linux-kernel - Re: [PATCH] perf tools pt: Remove obsolete paragraph in intel-pt.c

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Date:   Thu, 10 Nov 2016 10:11:27 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf tools pt: Remove obsolete paragraph in intel-pt.c

On 09/11/16 16:44, Arnaldo Carvalho de Melo wrote:
> Em Wed, Nov 09, 2016 at 04:01:12PM +0200, Adrian Hunter escreveu:
>> On 09/11/16 15:59, Arnaldo Carvalho de Melo wrote:
>>> Em Wed, Nov 09, 2016 at 10:14:26AM -0300, Arnaldo Carvalho de Melo escreveu:
>>> +++ b/tools/perf/Documentation/intel-pt.txt
>>> @@ -546,6 +546,18 @@ mode by using the --per-thread option.
>>>  Privileged vs non-privileged users
>>>  ----------------------------------
> 
>>> +The v4.2 kernel introduced support for a context switch metadata event,
>>> +PERF_RECORD_SWITCH, which allows unprivileged users to see when their processes
>>> +are scheduled out and in, just not by whom, which is left for the
>>> +PERF_RECORD_SWITCH_CPU_WIDE, that is only accessible in system wide context,
>>> +which in turn requires CAP_SYS_ADMIN.
>>> +
>>> +Please see the 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context
>>> +switches") commit, that introduces these metadata events for further info.
>>> +
>>> +When working with kernels < v4.2, the following considerations must be taken,
>>> +as the sched:sched_switch tracepoints will be used to receive such information:
>>> +
>>>  Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users
>>>  have memory limits imposed upon them.  That affects what buffer sizes they can
>>>  have as outlined above.
>  
>> Maybe put that last paragraph about memory limits above the new text.
> 
> Ok, adding your Acked-by, it now stands as, please ack:

Yes that is good. Thank you!

> 
> commit c955ced680270b95a3de6a2434311befaeaea948
> Author: Arnaldo Carvalho de Melo <acme@...hat.com>
> Date:   Wed Nov 9 11:04:05 2016 -0300
> 
>     perf intel-pt: Update documentation about context switch events
>     
>     Since the unprivileged sched switch event was added in perf, PT doesn't
>     need need perf_event_paranoid=-1 anymore for per cpu decoding.
>     
>     Add a note stating that that is only needed for kernels < 4.2.
>     
>     Reported-by: Andi Kleen <ak@...ux.intel.com>
>     Report-link: http://lkml.kernel.org/r/http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi42r@git.kernel.org
>     Acked-by: Adrian Hunter <adrian.hunter@...el.com>
>     Fixes: 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context switches")
>     Link: http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi42r@git.kernel.org
>     Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> 
> diff --git a/tools/perf/Documentation/intel-pt.txt b/tools/perf/Documentation/intel-pt.txt
> index c6c8318e38a2..b0b3007d3c9c 100644
> --- a/tools/perf/Documentation/intel-pt.txt
> +++ b/tools/perf/Documentation/intel-pt.txt
> @@ -550,6 +550,18 @@ Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users
>  have memory limits imposed upon them.  That affects what buffer sizes they can
>  have as outlined above.
>  
> +The v4.2 kernel introduced support for a context switch metadata event,
> +PERF_RECORD_SWITCH, which allows unprivileged users to see when their processes
> +are scheduled out and in, just not by whom, which is left for the
> +PERF_RECORD_SWITCH_CPU_WIDE, that is only accessible in system wide context,
> +which in turn requires CAP_SYS_ADMIN.
> +
> +Please see the 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context
> +switches") commit, that introduces these metadata events for further info.
> +
> +When working with kernels < v4.2, the following considerations must be taken,
> +as the sched:sched_switch tracepoints will be used to receive such information:
> +
>  Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users are
>  not permitted to use tracepoints which means there is insufficient side-band
>  information to decode Intel PT in per-cpu mode, and potentially workload-only
> @@ -564,8 +576,11 @@ sched_switch tracepoint
>  -----------------------
>  
>  The sched_switch tracepoint is used to provide side-band data for Intel PT
> -decoding.  sched_switch events are automatically added. e.g. the second event
> -shown below
> +decoding in kernels where the PERF_RECORD_SWITCH metadata event isn't
> +available.
> +
> +The sched_switch events are automatically added. e.g. the second event shown
> +below:
>  
>  	$ perf record -vv -e intel_pt//u uname
>  	------------------------------------------------------------
>