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... > 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. > 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? > 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 :) Vlastimil > -- 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>
