Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH] xen-evtchn: Bind dyn evtchn:qemu-dm interrupt to next online VCPU | From | Anoob Soman <> | Date | Mon, 5 Jun 2017 15:49:42 +0100 |
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On 05/06/17 15:10, Boris Ostrovsky wrote: >> The reason for percpu instead of global, was to avoid locking. We can >> have a global variable (last_cpu) without locking, but value of >> last_cpu wont be consistent, without locks. Moreover, since >> irq_affinity is also used in the calculation of cpu to bind, having a >> percpu or global wouldn't really matter, as the result (selected_cpu) >> is more likely to be random (because different irqs can have different >> affinity). What do you guys suggest. > Doesn't initial affinity (which is what we expect here since irqbalance > has not run yet) typically cover all guest VCPUs?
Yes, initial affinity covers all online VCPUs. But there is a small chance that initial affinity might change, before evtch_bind_interdom_next_vcpu is called. For example, I could run a script to change irq affinity, just when irq sysfs entry appears. This is the reason that I thought it would be sensible (based on your suggestion) to include irq_affinity to calculate the next VCPU. If you think, changing irq_affinity between request_irq() and evtch_bind_interdom_next_vcpu is virtually impossible, then we can drop affinity and just use cpu_online_mask.
>> >> I think we would still require spin_lock(). spin_lock is for irq_desc. > If you are trying to protect affinity then it may well change after you > drop the lock. > > In fact, don't you have a race here? If we offline a VCPU we will (by > way of cpu_disable_common()->fixup_irqs()) update affinity to reflect > that a CPU is gone and there is a chance that xen_rebind_evtchn_to_cpu() > will happen after that. > > So, contrary to what I said earlier ;-) not only do you need the lock, > but you should hold it across xen_rebind_evtchn_to_cpu() call. Does this > make sense?
Yes, you are correct. .irq_set_affinity pretty much does the same thing.
The code will now looks like this. raw_spin_lock_irqsave(lock, flags); percpu read select_cpu percpu write xen_rebind_evtchn_to_cpu(evtchn, selected_cpu) raw_spin_unlock_irqsave(lock, flags);
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