From: Kyle Huey <me@kylehuey.com>
To: "Jin, Yao" <yao.jin@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
stable@vger.kernel.org,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Arnaldo Carvalho de Melo <acme@redhat.com>,
Jiri Olsa <jolsa@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Namhyung Kim <namhyung@kernel.org>,
Stephane Eranian <eranian@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Vince Weaver <vincent.weaver@maine.edu>,
acme@kernel.org, jolsa@kernel.org, kan.liang@intel.com,
Mark Rutland <mark.rutland@arm.com>,
Will Deacon <will.deacon@arm.com>,
gregkh@linuxfoundation.org,
"Robert O'Callahan" <robert@ocallahan.org>,
open list <linux-kernel@vger.kernel.org>,
yao.jin@intel.com
Subject: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region
Date: Wed, 28 Jun 2017 00:30:25 -0700 [thread overview]
Message-ID: <CAP045ArFWEoQd43gSFo7rSm1gB8-mhiAg24FopuUkU4zm-M3AA@mail.gmail.com> (raw)
In-Reply-To: <d66c1793-8498-b27b-8473-1685293550b4@linux.intel.com>
On Tue, Jun 27, 2017 at 10:35 PM, Jin, Yao <yao.jin@linux.intel.com> wrote:
> Hi Kyle,
>
> I understand your requirement. Sorry I don't know the detail of rr debugger,
> but I guess if it just uses counter overflow to deliver a signal, could it
> set the counter without "exclude_kernel"?
Unfortunately we cannot. We depend on the counter value being exactly
the same between recording and replay, and dropping `exclude_kernel`
would introduce non-determinism.
> Another consideration is, I'm not sure if the kernel can accurately realize
> that if the interrupt is to trigger sampling or just deliver a signal.
> Userspace may know that but kernel may not.
After looking at this code a bit more, I think that changing the
`is_sample_allowed` check from an early return to a guard around the
invocation of `overflow_handler` would fix this. I believe, but have
not tested, that `perf_event_fasync` is what must run to deliver our
signal, while the `overflow_handler` is what copies the kernel RIP/etc
into the output buffer that you want to skip.
- Kyle
> Anyway let's listen to more comments from community.
>
> Thanks
>
> Jin Yao
>
>
>
> On 6/28/2017 12:51 PM, Kyle Huey wrote:
>>
>> On Tue, Jun 27, 2017 at 7:09 PM, Jin, Yao <yao.jin@linux.intel.com> wrote:
>>>
>>> Hi,
>>>
>>> In theory, the PMI interrupts in skid region should be dropped, right?
>>
>> No, why would they be dropped?
>>
>> My understanding of the situation is as follows:
>>
>> There is some time, call it t_0, where the hardware counter overflows.
>> The PMU triggers an interrupt, but this is not instantaneous. Call
>> the time when the interrupt is actually delivered t_1. Then t_1 - t_0
>> is the "skid".
>>
>> Note that if the counter is `exclude_kernel`, then at t_0 the CPU
>> *must* be running a userspace program. But by t_1, the CPU may be
>> doing something else. Your patch changed things so that if at t_1 the
>> CPU is in the kernel, then the interrupt is discarded. But rr has
>> programmed the counter to deliver a signal on overflow (via F_SETSIG
>> on the fd returned by perf_event_open). This change results in the
>> signal never being delivered, because the interrupt was ignored.
>> (More accurately, the signal is delivered the *next* time the counter
>> overflows, which is far past where we wanted to inject our
>> asynchronous event into our tracee.
>>
>> It seems to me that it might be reasonable to ignore the interrupt if
>> the purpose of the interrupt is to trigger sampling of the CPUs
>> register state. But if the interrupt will trigger some other
>> operation, such as a signal on an fd, then there's no reason to drop
>> it.
>>
>>> For a userspace debugger, is it the only choice that relies on the *skid*
>>> PMI interrupt?
>>
>> I don't understand this question, but hopefully the above clarified
>> things.
>>
>> - Kyle
>>
>>> Thanks
>>> Jin Yao
>>>
>>>
>>> On 6/28/2017 9:01 AM, Kyle Huey wrote:
>>>>
>>>> Sent again with LKML CCd, sorry for the noise.
