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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@linux.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@intel.com>
Fixes: 45ac1403f5 ("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@redhat.com>
This commit is contained in:
Arnaldo Carvalho de Melo 2016-11-09 11:04:05 -03:00
parent c56cb33b56
commit 699c12a7cc
1 changed files with 17 additions and 2 deletions
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19
tools/perf/Documentation/intel-pt.txt
View File

@ -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
------------------------------------------------------------