Commit d21670ac authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: reduce the number of spurious RCU_SOFTIRQ invocations 

Lai Jiangshan noted that up to 10% of the RCU_SOFTIRQ are spurious, and
traced this down to the fact that the current grace-period machinery
will uselessly raise RCU_SOFTIRQ when a given CPU needs to go through
a quiescent state, but has not yet done so.  In this situation, there
might well be nothing that RCU_SOFTIRQ can do, and the overhead can be
worth worrying about in the ksoftirqd case.  This patch therefore avoids
raising RCU_SOFTIRQ in this situation.

Changes since v1 (http://lkml.org/lkml/2010/3/30/122

 from Lai Jiangshan):

o	Omit the rcu_qs_pending() prechecks, as they aren't that
	much less expensive than the quiescent-state checks.

o	Merge with the set_need_resched() patch that reduces IPIs.

o	Add the new n_rp_report_qs field to the rcu_pending tracing output.

o	Update the tracing documentation accordingly.

Signed-off-by: default avatarLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 4a90a068

Documentation/RCU/trace.txt

+19 −16
Original line number Diff line number Diff line
@@ -256,23 +256,23 @@ o Each element of the form "1/1 0:127 ^0" represents one struct 
The output of "cat rcu/rcu_pending" looks as follows:



rcu_sched:

  0 np=255892 qsp=53936 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741

  1 np=261224 qsp=54638 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792

  2 np=237496 qsp=49664 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629

  3 np=236249 qsp=48766 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723

  4 np=221310 qsp=46850 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110

  5 np=237332 qsp=48449 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456

  6 np=219995 qsp=46718 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834

  7 np=249893 qsp=49390 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888

  0 np=255892 qsp=53936 rpq=85 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741

  1 np=261224 qsp=54638 rpq=33 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792

  2 np=237496 qsp=49664 rpq=23 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629

  3 np=236249 qsp=48766 rpq=98 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723

  4 np=221310 qsp=46850 rpq=7 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110

  5 np=237332 qsp=48449 rpq=9 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456

  6 np=219995 qsp=46718 rpq=12 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834

  7 np=249893 qsp=49390 rpq=42 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888

rcu_bh:

  0 np=146741 qsp=1419 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314

  1 np=155792 qsp=12597 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180

  2 np=136629 qsp=18680 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936

  3 np=137723 qsp=2843 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863

  4 np=123110 qsp=12433 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671

  5 np=137456 qsp=4210 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235

  6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921

  7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542

  0 np=146741 qsp=1419 rpq=6 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314

  1 np=155792 qsp=12597 rpq=3 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180

  2 np=136629 qsp=18680 rpq=1 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936

  3 np=137723 qsp=2843 rpq=0 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863

  4 np=123110 qsp=12433 rpq=0 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671

  5 np=137456 qsp=4210 rpq=1 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235

  6 np=120834 qsp=9902 rpq=2 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921

  7 np=144888 qsp=26336 rpq=0 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542



As always, this is once again split into "rcu_sched" and "rcu_bh"

portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional
@@ -284,6 +284,9 @@ o "np" is the number of times that __rcu_pending() has been invoked 
o	"qsp" is the number of times that the RCU was waiting for a

	quiescent state from this CPU.



o	"rpq" is the number of times that the CPU had passed through

	a quiescent state, but not yet reported it to RCU.



o	"cbr" is the number of times that this CPU had RCU callbacks

	that had passed through a grace period, and were thus ready

	to be invoked.

kernel/rcutree.c

+6 −5
Original line number Diff line number Diff line
@@ -1161,8 +1161,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp) 
 */

void rcu_check_callbacks(int cpu, int user)

{

	if (!rcu_pending(cpu))

		return; /* if nothing for RCU to do. */

	if (user ||

	    (idle_cpu(cpu) && rcu_scheduler_active &&

	     !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
@@ -1194,6 +1192,7 @@ void rcu_check_callbacks(int cpu, int user) 
		rcu_bh_qs(cpu);

	}

	rcu_preempt_check_callbacks(cpu);

	if (rcu_pending(cpu))

		raise_softirq(RCU_SOFTIRQ);

}
@@ -1534,18 +1533,20 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp) 
	check_cpu_stall(rsp, rdp);



	/* Is the RCU core waiting for a quiescent state from this CPU? */

	if (rdp->qs_pending) {

	if (rdp->qs_pending && !rdp->passed_quiesc) {



		/*

		 * If force_quiescent_state() coming soon and this CPU

		 * needs a quiescent state, and this is either RCU-sched

		 * or RCU-bh, force a local reschedule.

		 */

		rdp->n_rp_qs_pending++;

		if (!rdp->preemptable &&

		    ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs) - 1,

				 jiffies))

			set_need_resched();

		rdp->n_rp_qs_pending++;

	} else if (rdp->qs_pending && rdp->passed_quiesc) {

		rdp->n_rp_report_qs++;

		return 1;

	}

kernel/rcutree.h

+1 −0
Original line number Diff line number Diff line
@@ -223,6 +223,7 @@ struct rcu_data { 
	/* 5) __rcu_pending() statistics. */

	unsigned long n_rcu_pending;	/* rcu_pending() calls since boot. */

	unsigned long n_rp_qs_pending;

	unsigned long n_rp_report_qs;

	unsigned long n_rp_cb_ready;

	unsigned long n_rp_cpu_needs_gp;

	unsigned long n_rp_gp_completed;

kernel/rcutree_trace.c

+3 −1
Original line number Diff line number Diff line
@@ -241,11 +241,13 @@ static const struct file_operations rcugp_fops = { 
static void print_one_rcu_pending(struct seq_file *m, struct rcu_data *rdp)

{

	seq_printf(m, "%3d%cnp=%ld "

		   "qsp=%ld cbr=%ld cng=%ld gpc=%ld gps=%ld nf=%ld nn=%ld\n",

		   "qsp=%ld rpq=%ld cbr=%ld cng=%ld "

		   "gpc=%ld gps=%ld nf=%ld nn=%ld\n",

		   rdp->cpu,

		   cpu_is_offline(rdp->cpu) ? '!' : ' ',

		   rdp->n_rcu_pending,

		   rdp->n_rp_qs_pending,

		   rdp->n_rp_report_qs,

		   rdp->n_rp_cb_ready,

		   rdp->n_rp_cpu_needs_gp,

		   rdp->n_rp_gp_completed,

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