>>>>
>>>> - Kyle
>>>>
>>>> On Tue, Jun 27, 2017 at 5:38 PM, Kyle Huey <me@kylehuey.com> wrote:
>>>>>
>>>>> cc1582c231ea introduced a regression in v4.12.0-rc5, and appears to be
>>>>> a candidate for backporting to stable branches.
>>>>>
>>>>> rr, a userspace record and replay debugger[0], uses the PMU interrupt
>>>>> to stop a program during replay to inject asynchronous events such as
>>>>> signals. We are counting retired conditional branches in userspace
>>>>> only. This changeset causes the kernel to drop interrupts on the
>>>>> floor if, during the PMU interrupt's "skid" region, the CPU enters
>>>>> kernel mode for whatever reason. When replaying traces of complex
>>>>> programs such as Firefox, we intermittently fail to deliver
>>>>> asynchronous events on time, leading the replay to diverge from the
>>>>> recorded state.
>>>>>
>>>>> It seems like this change should, at a bare minimum, be limited to
>>>>> counters that actually perform sampling of register state when the
>>>>> interrupt fires. In our case, with the retired conditional branches
>>>>> counter restricted to counting userspace events only, it makes no
>>>>> difference that the PMU interrupt happened to be delivered in the
>>>>> kernel.
>>>>>
>>>>> As this makes rr unusable on complex applications and cannot be
>>>>> efficiently worked around, we would appreciate this being addressed
>>>>> before 4.12 is finalized, and the regression not being introduced to
>>>>> stable branches.
>>>>>
>>>>> Thanks,
>>>>>
>>>>> - Kyle
>>>>>
>>>>> [0] http://rr-project.org/
>>>
>>>
>
next prev parent reply other threads:[~2017-06-28 7:30 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAP045Ap8cMx6mzSgcQ3n3bnh_8GJuCp7_KZe_5ZTCR_K6cPTLw@mail.gmail.com>
2017-06-28 1:01 ` [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region Kyle Huey
2017-06-28 2:09 ` Jin, Yao
2017-06-28 4:51 ` Kyle Huey
2017-06-28 5:35 ` Jin, Yao
2017-06-28 7:30 ` Kyle Huey [this message]
2017-06-28 10:12 ` Mark Rutland
2017-06-28 10:56 ` [PATCH] perf/core: generate overflow signal when samples are dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region) Mark Rutland
2017-06-28 12:40 ` Vince Weaver
2017-06-28 13:07 ` Mark Rutland
2017-06-29 8:13 ` Alexey Budankov
2017-06-29 8:25 ` Alexey Budankov
2017-06-28 16:48 ` Kyle Huey
2017-06-28 17:49 ` Mark Rutland
2017-06-28 22:55 ` Kyle Huey
2017-06-29 0:27 ` Jin, Yao
2017-06-30 17:44 ` Kyle Huey
2017-07-04 9:03 ` Peter Zijlstra
2017-07-04 9:33 ` Mark Rutland
2017-07-04 9:46 ` Peter Zijlstra
2017-07-04 10:21 ` Mark Rutland
2017-07-06 5:07 ` Robert O'Callahan
2017-07-11 2:03 ` Kyle Huey
2017-07-11 9:03 ` Ingo Molnar
2017-07-11 13:07 ` Jin, Yao
2017-07-12 7:57 ` Ingo Molnar
2017-07-11 14:26 ` Mark Rutland
2017-07-11 15:32 ` Kyle Huey
2017-07-18 0:07 ` Kyle Huey
2017-06-29 8:12 ` Ingo Molnar
2017-07-04 9:06 ` Peter Zijlstra
2017-07-04 10:04 ` Ingo Molnar
2017-07-11 9:03 ` [tip:perf/urgent] Revert "perf/core: Drop kernel samples even though :u is specified" tip-bot for Ingo Molnar
2017-06-28 16:46 ` [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region Kyle Huey
2017-06-28 17:19 ` Mark Rutland
2017-06-28 17:36 ` Kyle Huey
2017-06-28 17:52 ` Mark Rutland
2017-06-28 17:48 ` Robert O'Callahan
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