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

[Qian Cai]

On Wed, 2019-09-04 at 23:48 +0900, Sergey Senozhatsky wrote:
> 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][ÂÂÂC31]ÂÂrun_ksoftirqd+0x1f/0x40
> > [83605.577256][ÂÂÂC31]ÂÂsmpboot_thread_fn+0x255/0x440
> > [83605.577256][ÂÂÂC31]ÂÂkthread+0x1df/0x200
> > [83605.577256][ÂÂÂC31]ÂÂret_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.

No, I don't use netcon. Just thought to mention it anyway since there could
other people use it.

> 
> 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