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For perf recording, it retrieves process info by iterating nodes in proc
fs. If we run perf in a non-root PID namespace with command:
# unshare --fork --pid perf record -e cycles -a -- test_program
... in this case, unshare command creates a child PID namespace and
launches perf tool in it, but the issue is the proc fs is not mounted
for the non-root PID namespace, this leads to the perf tool gathering
process info from its parent PID namespace.
We can use below command to observe the process nodes under proc fs:
# unshare --pid --fork ls /proc
1 137 1968 2128 3 342 48 62 78 crypto kcore net uptime
10 138 2 2142 30 35 49 63 8 devices keys pagetypeinfo version
11 139 20 2143 304 36 50 64 82 device-tree key-users partitions vmallocinfo
12 14 2011 22 305 37 51 65 83 diskstats kmsg self vmstat
128 140 2038 23 307 39 52 656 84 driver kpagecgroup slabinfo zoneinfo
129 15 2074 24 309 4 53 67 9 execdomains kpagecount softirqs
13 16 2094 241 31 40 54 68 asound fb kpageflags stat
130 164 2096 242 310 41 55 69 buddyinfo filesystems loadavg swaps
131 17 2098 25 317 42 56 70 bus fs locks sys
132 175 21 26 32 43 57 71 cgroups interrupts meminfo sysrq-trigger
133 179 2102 263 329 44 58 75 cmdline iomem misc sysvipc
134 1875 2103 27 330 45 59 76 config.gz ioports modules thread-self
135 19 2117 29 333 46 6 77 consoles irq mounts timer_list
136 1941 2121 298 34 47 60 773 cpuinfo kallsyms mtd tty
So it shows many existed tasks, since unshared command has not mounted
the proc fs for the new created PID namespace, it still accesses the
proc fs of the root PID namespace. This leads to two prominent issues:
- Firstly, PID values are mismatched between thread info and samples.
The gathered thread info are coming from the proc fs of the root PID
namespace, but samples record its PID from the child PID namespace.
- The second issue is profiled program 'test_program' returns its forked
PID number from the child PID namespace, perf tool wrongly uses this
PID number to retrieve the process info via the proc fs of the root
PID namespace.
To avoid issues, we need to mount proc fs for the child PID namespace
with the option '--mount-proc' when use unshare command:
# unshare --fork --pid --mount-proc perf record -e cycles -a -- test_program
Conversely, when the proc fs of the root PID namespace is used by child
namespace, perf tool can detect the multiple PID levels and
nsinfo__is_in_root_namespace() returns false, this patch reports error
for this case:
# unshare --fork --pid perf record -e cycles -a -- test_program
Couldn't synthesize bpf events.
Perf runs in non-root PID namespace but it tries to gather process info from its parent PID namespace.
Please mount the proc file system properly, e.g. add the option '--mount-proc' for unshare command.
Reviewed-by: James Clark <james.clark@arm.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20211224124014.2492751-1-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
bc9c806e52