On (11/09/17 00:06), Steven Rostedt wrote: > What does safe context mean? "safe" means that we don't cause lockups, stalls, sched throttlings, etc. by doing console_unlock() from that context [task]. > Do we really want to allow the printk thread to sleep when there's more > to print? What happens if there's a crash at that moment? How do we safely > flush out all the data when the printk thread is sleeping? printk-kthread does not schedule with the console_sem locked. one of the changes to console_unlock() introduced with printk-kthread, which we can't have without offloading. > Now we could have something that uses both nicely. When the > printk_thread wakes up (we need to figure out when to do that), then it > could constantly take over. certainly we can have a better hand-off scheme in printk-kthread patch set. > > CPU1 CPU2 > ---- ---- > console_unlock() > start printing a lot > (more than one, wake up printk_thread) > > printk thread wakes up > > becomes the waiter > > sees waiter hands off > > starts printing > > printk() > becomes waiter > > sees waiter hands off > then becomes new waiter! <-- key > > starts printing > sees waiter hands off > continues printing there are corners cases here. learned the hard way. real reproducers do exist. wake_up_process() may enqueue printk_thread on the same rq that current printk task is running on. so if your printk(), for instance, is from IRQ then offloading won't happen. > That is, we keep the waiter logic, and if anyone starts printing too > much, it wakes up the printk thread (hopefully on another CPU, or the > printk thread should migrate) when the printk thread starts running it it must migrate, yes. currently I'm playing games with the affinity mask of printk-kthread when I do offloading. > becomes the new waiter if the console lock is still held (just like in > printk). Then it gets handed off the printk. We could just have the > printk thread keep going, though I'm not sure I would want to let it > schedule while printing. yes, scheduling under console_sem is not right. we don't want this. not anymore, at least. > But it could also hand off printks (like > above), but then take it back immediately. This would mean that a > printk caller from a "critical" path will only get to do one message, > before the printk thread asks for it again. > > Perhaps we could have more than one printk thread that migrates around, > and they each hand off the printing. This makes sure the printing > always happens and that it never stops due to the console_lock holder > sleeping and we never lock up one CPU that does printing. This would > work with just two printk threads. When one starts a printk loop, > another one wakes up on another CPU and becomes the waiter to get the > handoff of the console_lock. Then the first could schedule out (migrate > if the current CPU is busy), and take over. In fact, this would > basically have two CPUs bouncing back and forth to do the printing. can be. I pushed it much further, once. [probably too far]. and had per-CPU printk-kthreads :) > This gives us our cake and we get to eat it too. > > One, printing never stops (no scheduling out), as there's two threads > to share the load (obiously only on SMP machines). > > There's no lock up. There's two threads that print a little, pass off > the console lock, do a cond_resched(), then takes over again. > > Bascially, what I'm saying is that this is not two different solutions. > There is two algorithms that can work together to give us reliable > output and not lock up the system in doing so. sure, I understand. -ss -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
