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][ 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. 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
