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Subject[PATCH 1/2] locking/qspinlock_stat: Count instances of nested lock slowpaths
Date
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@redhat.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
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