Re: [External] Re: [PATCH v2] io_uring: add IORING_SETUP_SQTHREAD_STATS flag to enable sqthread stats collection

[Jens Axboe <axboe@kernel.dk>]

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On 10/21/25 2:54 AM, Fengnan Chang wrote:
> Jens Axboe <axboe@kernel.dk> ?2025?10?20??? 23:12???
>>
>> On 10/20/25 5:30 AM, Fengnan Chang wrote:
>>> In previous versions, getrusage was always called in sqrthread
>>> to count work time, but this could incur some overhead.
>>> This patch turn off stats by default, and introduces a new flag
>>> IORING_SETUP_SQTHREAD_STATS that allows user to enable the
>>> collection of statistics in the sqthread.
>>>
>>> ./t/io_uring -p1 -d128 -b4096 -s32 -c1 -F1 -B1 -R1 -X1 -n1 ./testfile
>>> IOPS base: 570K, patch: 590K
>>>
>>> ./t/io_uring -p1 -d128 -b4096 -s32 -c1 -F1 -B1 -R1 -X1 -n1 /dev/nvme1n1
>>> IOPS base: 826K, patch: 889K
>>
>> That's a crazy amount of overhead indeed. I'm assuming this is
>> because the task has lots of threads? And/or going through the retry
>> a lot? Might make more sense to have a cheaper and more rough
>> getrusage() instead? All we really use is ru_stime.{tv_sec,tv_nsec}.
>> Should it be using RUSAGE_THREAD? Correct me if I'm wrong, but using
>> PTHREAD_SELF actually seems wrong as-is.
> 
> Only one sqpoll thread, no retry.  Only enable sq_thread_poll by default in
>  ./t/io_uring.c to test.
> Yeh, getrusage is not correct, I'll try to find a cheaper way.
> 
>>
>> In any case, I don't think a setup flag is the right choice here. That
>> space is fairly limited, and SQPOLL is also a bit of an esoteric
>> feature. Hence not sure a setup flag is the right approach. Would
>> probably make more sense to have a REGISTER opcode to get/set various
>> features like this, I'm sure it's not the last thing like this we'll run
>> into. But as mentioned, I do think just having a more light weight
>> getrusage would perhaps be prudent.
> Get your point,  I'll make it in REGISTER opcode.

My main point was actually "perhaps we're doing something stupid, maybe
let's try and do the accounting in a less stupid way". For example,
right now it's just blindly getting the time all of the time. That seems
very stupid. And rather than add a knob for this, perhaps we can just
fix it?

I took a quick stab at it, see the attached two patches. I'll send them
out as well. With those, accounting should still be fine, but all the
overhead is essentially gone. Hence no need for a knob...

-- 
Jens Axboe

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From ad005f741645fc7da5d16ac0768419bb3393e22b Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Tue, 21 Oct 2025 11:44:39 -0600
Subject: [PATCH 2/2] io_uring/sqpoll: be smarter on when to update the stime
 usage

The current approach is a bit naive, and hence calls the time querying
way too often. Only start the "doing work" timer when there's actual
work to do, and then use that information to terminate (and account) the
work time once done. This greatly reduces the frequency of these calls,
when they cannot have changed anyway.

Running a basic random reader that is setup to use SQPOLL, a profile
before this change shows these as the top cycle consumers:

+   32.60%  iou-sqp-1074  [kernel.kallsyms]  [k] thread_group_cputime_adjusted
+   19.97%  iou-sqp-1074  [kernel.kallsyms]  [k] thread_group_cputime
+   12.20%  io_uring      io_uring           [.] submitter_uring_fn
+    4.13%  iou-sqp-1074  [kernel.kallsyms]  [k] getrusage
+    2.45%  iou-sqp-1074  [kernel.kallsyms]  [k] io_submit_sqes
+    2.18%  iou-sqp-1074  [kernel.kallsyms]  [k] __pi_memset_generic
+    2.09%  iou-sqp-1074  [kernel.kallsyms]  [k] cputime_adjust

and after this change, top of profile looks as follows:

