Messages in this thread | | | Date | Thu, 3 Jun 2010 10:52:51 +0200 | From | Andi Kleen <> | Subject | Re: [PATCH] use unfair spinlock when running on hypervisor. |
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On Thu, Jun 03, 2010 at 09:50:51AM +0530, Srivatsa Vaddagiri wrote: > On Wed, Jun 02, 2010 at 12:00:27PM +0300, Avi Kivity wrote: > > > > There are two separate problems: the more general problem is that > > the hypervisor can put a vcpu to sleep while holding a lock, causing > > other vcpus to spin until the end of their time slice. This can > > only be addressed with hypervisor help. > > Fyi - I have a early patch ready to address this issue. Basically I am using > host-kernel memory (mmap'ed into guest as io-memory via ivshmem driver) to hint > host whenever guest is in spin-lock'ed section, which is read by host scheduler > to defer preemption.
Looks like a ni.ce simple way to handle this for the kernel.
However I suspect user space will hit the same issue sooner or later. I assume your way is not easily extensable to futexes?
> One pathological case where this may actually hurt is routines in guest like > flush_tlb_others_ipi() which take a spinlock and then enter a while() loop > waiting for other cpus to ack something. In this case, deferring preemption just > because guest is in critical section actually hurts! Hopefully the upper bound > for deferring preemtion and the fact that such routines may not be frequently > hit should help alleviate such situations.
So do you defer during the whole spinlock region or just during the spin?
I assume the the first?
-Andi -- ak@linux.intel.com -- Speaking for myself only.
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