Re: [PATCH] net: sch: eliminate unnecessary RCU waits in mini_qdisc_pair_swap()

On Mon, Oct 25, 2021 at 12:48:28PM -0700, Jakub Kicinski wrote:

On Fri, 22 Oct 2021 11:17:46 -0500 Seth Forshee wrote:

quoted

From: Seth Forshee <redacted>

Currently rcu_barrier() is used to ensure that no readers of the
inactive mini_Qdisc buffer remain before it is reused. This waits for
any pending RCU callbacks to complete, when all that is actually
required is to wait for one RCU grace period to elapse after the buffer
was made inactive. This means that using rcu_barrier() may result in
unnecessary waits.

To improve this, store the current RCU state when a buffer is made
inactive and use poll_state_synchronize_rcu() to check whether a full
grace period has elapsed before reusing it. If a full grace period has
not elapsed, wait for a grace period to elapse, and in the non-RT case
use synchronize_rcu_expedited() to hasten it.

Since this approach eliminates the RCU callback it is no longer
necessary to synchronize_rcu() in the tp_head==NULL case. However, the
RCU state should still be saved for the previously active buffer.

Before this change I would typically see mini_qdisc_pair_swap() take
tens of milliseconds to complete. After this change it typcially
finishes in less than 1 ms, and often it takes just a few microseconds.

Thanks to Paul for walking me through the options for improving this.

Cc: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Seth Forshee <redacted>

LGTM, but please rebase and retest on top of latest net-next.

Will do.

quoted

 void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
 			  struct tcf_proto *tp_head)
 {

@@ -1423,28 +1419,30 @@ void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
 
 	if (!tp_head) {
 		RCU_INIT_POINTER(*miniqp->p_miniq, NULL);
-		/* Wait for flying RCU callback before it is freed. */
-		rcu_barrier();
-		return;
-	}
+	} else {
+		miniq = !miniq_old || miniq_old == &miniqp->miniq2 ?
+			&miniqp->miniq1 : &miniqp->miniq2;
 
-	miniq = !miniq_old || miniq_old == &miniqp->miniq2 ?
-		&miniqp->miniq1 : &miniqp->miniq2;

nit: any reason this doesn't read:

	miniq = miniq_old != &miniqp->miniq1 ? 
		&miniqp->miniq1 : &miniqp->miniq2;

Surely it's not equal to miniq1 or miniq2 if it's NULL.

I agree, that looks simpler and functionally equivalent. It seems
off-topic for this patch though; I'm only touching that line to change
the indentation.

Thanks,
Seth