[PATCH] io_uring: show sqthread pid and cpu in fdinfo

[Joseph Qi <joseph.qi@linux.alibaba.com>]

In most cases we'll specify IORING_SETUP_SQPOLL and run multiple
io_uring instances in a host. Since all sqthreads are named
"io_uring-sq", it's hard to distinguish the relations between
application process and its io_uring sqthread.
With this patch, application can get its corresponding sqthread pid
and cpu through show_fdinfo.
Steps:
1. Get io_uring fd first.
$ ls -l /proc/<pid>/fd | grep -w io_uring
2. Then get io_uring instance related info, including corresponding
sqthread pid and cpu.
$ cat /proc/<pid>/fdinfo/<io_uring_fd>

pos:	0
flags:	02000002
mnt_id:	13
SqThread:	6929
SqThreadCpu:	2
UserFiles:	1
    0: testfile
UserBufs:	0
PollList:

Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
---
1. Go to upstream/4.9/4.19？(N)
2. Backport from upstream？(N)
3. Summarized pre-commmit testing in Aone (Y)
4. Aone URL (https://aone.alibaba-inc.com/task/29517834)
5. Use CONFIG_xxx ? (N)
6. Use sysfs or boot parameter to turn off code changes? (N)
7. Any kernel-userspace API/ABI changes? (N)
8. I have full maintenance commitments for my 50+ code changes (Y)

 fs/io_uring.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index c718ac0..5171de0 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -7934,6 +7934,10 @@ static void __io_uring_show_fdinfo(struct io_ring_ctx *ctx, struct seq_file *m)
 	int i;
 
 	mutex_lock(&ctx->uring_lock);
+	seq_printf(m, "SqThread:\t%d\n", (ctx->flags & IORING_SETUP_SQPOLL) ?
+					 task_pid_nr(ctx->sqo_thread) : -1);
+	seq_printf(m, "SqThreadCpu:\t%d\n", (ctx->flags & IORING_SETUP_SQPOLL) ?
+					    task_cpu(ctx->sqo_thread) : -1);
 	seq_printf(m, "UserFiles:\t%u\n", ctx->nr_user_files);
 	for (i = 0; i < ctx->nr_user_files; i++) {
 		struct fixed_file_table *table;
-- 
1.8.3.1