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
