HACK/WIP: locking/atomic: allow statistics to be collected for atomics
When analysing some workloads it can be helpful to gather statistics for
the various atomic/atomic64/atomic_long operations, such as the number
of times an atomic is used, or the number of retries required by cmpxchg
loops.
This patch adds some basic statistics for atomic operations, which are
exposed in aggregate under debugfs.
To minimize cross-cpu contention on shared counters, per-cpu buckets are
used. CPUs are allowed to write to other CPUs' buckets (and e.g. may do
so if preempted), but in the common case this should minimize
contention.
This is *VERY* incomplete, and is not intended for use in any production
environment. Enabling these statistics *WILL* pertub workloads.
For example, shortly after booting a VM, the top 10 atomics can be found
with:
| # mount -t debugfs none /sys/kernel/debug/
| # cat /sys/kernel/debug/atomic_stats | (head -n 1; sort -k 2 -rn) | head -n 11
| operation used fail loop
| atomic_long_set 250053 0 0
| atomic_try_cmpxchg_acquire 231703 161 0
| atomic_long_try_cmpxchg_acquire 124637 8 0
| atomic_dec_and_test 117357 105789 0
| atomic_long_fetch_add_release 104879 0 0
| atomic_long_read 74008 0 0
| atomic_add_unless 46777 155 0
| atomic_add_negative 41078 35447 0
| atomic_inc_and_test 40172 36428 0
| atomic_long_add 15050 0 0
TODO: implement loop counting.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
10 files changed
