[tip:locking/core] locking/qspinlock_stat: Count instances of nested lock slowpaths

[tip-bot for Waiman Long <tipbot@zytor.com>]

Commit-ID:  1222109a53637f96c581224198b86856d503f892
Gitweb:     https://git.kernel.org/tip/1222109a53637f96c581224198b86856d503f892
Author:     Waiman Long <longman@redhat.com>
AuthorDate: Tue, 16 Oct 2018 09:45:06 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 17 Oct 2018 08:37:31 +0200

locking/qspinlock_stat: Count instances of nested lock slowpaths

Queued spinlock supports up to 4 levels of lock slowpath nesting -
user context, soft IRQ, hard IRQ and NMI. However, we are not sure how
often the nesting happens.

So add 3 more per-CPU stat counters to track the number of instances where
nesting index goes to 1, 2 and 3 respectively.

On a dual-socket 64-core 128-thread Zen server, the following were the
new stat counter values under different circumstances:

         State                         slowpath   index1   index2   index3
         -----                         --------   ------   ------   -------
  After bootup                         1,012,150    82       0        0
  After parallel build + perf-top    125,195,009    82       0        0

So the chance of having more than 2 levels of nesting is extremely low.

[ mingo: Minor changelog edits. ]

Signed-off-by: Waiman Long <longman@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1539697507-28084-1-git-send-email-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/qspinlock.c      | 5 +++++
 kernel/locking/qspinlock_stat.h | 6 ++++++
 2 files changed, 11 insertions(+)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 341ca666bc60..ce6af1ee2cac 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -396,6 +396,11 @@ pv_queue:
 
 	node += idx;
 
+	/*
+	 * Keep counts of non-zero index values:
+	 */
+	qstat_inc(qstat_lock_idx1 + idx - 1, idx);
+
 	/*
 	 * Ensure that we increment the head node->count before initialising
 	 * the actual node. If the compiler is kind enough to reorder these
diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index 6bd78c0740fc..42d3d8dc8f49 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -55,6 +55,9 @@ enum qlock_stats {
 	qstat_pv_wait_node,
 	qstat_lock_pending,
 	qstat_lock_slowpath,
+	qstat_lock_idx1,
+	qstat_lock_idx2,
+	qstat_lock_idx3,
 	qstat_num,	/* Total number of statistical counters */
 	qstat_reset_cnts = qstat_num,
 };
@@ -82,6 +85,9 @@ static const char * const qstat_names[qstat_num + 1] = {
 	[qstat_pv_wait_node]       = "pv_wait_node",
 	[qstat_lock_pending]       = "lock_pending",
 	[qstat_lock_slowpath]      = "lock_slowpath",
+	[qstat_lock_idx1]	   = "lock_index1",
+	[qstat_lock_idx2]	   = "lock_index2",
+	[qstat_lock_idx3]	   = "lock_index3",
 	[qstat_reset_cnts]         = "reset_counters",
 };