Messages in this thread | | | Subject | Re: [PATCH] mm: don't warn about allocations which stall for too long | From | Vlastimil Babka <> | Date | Wed, 1 Nov 2017 18:42:25 +0100 |
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On 11/01/2017 04:33 PM, Steven Rostedt wrote: > On Wed, 1 Nov 2017 09:30:05 +0100 > Vlastimil Babka <vbabka@suse.cz> 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 >
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