+   36.23%  io_uring     io_uring           [.] submitter_uring_fn
+   23.26%  iou-sqp-819  [kernel.kallsyms]  [k] io_sq_thread
+   10.14%  iou-sqp-819  [kernel.kallsyms]  [k] io_sq_tw
+    6.52%  iou-sqp-819  [kernel.kallsyms]  [k] tctx_task_work_run
+    4.82%  iou-sqp-819  [kernel.kallsyms]  [k] nvme_submit_cmds.part.0
+    2.91%  iou-sqp-819  [kernel.kallsyms]  [k] io_submit_sqes
[...]
     0.02%  iou-sqp-819  [kernel.kallsyms]  [k] cputime_adjust                      ▒

where it's spending the cycles on things that actually matter.

Reported-by: Fengnan Chang <changfengnan@bytedance.com>
Cc: stable@vger.kernel.org
Fixes: 3fcb9d17206e ("io_uring/sqpoll: statistics of the true utilization of sq threads")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/sqpoll.c | 43 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 32 insertions(+), 11 deletions(-)

diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
index 8705b0aa82e0..f6916f46c047 100644
--- a/io_uring/sqpoll.c
+++ b/io_uring/sqpoll.c
@@ -170,6 +170,11 @@ static inline bool io_sqd_events_pending(struct io_sq_data *sqd)
 	return READ_ONCE(sqd->state);
 }
 
+struct io_sq_time {
+	bool started;
+	struct timespec64 ts;
+};
+
 struct timespec64 io_sq_cpu_time(struct task_struct *tsk)
 {
 	u64 utime, stime;
@@ -178,15 +183,27 @@ struct timespec64 io_sq_cpu_time(struct task_struct *tsk)
 	return ns_to_timespec64(stime);
 }
 
-static void io_sq_update_worktime(struct io_sq_data *sqd, struct timespec64 start)
+static void io_sq_update_worktime(struct io_sq_data *sqd, struct io_sq_time *ist)
 {
 	struct timespec64 ts;
 
-	ts = timespec64_sub(io_sq_cpu_time(current), start);
+	if (!ist->started)
+		return;
+	ist->started = false;
+	ts = timespec64_sub(io_sq_cpu_time(current), ist->ts);
 	sqd->work_time += ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
 }
 
-static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
+static void io_sq_start_worktime(struct io_sq_time *ist)
+{
+	if (ist->started)
+		return;
+	ist->started = true;
+	ist->ts = io_sq_cpu_time(current);
+}
+
+static int __io_sq_thread(struct io_ring_ctx *ctx, struct io_sq_data *sqd,
+			  bool cap_entries, struct io_sq_time *ist)
 {
 	unsigned int to_submit;
 	int ret = 0;
@@ -199,6 +216,8 @@ static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
 	if (to_submit || !wq_list_empty(&ctx->iopoll_list)) {
 		const struct cred *creds = NULL;
 
+		io_sq_start_worktime(ist);
+
 		if (ctx->sq_creds != current_cred())
 			creds = override_creds(ctx->sq_creds);
 
@@ -277,7 +296,6 @@ static int io_sq_thread(void *data)
 	struct llist_node *retry_list = NULL;
 	struct io_sq_data *sqd = data;
 	struct io_ring_ctx *ctx;
-	struct timespec64 start;
 	unsigned long timeout = 0;
 	char buf[TASK_COMM_LEN] = {};
 	DEFINE_WAIT(wait);
@@ -315,6 +333,7 @@ static int io_sq_thread(void *data)
 	mutex_lock(&sqd->lock);
 	while (1) {
 		bool cap_entries, sqt_spin = false;
+		struct io_sq_time ist = { };
 
 		if (io_sqd_events_pending(sqd) || signal_pending(current)) {
 			if (io_sqd_handle_event(sqd))
@@ -323,9 +342,8 @@ static int io_sq_thread(void *data)
 		}
 
 		cap_entries = !list_is_singular(&sqd->ctx_list);
-		start = io_sq_cpu_time(current);
 		list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
-			int ret = __io_sq_thread(ctx, cap_entries);
+			int ret = __io_sq_thread(ctx, sqd, cap_entries, &ist);
 
 			if (!sqt_spin && (ret > 0 || !wq_list_empty(&ctx->iopoll_list)))
 				sqt_spin = true;
@@ -333,15 +351,18 @@ static int io_sq_thread(void *data)
 		if (io_sq_tw(&retry_list, IORING_TW_CAP_ENTRIES_VALUE))
 			sqt_spin = true;
 
