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 <a...@redhat.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 <a...@linux.intel.com>
> Report-link:
> http://lkml.kernel.org/r/http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi...@git.kernel.org
> Acked-by: Adrian Hunter <adrian.hun...@intel.com>
> Fixes: 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context
> switches")
> Link: http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi...@git.kernel.org
> Signed-off-by: Arnaldo Carvalho de Melo <a...@redhat.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
> ------------------------------------------------------------
>
