linux-kernel - [PATCH 1/2] locking/qspinlock_stat: Count instances of nested lock slowpaths

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Date:   Tue, 16 Oct 2018 09:45:06 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: [PATCH 1/2] 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 3 more per-cpu stat counters are added
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 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 needing more than 2 levels of nesting is extremely rare.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 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 bfaeb05..7099bbe 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -378,6 +378,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	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
 	 * stores, then an IRQ could overwrite our assignments.
diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index 6bd78c0..42d3d8d 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 @@ enum qlock_stats {
 	[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",
 };
 
-- 
1.8.3.1