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

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