On 01/18/2019 05:02 AM, Peter Zijlstra wrote: > >> e.g. We can't take an SError during the SError handler. >> >> But we can take this SError/NMI on another CPU while the first one is still >> running the handler. >> >> These multiple NMIlike notifications mean having multiple locks/fixmap-slots, >> one per notification. This is where the qspinlock node limit comes in, as we >> could have more than 4 contexts. > Right; so Waiman was going to do a patch that reverts to test-and-set or > something along those lines once we hit the queue limit, which seems > like a good way out. Actually hitting that nesting level should be > exceedingly rare.
Yes, I am working on a patch to support arbitrary levels of nesting. It is easy for PV qspinlock as lock stealing is supported. For native qspinlock, we cannot do lock stealing without incurring a certain amount of overhead in the regular slowpath code. It was up to 10% in my own testing. So I am exploring an alternative that can do the job without incurring any noticeable performance degradation in the slowpath. I ran into a race condition which I am still trying to find out where that comes from. Hopefully, I will have something to post next week. Cheers, Longman
