linux-imx

perf sched replay: Increase the MAX_PID value to fix assertion failure problem

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@huawei.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1427809596-29559-3-git-send-email-yunlong.song@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>