (09/04/19 16:42), Qian Cai wrote:
> > > Let me think more.
> > 
> > To summary, those look to me are all good long-term improvement that would
> > reduce the likelihood of this kind of livelock in general especially for
> > other
> > unknown allocations that happen while processing softirqs, but it is still
> > up to
> > the air if it fixes it 100% in all situations as printk() is going to take
> > more
> > time
> 
> Well. So. I guess that we don't need irq_work most of the time.
> 
> We need to queue irq_work for "safe" wake_up_interruptible(), when we
> know that we can deadlock in scheduler. IOW, only when we are invoked
> from the scheduler. Scheduler has printk_deferred(), which tells printk()
> that it cannot do wake_up_interruptible(). Otherwise we can just use
> normal wake_up_process() and don't need that irq_work->wake_up_interruptible()
> indirection. The parts of the scheduler, which by mistake call plain printk()
> from under pi_lock or rq_lock have chances to deadlock anyway and should
> be switched to printk_deferred().
> 
> I think we can queue significantly much less irq_work-s from printk().
> 
> Petr, Steven, what do you think?
> 
> Something like this. Call wake_up_interruptible(), switch to
> wake_up_klogd() only when called from sched code.
> 
> ---
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index cd51aa7d08a9..89cb47882254 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2027,8 +2027,11 @@ asmlinkage int vprintk_emit(int facility, int level,
> Â	pending_output = (curr_log_seq != log_next_seq);
> Â	logbuf_unlock_irqrestore(flags);
> Â
> +	if (!pending_output)
> +		return printed_len;
> +
> Â	/* If called from the scheduler, we can not call up(). */
> -	if (!in_sched && pending_output) {
> +	if (!in_sched) {
> Â		/*
> Â		Â* Disable preemption to avoid being preempted while holding
> Â		Â* console_sem which would prevent anyone from printing to
> @@ -2043,10 +2046,11 @@ asmlinkage int vprintk_emit(int facility, int level,
> Â		if (console_trylock_spinning())
> Â			console_unlock();
> Â		preempt_enable();
> -	}
> Â
> -	if (pending_output)
> +		wake_up_interruptible(&log_wait);
> +	} else {
> Â		wake_up_klogd();
> +	}
> Â	return printed_len;
> Â}
> ÂEXPORT_SYMBOL(vprintk_emit);
> ---
> 
> > and could deal with console hardware that involve irq_exit() anyway.
> 
> printk->console_driver->write() does not involve irq.

Hmm, from the article,

https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter

"Since transmission of a single or multiple characters may take a long time
relative to CPU speeds, a UART maintains a flag showing busy status so that the
host system knows if there is at least one character in the transmit buffer or
shift register; "ready for next character(s)" may also be signaled with an
interrupt."