On Mon, Feb 12, 2007 at 07:22:09AM +0100, Jens Axboe wrote:
> On Sun, Feb 11 2007, Paul E. McKenney wrote:
> > This patch optimizes the "quick" RCU update-side fastpath, so that in the
> > absence of readers, synchronize_qrcu() does four non-atomic comparisons
> > and three memory barriers, eliminating the need to acquire the global
> > lock in this case. Lightly tested. Algorithm has been validated for
> > the 3-reader-2-updater and 2-reader-3-updater cases -- 3-readers-3-updaters
> > case still to be done (I expect to get access to a large-memory machine
> > in the next few weeks -- need >>20GB).
> >
> > Not for inclusion. Patch is against Oleg's original patch, and likely
> > needs to be rediffed against Jen's patchstack. I will do this rediffing
> > later, first want an easy-to-test and easy-to-inpect version.
>
> I'd suggest just merging this optimization into the original QRCU patch.
> Once you are happy with the validation, I'll add it to the plug branch
> as well.
>
> Version against the plug branch below.
Thank you very much!!!
One way or another, I will get you something in a form friendly to your
patch stack.
Thanx, Paul
> diff --git a/kernel/srcu.c b/kernel/srcu.c
> index 53c6989..bfe347a 100644
> --- a/kernel/srcu.c
> +++ b/kernel/srcu.c
> @@ -324,28 +324,53 @@ void synchronize_qrcu(struct qrcu_struct *qp)
> {
> int idx;
>
> + smp_mb(); /* Force preceding change to happen before fastpath check. */
> +
> /*
> - * The following memory barrier is needed to ensure that
> - * any prior data-structure manipulation is seen by other
> - * CPUs to happen before picking up the value of
> - * qp->completed.
> + * Fastpath: If the two counters sum to "1" at a given point in
> + * time, there are no readers. However, it takes two separate
> + * loads to sample both counters, which won't occur simultaneously.
> + * So we might race with a counter switch, so that we might see
> + * ctr[0]==0, then the counter might switch, then we might see
> + * ctr[1]==1 (unbeknownst to us because there is a reader still
> + * there). So we do a read memory barrier and recheck. If the
> + * same race happens again, there must have been a second counter
> + * switch. This second counter switch could not have happened
> + * until all preceding readers finished, so if the condition
> + * is true both times, we may safely proceed.
> + *
> + * This relies critically on the atomic increment and atomic
> + * decrement being seen as executing in order.
> */
> - smp_mb();
> +
> + if (atomic_read(&qp->ctr[0]) + atomic_read(&qp->ctr[1]) <= 1) {
> + smp_rmb(); /* Keep two checks independent. */
> + if (atomic_read(&qp->ctr[0]) + atomic_read(&qp->ctr[1]) <= 1)
> + goto out;
> + }
> +
> mutex_lock(&qp->mutex);
>
> idx = qp->completed & 0x1;
> if (atomic_read(qp->ctr + idx) == 1)
> - goto out;
> + goto out_unlock;
>
> atomic_inc(qp->ctr + (idx ^ 0x1));
> - /* Reduce the likelihood that qrcu_read_lock() will loop */
> +
> + /*
> + * Prevent subsequent decrement from being seen before previous
> + * increment -- such an inversion could cause the fastpath
> + * above to falsely conclude that there were no readers. Also,
> + * reduce the likelihood that qrcu_read_lock() will loop.
> + */
> smp_mb__after_atomic_inc();
> qp->completed++;
>
> atomic_dec(qp->ctr + idx);
> __wait_event(qp->wq, !atomic_read(qp->ctr + idx));
> -out:
> +out_unlock:
> mutex_unlock(&qp->mutex);
> +out:
> smp_mb();
> /*
> * The above smp_mb() is needed in the case that we
>
> --
> Jens Axboe
>
-
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