Re: [RFC PATCH] QRCU fastpath optimization

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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
>