On Wed, 1 Nov 2017 18:42:25 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote: > On 11/01/2017 04:33 PM, Steven Rostedt wrote: > > On Wed, 1 Nov 2017 09:30:05 +0100 > > Vlastimil Babka <vbabka@xxxxxxx> wrote: > > > >> > >> But still, it seems to me that the scheme only works as long as there > >> are printk()'s coming with some reasonable frequency. There's still a > >> corner case when a storm of printk()'s can come that will fill the ring > >> buffers, and while during the storm the printing will be distributed > >> between CPUs nicely, the last unfortunate CPU after the storm subsides > >> will be left with a large accumulated buffer to print, and there will be > >> no waiters to take over if there are no more printk()'s coming. What > >> then, should it detect such situation and defer the flushing? > > > > No! > > > > If such a case happened, that means the system is doing something > > really stupid. > > Hm, what about e.g. a soft lockup that triggers backtraces from all > CPU's? Yes, having softlockups is "stupid" but sometimes they do happen > and the system still recovers (just some looping operation is missing > cond_resched() and took longer than expected). It would be sad if it > didn't recover because of a printk() issue... I still think such a case would not be huge for the last printer. > > > Btw, each printk that takes over, does one message, so the last one to > > take over, shouldn't have a full buffer anyway. > > There might be multiple messages per each CPU, e.g. the softlockup > backtraces. And each one does multiple printks, still spreading the love around. > > > But still, if you have such a hypothetical situation, the system should > > just crash. The printk is still bounded by the length of the buffer. > > Although it is slow, it will finish. > > Finish, but with single CPU doing the printing, which is wrong? I don't think so. This is all hypothetical anyway. I need to implement my solution, and then lets see if this can actually happen. > > > Which is not the case with the > > current situation. And the current situation (as which this patch > > demonstrates) does happen today and is not hypothetical. > > Yep, so ideally it can be fixed without corner cases :) If there is any corner cases. I guess the test would be to trigger a soft lockup on all CPUs to print out a dump at the same time. But then again, how is a soft lockup on all CPUs not any worse than a single CPU finishing up the buffer output? -- Steve -- 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>
