On Fri, Sep 04, 2015 at 08:21:28AM -0700, Linus Torvalds wrote: > On Fri, Sep 4, 2015 at 8:14 AM, Peter Zijlstra <pet...@infradead.org> wrote: > > > > The reason we chose to revert to a test-and-set is because regular fair > > locks, like the ticket and the queue thing, have horrible behaviour > > under vcpu preemption. > > Right. However, with our old ticket locks, that's what we got when you > didn't ask for paravirt support. No?
Indeed. > And even ignoring the "implementation was crap" issue, some people may > well want their kernels to be "bare hardware" kernels even under a > hypervisor. It may be a slim hypervisor that gives you all the cpus, > or it may just be a system that is just sufficiently overprovisioned, > so you don't get vcpu preemption in practice. Fair enough; I had not considered the slim hypervisor case. Should I place the virt_spin_lock() thing under CONFIG_PARAVIRT (maybe even _SPINLOCKS) such that only paravirt enabled kernels when ran on a hypervisor that does not support paravirt patching (HyperV, VMware, etc..) revert to the test-and-set? > But it would be interesting to hear if just fixing the busy-looping to > not pound the lock with a constant stream of cmpxchg's is already > sufficient to fix the big picture problem. Dave replaced the cpu_relax() with a __delay(1) to match what spinlock-debug does and that fixed things for him. Of course, it would be good if he can try the proposed patch too. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
