Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: synchronize_qrcu_timeout() | Date | Wed, 27 Jun 2007 22:52:14 +0200 |
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On Wednesday, 27 June 2007 13:18, Oleg Nesterov wrote: > On 06/26, Paul E. McKenney wrote: > > > > On Mon, Jun 25, 2007 at 07:49:57PM +0400, Oleg Nesterov wrote: > > > On 06/25, Oleg Nesterov wrote: > > > > > > > > On 06/25, Paul E. McKenney wrote: > > > > > > > > > > On Mon, Jun 25, 2007 at 02:43:32PM +0400, Oleg Nesterov wrote: > > > > > > > > > > > > Sadly, you can't use srcu/qrcu because it doesn't handle timeouts. > > > > > > > > > > Interesting... So the thought is to have a synchronize_srcu_timeout() > > > > > or something similar that waited for a grace period to elapse or for > > > > > a timeout to expire, whichever comes first? It should not be too hard > > > > > to arrange something, if needed. > > > > > > > > Yes. As for qrcu (see http://marc.info/?t=116484476500001), I think it is easy. > > > > First, we add "int interrupted" into struct qrcu_struct, then something like this > > > > > > Even simpler, we don't need ->interrupted. > > > > I have to ask... > > > > What sorts of performance characteristics are needed here? The reason > > that I ask is because putting a straight synchronize_qrcu() into a > > workqueue (or something similar) and then using a timer to provide > > any needed wakeup seems a lot simpler than rearranging the innards of > > synchronize_qrcu(). > > Didn't think about that... But yes, we can implement synchronize_qrcu_timeout() > on top of synchronize_qrcu() as you suggested. Hovewer, this implementation > will add more code to .text compared to changing synchronize_qrcu(). > > Note also that we can't share the "context" of synchronize_qrcu_timeout() in > that case. Each caller of synchronize_qrcu_timeout() needs a separate > wait_queue_head_t + work_struct. This context can't live on stack, because > it should survive after the timeout. > > > If we change synchronize_qrcu() instead, we only add > > if (unlikely(atomic_read(qp->ctr + prv))) { > __wait_event_timeout(qp->wq, !atomic_read(qp->ctr + prv), tout); > if (unlikely(!tout)) > goto out; > } > > to the slow path. This has nearly zero perfomance penalty. Yes, we have > to wait if the previous call was interrupted by timeout. But in that case > we lost nothing, we spend the same time waiting for qp->mutex when the > previous call succeeds. > > > That said, I am not sure synchronize_Xrcu_timeout() is terribly useful. > Rafael could use it, but it is not better for this particular case. > Except the code re-use, which is always good.
Well, if there are more cases in which it's useful, we'll be able to modify this code to use it too in the future.
Greetings, Rafael
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