Re: Question about qspinlock nest

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jan 18, 2019 at 09:50:12AM -0500, Waiman Long wrote:
> 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.

Where does the overhead come from? Surely that's not just checking that
bound?