linux-kernel - Re: [PATCH v3] perf: map pages in advance

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Message-ID: <e489e48d-f9ef-453d-9b88-91368b9534d7@redhat.com>
Date: Fri, 20 Dec 2024 22:53:14 +0100
From: David Hildenbrand <david@...hat.com>
To: "Chen, Zide" <zide.chen@...el.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Kan Liang <kan.liang@...ux.intel.com>, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Matthew Wilcox <willy@...radead.org>, Yi Lai <yi1.lai@...ux.intel.com>
Subject: Re: [PATCH v3] perf: map pages in advance

On 20.12.24 22:29, David Hildenbrand wrote:
> On 20.12.24 20:36, Chen, Zide wrote:
>>
>>
>> On 12/20/2024 1:56 AM, David Hildenbrand wrote:
>>> On 20.12.24 10:31, Lorenzo Stoakes wrote:
>>>> On Thu, Dec 19, 2024 at 01:17:44PM -0800, Chen, Zide wrote:
>>>>
>>>>> With this patch, it seems perf tool has some problems in capturing the
>>>>> kernel data with Intel PT.
>>>>>
>>>>> Running the following commands, the size of perf.data is very small, and
>>>>> perf script can't find any valid records.
>>>>>
>>>>> perf record -e intel_pt//u -- /bin/ls
>>>>> perf script --insn-trace
>>>>>
>>>>>
>>>>
>>>> Hi,
>>>>
>>>> I'm on leave (and should really go back to relaxing :>), returning on 2nd
>>>> Jan so can't really dig into this.
>>>>
>>>> But I tried it on my intel box and it 'works on my machine' with and
>>>> without patch with commands provided, so I'm not sure this is actually a
>>>> product of this change (which shouldn't impact this).
>>>
>>> Zide Chen, can you try with and without this patch to see if it
>>> introduces the issue?
>>
>> Yes, I re-did the test on a SPR server, and the result is same. Without
>> the patch, it went well; But with it, "perf script --insn-trace" doesn't
>> show valid records.
>>
>> This time I tested it on the clean 6.13-rc1 tag, base commit
>> 40384c840ea1944d7c5a392e8975ed088ecf0b37
>>
>> Also, with this patch, running tools/perf/tests/shell/test_intel_pt.sh:
>>
>> Error:
>> The - data has no samples!
> 
> I just tested it on 6.13-rc1  vs. 6.13-rc1  with this patch.
> 
> Indeed, there is quite difference. Below are the main parts that changed, only.
> 
> We seem to be recording data, but maybe what we record gets corrupted somehow?

Huge parts of the new file are full of 0s. Either we are mapping the 
wrong pages, or reading from the pages (via PFNMAP) does not work as 
expected.

-- 
Cheers,

David / dhildenb