[PATCH tip/core/rcu 0/5] SRCU updates

[PATCH tip/core/rcu 0/5] SRCU updates

[Paul E. McKenney]

Hello!

This series provides minor SRCU updates:

1.	Use __func__ instead of hard-coded function names.

2.	Prevent sdp->srcu_gp_seq_needed_exp counter wrap, courtesy of
	Ildar Ismagilov.

3.	Reduce scans of srcu_data in counter wrap check, courtesy of
	Ildar Ismagilov.

4.	Remove dead code in srcu_gp_end(), courtesy of Byungchul Park.

5.	Remove SRCU throttling.

							Thanx, Paul

------------------------------------------------------------------------

 srcutree.c |   17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

[PATCH tip/core/rcu 1/5] srcu: Abstract function name

[Paul E. McKenney]

This commit moves to __func__ for function names in the name of better
resilience to change.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/srcutree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index d5cea81378cc..01f4a8bf0e03 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -386,7 +386,7 @@ void cleanup_srcu_struct(struct srcu_struct *sp)
 		flush_delayed_work(&per_cpu_ptr(sp->sda, cpu)->work);
 	if (WARN_ON(rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
 	    WARN_ON(srcu_readers_active(sp))) {
-		pr_info("cleanup_srcu_struct: Active srcu_struct %p state: %d\n", sp, rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)));
+		pr_info("%s: Active srcu_struct %p state: %d\n", __func__, sp, rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)));
 		return; /* Caller forgot to stop doing call_srcu()? */
 	}
 	free_percpu(sp->sda);
-- 
2.5.2

[PATCH tip/core/rcu 2/5] srcu: Prevent sdp->srcu_gp_seq_needed_exp counter wrap

[Paul E. McKenney]

From: Ildar Ismagilov <devix84@gmail.com>

SRCU checks each srcu_data structure's grace-period number for counter
wrap four times per cycle by default.  This frequency guarantees that
normal comparisons will detect potential wrap.  However, the expedited
grace-period number is not checked.  The consquences are not too horrible
(a failure to expedite a grace period when requested), but it would be
good to avoid such things.  This commit therefore adds this check to
the expedited grace-period number.

Signed-off-by: Ildar Ismagilov <devix84@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/srcutree.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 01f4a8bf0e03..bb9607baad9b 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -579,6 +579,9 @@ static void srcu_gp_end(struct srcu_struct *sp)
 				if (ULONG_CMP_GE(gpseq,
 						 sdp->srcu_gp_seq_needed + 100))
 					sdp->srcu_gp_seq_needed = gpseq;
+				if (ULONG_CMP_GE(gpseq,
+						 sdp->srcu_gp_seq_needed_exp + 100))
+					sdp->srcu_gp_seq_needed_exp = gpseq;
 				spin_unlock_irqrestore_rcu_node(sdp, flags);
 			}
 	}
-- 
2.5.2

[PATCH tip/core/rcu 3/5] srcu: Reduce scans of srcu_data in counter wrap check

[Paul E. McKenney]

From: Ildar Ismagilov <devix84@gmail.com>

Currently, given a multi-level srcu_node tree, SRCU can scan the full
set of srcu_data structures at each level when cleaning up after a grace
period.  This, though harmless otherwise, represents pointless overhead.
This commit therefore eliminates this overhead by scanning the srcu_data
structures only when traversing the leaf srcu_node structures.

Signed-off-by: Ildar Ismagilov <devix84@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/srcutree.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index bb9607baad9b..d582a2352cdb 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -527,6 +527,7 @@ static void srcu_gp_end(struct srcu_struct *sp)
 {
 	unsigned long cbdelay;
 	bool cbs;
+	bool last_lvl;
 	int cpu;
 	unsigned long flags;
 	unsigned long gpseq;
@@ -559,7 +560,8 @@ static void srcu_gp_end(struct srcu_struct *sp)
 	rcu_for_each_node_breadth_first(sp, snp) {
 		spin_lock_irq_rcu_node(snp);
 		cbs = false;
-		if (snp >= sp->level[rcu_num_lvls - 1])
+		last_lvl = snp >= sp->level[rcu_num_lvls - 1];
+		if (last_lvl)
 			cbs = snp->srcu_have_cbs[idx] == gpseq;
 		snp->srcu_have_cbs[idx] = gpseq;
 		rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
@@ -572,7 +574,7 @@ static void srcu_gp_end(struct srcu_struct *sp)
 			srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);
 
 		/* Occasionally prevent srcu_data counter wrap. */
-		if (!(gpseq & counter_wrap_check))
+		if (!(gpseq & counter_wrap_check) && last_lvl)
 			for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
 				sdp = per_cpu_ptr(sp->sda, cpu);
 				spin_lock_irqsave_rcu_node(sdp, flags);
-- 
2.5.2

[PATCH tip/core/rcu 4/5] srcu: Remove dead code in srcu_gp_end()

[Paul E. McKenney]

From: Byungchul Park <byungchul.park@lge.com>

Of course, compilers will optimize out a dead code. Anyway, remove
any dead code for better readibility.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/srcutree.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index d582a2352cdb..f962f4428c39 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -532,7 +532,6 @@ static void srcu_gp_end(struct srcu_struct *sp)
 	unsigned long flags;
 	unsigned long gpseq;
 	int idx;
-	int idxnext;
 	unsigned long mask;
 	struct srcu_data *sdp;
 	struct srcu_node *snp;
@@ -556,7 +555,6 @@ static void srcu_gp_end(struct srcu_struct *sp)
 
 	/* Initiate callback invocation as needed. */
 	idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
-	idxnext = (idx + 1) % ARRAY_SIZE(snp->srcu_have_cbs);
 	rcu_for_each_node_breadth_first(sp, snp) {
 		spin_lock_irq_rcu_node(snp);
 		cbs = false;
-- 
2.5.2

[PATCH tip/core/rcu 5/5] rcu: Remove SRCU throttling

[Paul E. McKenney]

The code in srcu_gp_end() inserts a delay every 0x3ff grace periods in
order to prevent SRCU grace-period work from consuming an entire CPU
when there is a long sequence of expedited SRCU grace-period requests.
However, all of SRCU's grace-period work is carried out in workqueues,
which are in turn within kthreads, which are automatically throttled as
needed by the scheduler.  In particular, if there is plenty of idle time,
there is no point in throttling.

This commit therefore removes the expedited SRCU grace-period throttling.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/srcutree.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index f962f4428c39..e2796f79a597 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -596,9 +596,7 @@ static void srcu_gp_end(struct srcu_struct *sp)
 	    ULONG_CMP_LT(gpseq, sp->srcu_gp_seq_needed)) {
 		srcu_gp_start(sp);
 		spin_unlock_irq_rcu_node(sp);
-		/* Throttle expedited grace periods: Should be rare! */
-		srcu_reschedule(sp, rcu_seq_ctr(gpseq) & 0x3ff
-				    ? 0 : SRCU_INTERVAL);
+		srcu_reschedule(sp, 0);
 	} else {
 		spin_unlock_irq_rcu_node(sp);
 	}
-- 
2.5.2