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From: James Clark <james.clark@arm.com>
To: Will Deacon <will@kernel.org>, German Gomez <german.gomez@arm.com>
Cc: Leo Yan <leo.yan@linaro.org>,
	linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	John Garry <john.garry@huawei.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Mike Leach <mike.leach@linaro.org>,
	linux-arm-kernel@lists.infradead.org, coresight@lists.linaro.org
Subject: Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback
Date: Tue, 12 Oct 2021 09:47:27 +0100	[thread overview]
Message-ID: <55f8978f-e656-5d5f-94e2-f4be4a70656d@arm.com> (raw)
In-Reply-To: <20211012081948.GA5156@willie-the-truck>



On 12/10/2021 09:19, Will Deacon wrote:
> On Mon, Oct 11, 2021 at 04:55:37PM +0100, German Gomez wrote:
>> Hi Leo,
>>
>> On 06/10/2021 10:51, Leo Yan wrote:
>>> On Wed, Oct 06, 2021 at 10:35:20AM +0100, German Gomez wrote:
>>>
>>> [...]
>>>
>>>>> So simply say, I think the head pointer monotonically increasing is
>>>>> the right thing to do in Arm SPE driver.
>>>> I will talk to James about how we can proceed on this.
>>> Thanks!
>>
>> I took this offline with James and, though it looks possible to patch
>> the SPE driver to have a monotonically increasing head pointer in order
>> to simplify the handling in the perf tool, it could be a breaking change
>> for users of the perf_event_open syscall that currently rely on the way
>> it works now.
>>
>> An alternative way we considered to simplify the patch is to change the
>> logic inside the find_snapshot callback so that it records the entire
>> contents of the aux buffer every time.
>>
>> What do you think?
> 
> What does intel-pt do?

Intel-pt has a wrapped head, which is why it has the intel_pt_find_snapshot()
function in perf to try to not save any zeros from the buffer that haven't
been written yet. (With a wrapped head pointer it's impossible to tell).

Coresight has a monotonically increasing head pointer so it is possible to
tell. Recently, Leo removed the Coresight version of find_snapshot() for this
reason.

It would be nice to do the same for SPE because that function has a heuristic
and is also slow, but I imagine that not returning wrapped head pointers could
break anything that expects them.

James
 
> 
> Will
> 

  reply	other threads:[~2021-10-12  8:47 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-16 15:46 [PATCH 1/5] perf cs-etm: Print size using consistent format German Gomez
2021-09-16 15:46 ` [PATCH 2/5] perf arm-spe: " German Gomez
2021-09-23 13:35   ` Leo Yan
2021-09-16 15:46 ` [PATCH 3/5] perf arm-spe: Add snapshot mode support German Gomez
2021-10-20 12:48   ` Leo Yan
2021-09-16 15:46 ` [PATCH 4/5] perf arm-spe: Implement find_snapshot callback German Gomez
2021-09-23 13:50   ` Leo Yan
2021-09-23 14:40     ` Leo Yan
2021-09-30 12:26       ` German Gomez
2021-10-04 12:27         ` Leo Yan
2021-10-06  9:35           ` German Gomez
2021-10-06  9:51             ` Leo Yan
2021-10-11 15:55               ` German Gomez
2021-10-12  8:19                 ` Will Deacon
2021-10-12  8:47                   ` James Clark [this message]
2021-10-13  0:39                 ` Leo Yan
2021-10-13  7:51                   ` Will Deacon
2021-10-15 12:33                     ` German Gomez
2021-10-15 14:16                       ` Leo Yan
2021-10-15 14:41                         ` German Gomez
2021-10-17  6:13                       ` Leo Yan
2021-10-19  9:23                         ` German Gomez
2021-10-19 13:12                           ` Leo Yan
2021-11-02 11:02                         ` German Gomez
2021-10-17 12:05   ` Leo Yan
2021-10-17 12:36     ` Leo Yan
2021-10-19 17:34     ` German Gomez
2021-10-20 13:25       ` Leo Yan
2021-09-16 15:46 ` [PATCH 5/5] perf arm-spe: Snapshot mode test German Gomez
2021-10-20 13:13   ` Leo Yan
2021-10-20 15:06     ` German Gomez
2021-11-02 14:07     ` James Clark
2021-11-02 15:37       ` James Clark
2021-11-09 13:26         ` German Gomez
2021-09-23 13:35 ` [PATCH 1/5] perf cs-etm: Print size using consistent format Leo Yan
2021-09-23 16:24 ` Mathieu Poirier
2021-09-30 12:09   ` German Gomez
2021-09-30 16:30     ` Mathieu Poirier

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