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

On (09/04/19 16:42), Qian Cai wrote:

quoted

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,
 	pending_output = (curr_log_seq != log_next_seq);
 	logbuf_unlock_irqrestore(flags);
 
+	if (!pending_output)
+		return printed_len;
+
 	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched && pending_output) {
+	if (!in_sched) {
 		/*
 		 * Disable preemption to avoid being preempted while holding
 		 * console_sem which would prevent anyone from printing to

@@ -2043,10 +2046,11 @@ asmlinkage int vprintk_emit(int facility, int level,
 		if (console_trylock_spinning())
 			console_unlock();
 		preempt_enable();
-	}
 
-	if (pending_output)
+		wake_up_interruptible(&log_wait);
+	} else {
 		wake_up_klogd();
+	}
 	return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);
---

and could deal with console hardware that involve irq_exit() anyway.

printk->console_driver->write() does not involve irq.

On the other hand, adding __GPF_NOWARN in the build_skb() allocation will fix
this known NET_TX_SOFTIRQ case which is common when softirqd involved at least
in short-term. It even have a benefit to reduce the overall warn_alloc() noise
out there.

That's not up to me to decide.

	-ss