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

[Sergey Senozhatsky]

On (09/04/19 08:14), Qian Cai wrote:
> > Plus one more check - waitqueue_active(&log_wait). printk() adds
> > pending irq_work only if there is a user-space process sleeping on
> > log_wait and irq_work is not already scheduled. If the syslog is
> > active or there is noone to wakeup then we don't queue irq_work.
> 
> Another possibility for this potential livelock is that those printk() from
> warn_alloc(), dump_stack() and show_mem() increase the time it needs to process
> build_skb() allocation failures significantly under memory pressure. As the
> result, ksoftirqd() could be rescheduled during that time via a different CPU
> (this is a large x86 NUMA system anyway),
> 
> [83605.577256][   C31]  run_ksoftirqd+0x1f/0x40
> [83605.577256][   C31]  smpboot_thread_fn+0x255/0x440
> [83605.577256][   C31]  kthread+0x1df/0x200
> [83605.577256][   C31]  ret_from_fork+0x35/0x40

Hum hum hum...

So I can, _probably_, think of several patches.

First, move wake_up_klogd() back to console_unlock().

Second, move `printk_pending' out of per-CPU region and make it global.
So we will have just one printk irq_work scheduled across all CPUs;
currently we have one irq_work per CPU. I think I sent a patch a long
long time ago, but we never discussed it, as far as I remember.

> In addition, those printk() will deal with console drivers or even a networking
> console, so it is probably not unusual that it could call irq_exit()-
>__do_softirq() at one point and then this livelock.

Do you use netcon? Because this, theoretically, can open up one more
vector. netcon allocates skbs from ->write() path. We call con drivers'
->write() from printk_safe context, so should netcon skb allocation
warn we will scedule one more irq_work on that CPU to flush per-CPU
printk_safe buffer.

If this is the case, then we can stop calling console_driver() under
printk_safe. I sent a patch a while ago, but we agreed to keep the
things the way they are, fot the time being.

Let me think more.

	-ss