Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH 1/2] locking/qspinlock_stat: Count instances of nested lock slowpaths | Date | Tue, 16 Oct 2018 09:45:06 -0400 |
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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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