[PATCH 3/9] perf sched replay: Alloc the memory of pid_to_task dynamically to adapt to the unexpected change of pid_max

[Yunlong Song]

The current memory allocation of struct task_desc *pid_to_task[MAX_PID]
is in a permanent and preset way, and it has two problems:

Problem 1: If the pid_max, which is the max number of pids in the system,
is much smaller than MAX_PID (1024*1000), then it causes a waste of stack
memory. This may happen in the case where the number of cpu cores is
much smaller than 1000.

Problem 2: If the pid_max is changed from the default value to a value
larger than MAX_PID, then it will cause assertion failure problem. The
maximum value of pid_max can be set to pid_max_max (see pidmap_init
defined in kernel/pid.c), which equals to PID_MAX_LIMIT. In x86_64,
PID_MAX_LIMIT is 4*1024*1024 (defined in include/linux/threads.h). This
value is much larger than MAX_PID, and will take up 32768 Kbytes
(4*1024*1024*8/1024) for memory allocation of pid_to_task, which is much
larger than the default 8192 Kbytes of the stack size of calling process.

Due to these two problems, we use calloc to allocate the memory of
pid_to_task dynamically.

Example:

Test environment: x86_64 with 160 cores

 $ cat /proc/sys/kernel/pid_max
 163840
 $ echo 1025000 > /proc/sys/kernel/pid_max
 $ cat /proc/sys/kernel/pid_max
 1025000

Run some applications until the pid of some process is greater than
the value of MAX_PID (1024*1000).

Before this patch:

 $ perf sched replay
 run measurement overhead: 221 nsecs
 sleep measurement overhead: 55480 nsecs
 the run test took 1000008 nsecs
 the sleep test took 1063151 nsecs
 perf: builtin-sched.c:330: register_pid: Assertion `!(pid >= 1024000)'
 failed.
 Aborted

After this patch:

 $ perf sched replay
 run measurement overhead: 221 nsecs
 sleep measurement overhead: 55435 nsecs
 the run test took 1000004 nsecs
 the sleep test took 1059312 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 | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index c466104..20d887b 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -23,6 +23,7 @@
 #include <semaphore.h>
 #include <pthread.h>
 #include <math.h>
+#include <api/fs/fs.h>
 
 #define PR_SET_NAME		15               /* Set process name */
 #define MAX_CPUS		4096
@@ -124,7 +125,7 @@ struct perf_sched {
 	struct perf_tool tool;
 	const char	 *sort_order;
 	unsigned long	 nr_tasks;
-	struct task_desc *pid_to_task[MAX_PID];
+	struct task_desc **pid_to_task;
 	struct task_desc **tasks;
 	const struct trace_sched_handler *tp_handler;
 	pthread_mutex_t	 start_work_mutex;
@@ -326,8 +327,14 @@ static struct task_desc *register_pid(struct perf_sched *sched,
 				      unsigned long pid, const char *comm)
 {
 	struct task_desc *task;
+	static int pid_max;
 
-	BUG_ON(pid >= MAX_PID);
+	if (sched->pid_to_task == NULL) {
+		if (sysctl__read_int("kernel/pid_max", &pid_max) < 0)
+			pid_max = MAX_PID;
+		BUG_ON((sched->pid_to_task = calloc(pid_max, sizeof(struct task_desc *))) == NULL);
+	}
+	BUG_ON(pid >= (unsigned long)pid_max);
 
 	task = sched->pid_to_task[pid];
 
-- 
1.8.5.2

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