[PATCH 2/9] perf sched replay: Increase the MAX_PID value to fix assertion failure problem

[Yunlong Song]

Current MAX_PID is only 65536, which will cause assertion failure problem
when CPU cores are more than 64 in x86_64. This is because the pid_max
value in x86_64 is at least PIDS_PER_CPU_DEFAULT * num_possible_cpus()
(see function pidmap_init defined in kernel/pid.c), where
PIDS_PER_CPU_DEFAULT is 1024 (defined in include/linux/threads.h). Thus
for MAX_PID = 65536, the correspoinding CPU cores are 65536/1024=64.
This is obviously not enough at all for x86_64, and will cause an
assertion failure problem due to BUG_ON(pid >= MAX_PID) in the codes.

We increase MAX_PID value from 65536 to 1024*1000, which can be used in
x86_64 with 1000 cores. This number is finally decided according to the
limitation of stack size of calling process. Use 'ulimit -a', the result
shows the stack size of any process is 8192 Kbytes, which is defined in
include/uapi/linux/resource.h (#define _STK_LIM (8*1024*1024)). Thus we
choose a large enough value for MAX_PID, and make it satisfy to the
limitation of the stack size, i.e., making the perf process take up a
memory space just smaller than 8192 Kbytes. We have calculated and
tested that 1024*1000 is OK for MAX_PID. This means perf sched replay
can now be used with at most 1000 cores in x86_64 without any assertion
failure problem.

Example:

Test environment: x86_64 with 160 cores

 $ cat /proc/sys/kernel/pid_max
 163840

Before this patch:

 $ perf sched replay
 run measurement overhead: 240 nsecs
 sleep measurement overhead: 55379 nsecs
 the run test took 1000004 nsecs
 the sleep test took 1059424 nsecs
 perf: builtin-sched.c:330: register_pid: Assertion `!(pid >= 65536)'
 failed.
 Aborted

After this patch:

 $ perf sched replay
 run measurement overhead: 221 nsecs
 sleep measurement overhead: 55397 nsecs
 the run test took 999920 nsecs
 the sleep test took 1053313 nsecs
 nr_run_events:        10
 nr_sleep_events:      1562
 nr_wakeup_events:     5
 task      0 (                  :1:         1), nr_events: 1
 task      1 (                  :2:         2), nr_events: 1
 task      2 (                  :3:         3), nr_events: 1
 task      3 (                  :5:         5), nr_events: 1
 ...

Signed-off-by: Yunlong Song <yunlong.song@xxxxxxxxxx>
---
 tools/perf/builtin-sched.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index a1893e8..c466104 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -28,7 +28,7 @@
 #define MAX_CPUS		4096
 #define COMM_LEN		20
 #define SYM_LEN			129
-#define MAX_PID			65536
+#define MAX_PID			1024000
 
 struct sched_atom;
 
-- 
1.8.5.2

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