[PATCH 5/9] perf sched replay: Fix the segmentation fault problem caused by pr_err in threads

[Yunlong Song]

The pr_err in self_open_counters() prints error message to stderr.
Unlike stdout, stderr uses memory buffer on the stack of each calling
process. The pr_err in self_open_counters() works in a thread called
thread_func created in function create_tasks, which concurrently creates
sched->nr_tasks threads. If the error happens and pr_err prints the
error message in each of these threads, the stack size of the perf
process (default is 8192 kbytes) will quickly run out and the
segmentation fault will happen then. To solve this problem, pr_err with
self_open_counters() should be moved from newly created threads to the
old main thread of the perf process. Then the pr_err can work in a
stable situation without the strange segmentation fault problem.

Example:

Test environment: x86_64 with 160 cores

Before this patch:

 $ perf sched replay
 ...
 task   1549 (             :163132:    163132), nr_events: 1
 task   1550 (             :163540:    163540), nr_events: 1
 task   1551 (           <unknown>:         0), nr_events: 10
 Segmentation fault

After this patch:

 $ perf sched replay
 ...
 task   1549 (             :163132:    163132), nr_events: 1
 task   1550 (             :163540:    163540), nr_events: 1
 task   1551 (           <unknown>:         0), nr_events: 10
 ...

As shown above, the result continues without any segmentation fault.

Signed-off-by: Yunlong Song <yunlong.song@xxxxxxxxxx>
---
 tools/perf/builtin-sched.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index dd71481..7fe3b3c 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -472,6 +472,7 @@ static u64 get_cpu_usage_nsec_self(int fd)
 struct sched_thread_parms {
 	struct task_desc  *task;
 	struct perf_sched *sched;
+	int fd;
 };
 
 static void *thread_func(void *ctx)
@@ -482,13 +483,12 @@ static void *thread_func(void *ctx)
 	u64 cpu_usage_0, cpu_usage_1;
 	unsigned long i, ret;
 	char comm2[22];
-	int fd;
+	int fd = parms->fd;
 
 	zfree(&parms);
 
 	sprintf(comm2, ":%s", this_task->comm);
 	prctl(PR_SET_NAME, comm2);
-	fd = self_open_counters();
 	if (fd < 0)
 		return NULL;
 again:
@@ -540,6 +540,7 @@ static void create_tasks(struct perf_sched *sched)
 		BUG_ON(parms == NULL);
 		parms->task = task = sched->tasks[i];
 		parms->sched = sched;
+		parms->fd = self_open_counters();
 		sem_init(&task->sleep_sem, 0, 0);
 		sem_init(&task->ready_for_work, 0, 0);
 		sem_init(&task->work_done_sem, 0, 0);
-- 
1.8.5.2

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