Commit 3afe7fa5 authored by Joel Fernandes (Google)'s avatar Joel Fernandes (Google) Committed by Paul E. McKenney
Browse files

rcu/trace: Add tracing for how segcb list changes 



This commit adds tracing to track how the segcb list changes before/after
acceleration, during queuing and during dequeuing.

This tracing helped discover an optimization that avoided needless GP
requests when no callbacks were accelerated. The tracing overhead is
minimal as each segment's length is now stored in the respective segment.

Reviewed-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Reviewed-by: default avatarNeeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 68804cf1

include/trace/events/rcu.h

+26 −0
Original line number Diff line number Diff line
@@ -505,6 +505,32 @@ TRACE_EVENT_RCU(rcu_callback, 
		  __entry->qlen)

);



TRACE_EVENT_RCU(rcu_segcb_stats,



		TP_PROTO(struct rcu_segcblist *rs, const char *ctx),



		TP_ARGS(rs, ctx),



		TP_STRUCT__entry(

			__field(const char *, ctx)

			__array(unsigned long, gp_seq, RCU_CBLIST_NSEGS)

			__array(long, seglen, RCU_CBLIST_NSEGS)

		),



		TP_fast_assign(

			__entry->ctx = ctx;

			memcpy(__entry->seglen, rs->seglen, RCU_CBLIST_NSEGS * sizeof(long));

			memcpy(__entry->gp_seq, rs->gp_seq, RCU_CBLIST_NSEGS * sizeof(unsigned long));



		),



		TP_printk("%s seglen: (DONE=%ld, WAIT=%ld, NEXT_READY=%ld, NEXT=%ld) "

			  "gp_seq: (DONE=%lu, WAIT=%lu, NEXT_READY=%lu, NEXT=%lu)", __entry->ctx,

			  __entry->seglen[0], __entry->seglen[1], __entry->seglen[2], __entry->seglen[3],

			  __entry->gp_seq[0], __entry->gp_seq[1], __entry->gp_seq[2], __entry->gp_seq[3])



);



/*

 * Tracepoint for the registration of a single RCU callback of the special

 * kvfree() form.  The first argument is the RCU type, the second argument

kernel/rcu/tree.c

+9 −0
Original line number Diff line number Diff line
@@ -1495,6 +1495,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) 
	if (!rcu_segcblist_pend_cbs(&rdp->cblist))

		return false;



	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPreAcc"));



	/*

	 * Callbacks are often registered with incomplete grace-period

	 * information.  Something about the fact that getting exact
@@ -1515,6 +1517,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) 
	else

		trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccReadyCB"));



	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPostAcc"));



	return ret;

}
@@ -2471,11 +2475,14 @@ static void rcu_do_batch(struct rcu_data *rdp) 
	rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);

	if (offloaded)

		rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);



	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbDequeued"));

	rcu_nocb_unlock_irqrestore(rdp, flags);



	/* Invoke callbacks. */

	tick_dep_set_task(current, TICK_DEP_BIT_RCU);

	rhp = rcu_cblist_dequeue(&rcl);



	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {

		rcu_callback_t f;
@@ -2987,6 +2994,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func) 
		trace_rcu_callback(rcu_state.name, head,

				   rcu_segcblist_n_cbs(&rdp->cblist));



	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));



	/* Go handle any RCU core processing required. */

	if (unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {

		__call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */