Re: Question about qspinlock nest

[Waiman Long]

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