From: Will Deacon <will@kernel.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
Marco Elver <elver@google.com>,
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>,
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 v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting
Date: Fri, 22 Jul 2022 11:10:54 +0100 [thread overview]
Message-ID: <20220722101053.GA18284@willie-the-truck> (raw)
In-Reply-To: <CACT4Y+ZOXXqxhe4U3ZtQPCj2yrf6Qtjg1q0Kfq8+poAOxGgUew@mail.gmail.com>
On Fri, Jul 22, 2022 at 11:20:25AM +0200, Dmitry Vyukov wrote:
> On Fri, 22 Jul 2022 at 11:10, Will Deacon <will@kernel.org> wrote:
> > > [adding Will]
> > >
> > > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > > > Add KUnit test for hw_breakpoint constraints accounting, with various
> > > > interesting mixes of breakpoint targets (some care was taken to catch
> > > > interesting corner cases via bug-injection).
> > > >
> > > > The test cannot be built as a module because it requires access to
> > > > hw_breakpoint_slots(), which is not inlinable or exported on all
> > > > architectures.
> > > >
> > > > Signed-off-by: Marco Elver <elver@google.com>
> > >
> > > As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
> > > v5.19-rc7:
> > >
> > > | TAP version 14
> > > | 1..1
> > > | # Subtest: hw_breakpoint
> > > | 1..9
> > > | ok 1 - test_one_cpu
> > > | ok 2 - test_many_cpus
> > > | # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > | Expected IS_ERR(bp) to be false, but is true
> > > | not ok 3 - test_one_task_on_all_cpus
> > > | # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > | Expected IS_ERR(bp) to be false, but is true
> > > | not ok 4 - test_two_tasks_on_all_cpus
> > > | # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > | Expected IS_ERR(bp) to be false, but is true
> > > | not ok 5 - test_one_task_on_one_cpu
> > > | # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > | Expected IS_ERR(bp) to be false, but is true
> > > | not ok 6 - test_one_task_mixed
> > > | # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > | Expected IS_ERR(bp) to be false, but is true
> > > | not ok 7 - test_two_tasks_on_one_cpu
> > > | # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > | Expected IS_ERR(bp) to be false, but is true
> > > | not ok 8 - test_two_tasks_on_one_all_cpus
> > > | # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > | Expected IS_ERR(bp) to be false, but is true
> > > | not ok 9 - test_task_on_all_and_one_cpu
> > > | # hw_breakpoint: pass:2 fail:7 skip:0 total:9
> > > | # Totals: pass:2 fail:7 skip:0 total:9
> > >
> > > ... which seems to be becasue arm64 currently forbids per-task
> > > breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:
> > >
> > > /*
> > > * Disallow per-task kernel breakpoints since these would
> > > * complicate the stepping code.
> > > */
> > > if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
> > > return -EINVAL;
> > >
> > > ... which has been the case since day one in commit:
> > >
> > > 478fcb2cdb2351dc ("arm64: Debugging support")
> > >
> > > I'm not immediately sure what would be necessary to support per-task kernel
> > > breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> > > invasive.
> >
> > I would actually like to remove HW_BREAKPOINT completely for arm64 as it
> > doesn't really work and causes problems for other interfaces such as ptrace
> > and kgdb.
>
> Will it be a localized removal of code that will be easy to revert in
> future? Or will it touch lots of code here and there?
> Let's say we come up with a very important use case for HW_BREAKPOINT
> and will need to make it work on arm64 as well in future.
My (rough) plan is to implement a lower-level abstraction for handling the
underlying hardware resources, so we can layer consumers on top of that
instead of funneling through hw_breakpoint. So if we figure out how to make
bits of hw_breakpoint work on arm64, then it should just go on top.
The main pain point for hw_breakpoint is kernel-side {break,watch}points
and I think there are open design questions about how they should work
on arm64, particularly when considering the interaction with user
watchpoints triggering on uaccess routines and the possibility of hitting
a kernel watchpoint in irq context.
Will
next prev parent reply other threads:[~2022-07-22 10:11 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-04 15:05 [PATCH v3 00/14] perf/hw_breakpoint: Optimize for thousands of tasks Marco Elver
2022-07-04 15:05 ` [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting Marco Elver
2022-07-04 15:10 ` Dmitry Vyukov
2022-07-20 15:22 ` Ian Rogers
2022-07-21 16:22 ` Mark Rutland
2022-07-22 9:10 ` Will Deacon
2022-07-22 9:20 ` Dmitry Vyukov
2022-07-22 10:10 ` Will Deacon [this message]
2022-07-22 10:31 ` Dmitry Vyukov
2022-07-22 11:03 ` Will Deacon
2022-07-22 13:41 ` Dmitry Vyukov
2022-07-25 11:00 ` Marco Elver
2022-07-04 15:05 ` [PATCH v3 02/14] perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test Marco Elver
2022-07-04 15:09 ` Dmitry Vyukov
2022-07-20 15:22 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 03/14] perf/hw_breakpoint: Clean up headers Marco Elver
2022-07-20 15:23 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 04/14] perf/hw_breakpoint: Optimize list of per-task breakpoints Marco Elver
2022-07-20 15:29 ` Ian Rogers
2022-07-20 15:39 ` Marco Elver
2022-07-04 15:05 ` [PATCH v3 05/14] perf/hw_breakpoint: Mark data __ro_after_init Marco Elver
2022-07-20 15:30 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 06/14] perf/hw_breakpoint: Optimize constant number of breakpoint slots Marco Elver
2022-07-20 15:31 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 07/14] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable Marco Elver
2022-07-20 15:32 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 08/14] perf/hw_breakpoint: Remove useless code related to flexible breakpoints Marco Elver
2022-07-20 15:34 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 09/14] powerpc/hw_breakpoint: Avoid relying on caller synchronization Marco Elver
2022-07-20 15:35 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked() Marco Elver
2022-07-20 15:36 ` Ian Rogers
2022-08-17 12:47 ` Peter Zijlstra
2022-08-29 6:00 ` Marco Elver
2022-07-04 15:05 ` [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks Marco Elver
2022-07-20 15:38 ` Ian Rogers
2022-08-17 13:03 ` Peter Zijlstra
2022-08-17 13:14 ` Marco Elver
2022-08-29 8:38 ` Peter Zijlstra
2022-08-29 9:38 ` Marco Elver
2022-07-04 15:05 ` [PATCH v3 12/14] perf/hw_breakpoint: Introduce bp_slots_histogram Marco Elver
2022-07-20 15:40 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 13/14] perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets Marco Elver
2022-07-20 15:42 ` Ian Rogers
2022-07-04 15:05 ` [PATCH v3 14/14] perf/hw_breakpoint: Optimize toggle_bp_slot() " Marco Elver
2022-07-20 15:44 ` Ian Rogers
2022-07-12 13:39 ` [PATCH v3 00/14] perf/hw_breakpoint: Optimize for thousands of tasks Marco Elver
2022-07-20 15:47 ` Ian Rogers
2022-08-16 14:12 ` Marco Elver
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20220722101053.GA18284@willie-the-truck \
--to=will@kernel.org \
--cc=acme@kernel.org \
--cc=alexander.shishkin@linux.intel.com \
--cc=dvyukov@google.com \
--cc=elver@google.com \
--cc=frederic@kernel.org \
--cc=jolsa@redhat.com \
--cc=kasan-dev@googlegroups.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-perf-users@vger.kernel.org \
--cc=linux-sh@vger.kernel.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=mark.rutland@arm.com \
--cc=mingo@kernel.org \
--cc=mpe@ellerman.id.au \
--cc=namhyung@kernel.org \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).