Subject: Re: [PATCH 3/5] perf util: kernel profiling is disallowed only when perf_event_paranoid > 1

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Tue, 27 Aug 2019 10:44:04 -0300

Em Mon, Aug 26, 2019 at 09:39:14PM -0400, Igor Lubashev escreveu:
> Perf was too restrictive about sysctl kernel.perf_event_paranoid. The
> kernel only disallows profiling when perf_event_paranoid > 1. Make perf do
> the same.

Thanks for following up on this, I added these notes to this cset commit
log message:

--------------------------------- 8< ------------------------------------

perf evsel: Kernel profiling is disallowed only when perf_event_paranoid > 1

Perf was too restrictive about sysctl kernel.perf_event_paranoid. The
kernel only disallows profiling when perf_event_paranoid > 1. Make perf
do the same.

Committer testing:

For a non-root user:

  $ id
  uid=1000(acme) gid=1000(acme) groups=1000(acme),10(wheel) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
  $

Before:

We were restricting it to just userspace (:u suffix) even for a
workload started by the user:

  $ perf record sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.001 MB perf.data (8 samples) ]
  $ perf evlist
  cycles:u
  $ perf evlist -v
  cycles:u: size: 112, { sample_period, sample_freq }: 4000, sample_type: IP|TID|TIME|PERIOD, read_format: ID, disabled: 1, inherit: 1, exclude_kernel: 1, mmap: 1, comm: 1, freq: 1, enable_on_exec: 1, task: 1, precise_ip: 3, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1, ksymbol: 1, bpf_event: 1
  $ perf report --stdio
  # To display the perf.data header info, please use --header/--header-only options.
  #
  # Total Lost Samples: 0
  #
  # Samples: 8  of event 'cycles:u'
  # Event count (approx.): 1040396
  #
  # Overhead  Command  Shared Object     Symbol                
  # ........  .......  ................  ......................
  #
      68.36%  sleep    libc-2.29.so      [.] _dl_addr
      27.33%  sleep    ld-2.29.so        [.] dl_main
       3.80%  sleep    ld-2.29.so        [.] _dl_setup_hash
  #
  # (Tip: Order by the overhead of source file name and line number: perf report -s srcline)
  #
  $
  $ 

After:

When the kernel allows profiling the kernel in that scenario:

  $ perf record sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.023 MB perf.data (11 samples) ]
  $ perf evlist
  cycles
  $ perf evlist -v
  cycles: size: 112, { sample_period, sample_freq }: 4000, sample_type: IP|TID|TIME|PERIOD, read_format: ID, disabled: 1, inherit: 1, mmap: 1, comm: 1, freq: 1, enable_on_exec: 1, task: 1, precise_ip: 3, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1, ksymbol: 1, bpf_event: 1
  $
  $ perf report --stdio
  # To display the perf.data header info, please use --header/--header-only options.
  #
  # Total Lost Samples: 0
  #
  # Samples: 11  of event 'cycles'
  # Event count (approx.): 1601964
  #
  # Overhead  Command  Shared Object     Symbol                    
  # ........  .......  ................  ..........................
  #
      28.14%  sleep    [kernel.vmlinux]  [k] __rb_erase_color
      27.21%  sleep    [kernel.vmlinux]  [k] unmap_page_range
      27.20%  sleep    ld-2.29.so        [.] __tunable_get_val
      15.24%  sleep    [kernel.vmlinux]  [k] thp_get_unmapped_area
       1.96%  perf     [kernel.vmlinux]  [k] perf_event_exec
       0.22%  perf     [kernel.vmlinux]  [k] native_sched_clock
       0.02%  perf     [kernel.vmlinux]  [k] intel_bts_enable_local
       0.00%  perf     [kernel.vmlinux]  [k] native_write_msr
  #
  # (Tip: Boolean options have negative forms, e.g.: perf report --no-children)
  #
  $

Reported-by: Arnaldo Carvalho de Melo <[email-masked]>
Signed-off-by: Igor Lubashev <[email-masked]>
Tested-by: Arnaldo Carvalho de Melo <[email-masked]>
Cc: Alexander Shishkin <[email-masked]>
Cc: Alexey Budankov <[email-masked]>
Cc: James Morris <[email-masked]>
Cc: Jiri Olsa <[email-masked]>
Cc: Mathieu Poirier <[email-masked]>
Cc: Namhyung Kim <[email-masked]>
Cc: Peter Zijlstra <[email-masked]>
Cc: Suzuki Poulouse <[email-masked]>
Cc: [email-masked]
Link: http://lkml.kernel.org/r/[email-masked]
Signed-off-by: Arnaldo Carvalho de Melo <[email-masked]>

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Author:    Igor Lubashev <[email-masked]>
# Date:      Mon Aug 26 21:39:14 2019 -0400
#
# On branch perf/core
# Changes to be committed:
#	modified:   tools/perf/util/evsel.c
#
# Untracked files:
#	a
#	a.c
#	bla
#	f_mode
#	perf.data
#	perf.data.old
#	q
#
# ------------------------ >8 ------------------------
# Do not modify or remove the line above.
# Everything below it will be ignored.
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 7c704b8f0e5c..d4540bfe4574 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -282,7 +282,7 @@ struct evsel *perf_evsel__new_idx(struct perf_event_attr *attr, int idx)
 
 static bool perf_event_can_profile_kernel(void)
 {
-	return perf_event_paranoid_check(-1);
+	return perf_event_paranoid_check(1);
 }
 
 struct evsel *perf_evsel__new_cycles(bool precise)

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