Re: [PATCH 4/4] Convert irq notifiers lists to RCU locking.

Paul E. McKenney
2009-07-13T20:09:13Z

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.)

Thanx, Paul