From: Marco Elver <elver@google.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Frederic Weisbecker <frederic@kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
Michael Ellerman <mpe@ellerman.id.au>,
linuxppc-dev@lists.ozlabs.org, linux-perf-users@vger.kernel.org,
x86@kernel.org, linux-sh@vger.kernel.org,
kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 13/13] perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets
Date: Tue, 28 Jun 2022 18:00:34 +0200 [thread overview]
Message-ID: <CANpmjNOh9gzzC7sOOOk1q7Ssj2dFxczj1bmufarYS2KupZQthg@mail.gmail.com> (raw)
In-Reply-To: <CACT4Y+aJZzkYHc+YJRApOLG-NYe8zXMaqxpQgQQFAy5WY97Ttg@mail.gmail.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.
prev parent reply other threads:[~2022-06-28 16:02 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-28 9:58 [PATCH v2 00/13] perf/hw_breakpoint: Optimize for thousands of tasks Marco Elver
2022-06-28 9:58 ` [PATCH v2 01/13] perf/hw_breakpoint: Add KUnit test for constraints accounting Marco Elver
2022-06-28 12:53 ` Dmitry Vyukov
2022-06-28 13:26 ` Marco Elver
2022-06-28 9:58 ` [PATCH v2 02/13] perf/hw_breakpoint: Clean up headers Marco Elver
2022-06-28 9:58 ` [PATCH v2 03/13] perf/hw_breakpoint: Optimize list of per-task breakpoints Marco Elver
2022-06-28 13:08 ` Dmitry Vyukov
2022-06-28 14:53 ` Marco Elver
2022-06-28 15:27 ` Dmitry Vyukov
2022-06-28 9:58 ` [PATCH v2 04/13] perf/hw_breakpoint: Mark data __ro_after_init Marco Elver
2022-06-28 9:58 ` [PATCH v2 05/13] perf/hw_breakpoint: Optimize constant number of breakpoint slots Marco Elver
2022-06-28 9:58 ` [PATCH v2 06/13] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable Marco Elver
2022-06-28 13:16 ` Dmitry Vyukov
2022-06-28 9:58 ` [PATCH v2 07/13] perf/hw_breakpoint: Remove useless code related to flexible breakpoints Marco Elver
2022-06-28 13:18 ` Dmitry Vyukov
2022-06-28 9:58 ` [PATCH v2 08/13] powerpc/hw_breakpoint: Avoid relying on caller synchronization Marco Elver
2022-06-28 13:21 ` Dmitry Vyukov
2022-07-01 8:54 ` Christophe Leroy
2022-07-01 9:41 ` Marco Elver
2022-07-01 10:15 ` Christophe Leroy
2022-06-28 9:58 ` [PATCH v2 09/13] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked() Marco Elver
2022-06-28 14:44 ` Dmitry Vyukov
2022-06-28 9:58 ` [PATCH v2 10/13] perf/hw_breakpoint: Reduce contention with large number of tasks Marco Elver
2022-06-28 14:45 ` Dmitry Vyukov
2022-06-28 9:58 ` [PATCH v2 11/13] perf/hw_breakpoint: Introduce bp_slots_histogram Marco Elver
2022-06-28 14:52 ` Dmitry Vyukov
2022-06-28 9:58 ` [PATCH v2 12/13] perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets Marco Elver
2022-06-28 15:41 ` Dmitry Vyukov
2022-06-28 9:58 ` [PATCH v2 13/13] perf/hw_breakpoint: Optimize toggle_bp_slot() " Marco Elver
2022-06-28 10:54 ` Marco Elver
2022-06-28 15:45 ` Dmitry Vyukov
2022-06-28 16:00 ` Marco Elver [this message]
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