Re: [PATCH v2 13/13] perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets

[Marco Elver <elver@google.com>]

On Tue, 28 Jun 2022 at 17:45, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@google.com> wrote:
> >
> > We can still see that a majority of the time is spent hashing task pointers:
> >
> >     ...
> >     16.98%  [kernel]       [k] rhashtable_jhash2
> >     ...
> >
> > Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus),
> > calling task_bp_pinned() for each CPU, even if task_bp_pinned() is
> > CPU-independent. The reason for this is to update the per-CPU
> > 'tsk_pinned' histogram.
> >
> > To optimize the CPU-independent case to O(1), keep a separate
> > CPU-independent 'tsk_pinned_all' histogram.
> >
> > The major source of complexity are transitions between "all
> > CPU-independent task breakpoints" and "mixed CPU-independent and
> > CPU-dependent task breakpoints". The code comments list all cases that
> > require handling.
> >
> > After this optimization:
> >
> >  | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
> >  |      Total time: 1.758 [sec]
> >  |
> >  |       34.336621 usecs/op
> >  |     4395.087500 usecs/op/cpu
> >
> >     38.08%  [kernel]       [k] queued_spin_lock_slowpath
> >     10.81%  [kernel]       [k] smp_cfm_core_cond
> >      3.01%  [kernel]       [k] update_sg_lb_stats
> >      2.58%  [kernel]       [k] osq_lock
> >      2.57%  [kernel]       [k] llist_reverse_order
> >      1.45%  [kernel]       [k] find_next_bit
> >      1.21%  [kernel]       [k] flush_tlb_func_common
> >      1.01%  [kernel]       [k] arch_install_hw_breakpoint
> >
> > Showing that the time spent hashing keys has become insignificant.
> >
> > With the given benchmark parameters, that's an improvement of 12%
> > compared with the old O(#cpus) version.
> >
> > And finally, using the less aggressive parameters from the preceding
> > changes, we now observe:
> >
> >  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
> >  |      Total time: 0.067 [sec]
> >  |
> >  |       35.292187 usecs/op
> >  |     2258.700000 usecs/op/cpu
> >
> > Which is an improvement of 12% compared to without the histogram
> > optimizations (baseline is 40 usecs/op). This is now on par with the
> > theoretical ideal (constraints disabled), and only 12% slower than no
> > breakpoints at all.
> >
> > Signed-off-by: Marco Elver <elver@google.com>
>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
>
> I don't see any bugs. But the code is quite complex. Does it make
> sense to add some asserts to the histogram type? E.g. counters don't
> underflow, weight is not negative (e.g. accidentally added -1 returned
> from task_bp_pinned()). Not sure if it will be enough to catch all
> types of bugs, though.
> Could kunit tests check that histograms are all 0's at the end?
>
> I am not just about the current code (which may be correct), but also
> future modifications to this code.

I'll think of some more options.

bp_slots_histogram_max*() already has asserts (WARN about underflow;
some with KCSAN help).

The main thing I did to raise my own confidence in the code is inject
bugs and see if the KUnit test catches it. If it didn't I extended the
tests. I'll do that some more maybe.