linux-kernel - Re: [PATCH] perf arm-spe: Fix incorrect sample timestamp in perf script

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Message-ID: <2af84c69-8528-9c4a-f666-2dd660bcd966@huawei.com>
Date:   Mon, 28 Jun 2021 14:30:56 +0800
From:   Yang Jihong <yangjihong1@...wei.com>
To:     Leo Yan <leo.yan@...aro.org>
CC:     <john.garry@...wei.com>, <will@...nel.org>,
        <mathieu.poirier@...aro.org>, <peterz@...radead.org>,
        <mingo@...hat.com>, <acme@...nel.org>, <mark.rutland@....com>,
        <alexander.shishkin@...ux.intel.com>, <jolsa@...hat.com>,
        <namhyung@...nel.org>, <james.clark@....com>,
        <andre.przywara@....com>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-perf-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf arm-spe: Fix incorrect sample timestamp in perf
 script

Hello Leo,

On 2021/6/28 12:07, Leo Yan wrote:
> On Mon, Jun 28, 2021 at 11:45:07AM +0800, Yang Jihong wrote:
> 
> [...]
> 
>> The timestamp does not match. Each timestamp has a difference of
>> 213.98296702 seconds. Is the command I executed wrong?
> 
> I don't understand what's your meaning "a difference of 213.98296702
> seconds".
> 
> But let me give an explaination with your below examples.  You could
> the command "perf script" outputs the timestamp 314938.859071370 which
> is the kernel's timestamp, if using the command "perf script -D", it
> outputs the value 31515284203839 which is the Arch timer's raw counter
> value.
> 
> In theory, the arch timer's counter is enabled at the very early
> time before kernel's booting (e.g. bootloaders, UEFI, etc...).  So for
> the kernel's timestamp, it should calibrate the timestamp and reduce
> the offset prior to the kernel's booting.  I think this is the reason
> why you observed 213.98296702 seconds difference (if compared with
> your own patch?)
> 
> If still see any issue, please let me know.  Thanks a lot for the
> testing!
> 
Thanks for the very detailed explanation.
To keep the correct relative time sequence between events in SPE and 
other events, we should use kernel timestamp instead of arch time. 
Therefore, we need to calibrate the time. Is this correct?

Thanks,
Jihong.
> Leo
> 
>> |symbol_address|perf script| perf script -D|
>> |ffffb7ee2a20e100|314938.859071370|31515284203839|
>> |ffffb7ee29f360e0|314938.859753820|31515284272084|
>> |fffe85dd87e0|314938.859820430|31515284278745|
>> |fffe85dd7a28|314938.859941110|31515284290813|
>> |fffe85dd7a28|314938.859948890|31515284291591|
>> |fffe85dc8f58|314938.859953610|31515284292063|
> .
>