On Tue, Jul 14, 2009 at 05:03:09AM -0700, Paul E. McKenney wrote: > On Tue, Jul 14, 2009 at 08:46:12AM +0300, Gleb Natapov wrote: > > On Mon, Jul 13, 2009 at 12:31:39PM -0700, Paul E. McKenney wrote: > > > On Mon, Jul 13, 2009 at 04:40:59PM +0300, Michael S. Tsirkin wrote: > > > > On Mon, Jul 13, 2009 at 04:32:34PM +0300, Gleb Natapov wrote: > > > > > Yeah I understand that other RCU read section may introduce delays too. > > > > > The question is how big the delay may be. > > > > > > > > I recall seeing the number "at least 3 jiffies" somewhere, but that > > > > might have changed since. > > > > > > A grace period lasts a handful of jiffies, depending on kernel > > > configuration and how long readers remain in a given RCU read-side > > > critical section. > > > > > > If a handful of jiffies is too long, there are patches that speed up > > > the grace period, down into the sub-hundred-microsecond range. > > > > > > > > I don't think multiple > > > > > milliseconds delay in device de-assignment is a big issue though. > > > > > > > > Right. My point was that since the sync is done under kvm lock, the > > > > guest can easily get blocked trying to get kvm lock meanwhile. > > > > > > I will ask the usual question -- can call_rcu() be used to move > > > the grace period out from under the lock? (Often this can be done, > > > but not always.) > > > > > It can't. Caller frees the data. > > I will then ask the usual next question... Can the freeing of the > data be moved from the caller? > Not really. But don't worry. In the case we discuss a small delay is not a problem. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
