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

From: Sergey Senozhatsky <[email-masked]>
Date: Thu, 5 Sep 2019 20:32:08 +0900

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,
 	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