Re: [PATCH 7/8] perf/hw_breakpoint: Optimize task_bp_pinned() if CPU-independent

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, 9 Jun 2022 at 13:31, Marco Elver <elver@google.com> wrote:
>
> Running the perf benchmark with (note: more aggressive parameters vs.
> preceding changes, but same host with 256 CPUs):
>
>  | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
>  |      Total time: 1.953 [sec]
>  |
>  |       38.146289 usecs/op
>  |     4882.725000 usecs/op/cpu
>
>     16.29%  [kernel]       [k] rhashtable_jhash2
>     16.19%  [kernel]       [k] osq_lock
>     14.22%  [kernel]       [k] queued_spin_lock_slowpath
>      8.58%  [kernel]       [k] task_bp_pinned
>      8.30%  [kernel]       [k] mutex_spin_on_owner
>      4.03%  [kernel]       [k] smp_cfm_core_cond
>      2.97%  [kernel]       [k] toggle_bp_slot
>      2.94%  [kernel]       [k] bcmp
>
> We can see that a majority of the time is now spent hashing task
> pointers to index into task_bps_ht in task_bp_pinned().
>
> However, if task_bp_pinned()'s computation is independent of any CPU,
> i.e. always `iter->cpu < 0`, the result for each invocation will be
> identical. With increasing CPU-count, this problem worsens.
>
> Instead, identify if every call to task_bp_pinned() is CPU-independent,
> and cache the result. Use the cached result instead of a call to
> task_bp_pinned(), now __task_bp_pinned(), with task_bp_pinned() deciding
> if the cached result can be used.
>
> After this optimization:
>
>     21.96%  [kernel]       [k] queued_spin_lock_slowpath
>     16.39%  [kernel]       [k] osq_lock
>      9.82%  [kernel]       [k] toggle_bp_slot
>      9.81%  [kernel]       [k] find_next_bit
>      4.93%  [kernel]       [k] mutex_spin_on_owner
>      4.71%  [kernel]       [k] smp_cfm_core_cond
>      4.30%  [kernel]       [k] __reserve_bp_slot
>      2.65%  [kernel]       [k] cpumask_next
>
> Showing that the time spent hashing keys has become insignificant.
>
> With the given benchmark parameters, however, we see no statistically
> significant improvement in performance on the test system with 256 CPUs.
> This is very likely due to the benchmark parameters being too aggressive
> and contention elsewhere becoming dominant.
>
> Indeed, when using the less aggressive parameters from the preceding
> changes, we now observe:
>
>  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
>  |      Total time: 0.071 [sec]
>  |
>  |       37.134896 usecs/op
>  |     2376.633333 usecs/op/cpu
>
> Which is an improvement of 12% compared to without this optimization
> (baseline is 42 usecs/op). This is now only 5% slower than the
> theoretical ideal (constraints disabled), and 18% slower than no
> breakpoints at all.
>
> [ While we're here, swap task_bp_pinned()'s bp and cpu arguments to be
>   more consistent with other functions (which have bp first, before the
>   cpu argument). ]

There are 3 main cases:
1. Per-cpu bp.
2. Per-task and per-cpu bp.
3. Per-task bp (on all cpus)
right?

For case 1 we still seem to do lots of unnecessary work in
fetch_bp_busy_slots() by iterating over all CPUs. We are going to bump
only the CPU's cpu_pinned, so that's the only CPU we need to
fetch/check.

For case 2 we also do lots of unnecessary work, again we also need to
check only 1 CPU (don't need cached_tbp_pinned). Also don't need to do
atomic_dec/inc on all other CPUs (they dec/inc the same variable).

Case 3 is the only one when we need to check all CPUs and
cached_tbp_pinned may be useful.
But I wonder if we could instead add a per-task
has_per_cpu_breakpoints flag. Then if the flag is set, we check all
CPUs as we do now (don't need cached_tbp_pinned). And if it's not set,
then we could optimize the code even more by making it O(1) instead of
O(N). Namely, we add global tsk_pinned for tasks that don't have
per-cpu breakpoints, and we update only that tsk_pinned instead of
iterating over all CPUs.
I think this will require adding cpu_pinned as well (similar to
tsk_pinned but aggregated over all CPUs).
Then the fast path capacity check can become just:

if (bp->hw.target && !bp->hw.target->has_per_cpu_breakpoints && bp->cpu < 0) {
  if (max_cpu_bp_pinned(type) + task_bp_pinned(-1 /*cpu*/, bp, type) +
hw_breakpoint_weight(bp) > nr_slots[type])
    return -ENOSPC;
}

Does it make any sense?