-		list_for_each_entry(ctx, &sqd->ctx_list, sqd_list)
-			if (io_napi(ctx))
+		list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
+			if (io_napi(ctx)) {
+				io_sq_start_worktime(&ist);
 				io_napi_sqpoll_busy_poll(ctx);
+			}
+		}
+
+		io_sq_update_worktime(sqd, &ist);
 
 		if (sqt_spin || !time_after(jiffies, timeout)) {
-			if (sqt_spin) {
-				io_sq_update_worktime(sqd, start);
+			if (sqt_spin)
 				timeout = jiffies + sqd->sq_thread_idle;
-			}
 			if (unlikely(need_resched())) {
 				mutex_unlock(&sqd->lock);
 				cond_resched();
-- 
2.51.0


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From 4052e20d3d63dac5e05135854e30c6b5f37fdd4a Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Tue, 21 Oct 2025 07:16:08 -0600
Subject: [PATCH 1/2] io_uring/sqpoll: switch away from getrusage() for CPU
 accounting

getrusage() does a lot more than what the SQPOLL accounting needs, the
latter only cares about (and uses) the stime. Rather than do a full
RUSAGE_SELF summation, just query the used stime instead.

Cc: stable@vger.kernel.org
Fixes: 3fcb9d17206e ("io_uring/sqpoll: statistics of the true utilization of sq threads")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/fdinfo.c |  9 +++++----
 io_uring/sqpoll.c | 34 ++++++++++++++++++++--------------
 io_uring/sqpoll.h |  1 +
 3 files changed, 26 insertions(+), 18 deletions(-)

diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
index ff3364531c77..966e06b078f6 100644
--- a/io_uring/fdinfo.c
+++ b/io_uring/fdinfo.c
@@ -59,7 +59,6 @@ static void __io_uring_show_fdinfo(struct io_ring_ctx *ctx, struct seq_file *m)
 {
 	struct io_overflow_cqe *ocqe;
 	struct io_rings *r = ctx->rings;
-	struct rusage sq_usage;
 	unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
 	unsigned int sq_head = READ_ONCE(r->sq.head);
 	unsigned int sq_tail = READ_ONCE(r->sq.tail);
@@ -152,14 +151,16 @@ static void __io_uring_show_fdinfo(struct io_ring_ctx *ctx, struct seq_file *m)
 		 * thread termination.
 		 */
 		if (tsk) {
+			struct timespec64 ts;
+
 			get_task_struct(tsk);
 			rcu_read_unlock();
-			getrusage(tsk, RUSAGE_SELF, &sq_usage);
+			ts = io_sq_cpu_time(tsk);
 			put_task_struct(tsk);
 			sq_pid = sq->task_pid;
 			sq_cpu = sq->sq_cpu;
-			sq_total_time = (sq_usage.ru_stime.tv_sec * 1000000
-					 + sq_usage.ru_stime.tv_usec);
+			sq_total_time = (ts.tv_sec * 1000000
+					 + ts.tv_nsec / 1000);
 			sq_work_time = sq->work_time;
 		} else {
 			rcu_read_unlock();
diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
index a3f11349ce06..8705b0aa82e0 100644
--- a/io_uring/sqpoll.c
+++ b/io_uring/sqpoll.c
@@ -11,6 +11,7 @@
 #include <linux/audit.h>
 #include <linux/security.h>
 #include <linux/cpuset.h>
+#include <linux/sched/cputime.h>
 #include <linux/io_uring.h>
 
 #include <uapi/linux/io_uring.h>
@@ -169,6 +170,22 @@ static inline bool io_sqd_events_pending(struct io_sq_data *sqd)
 	return READ_ONCE(sqd->state);
 }
 
+struct timespec64 io_sq_cpu_time(struct task_struct *tsk)
+{
+	u64 utime, stime;
+
+	task_cputime_adjusted(tsk, &utime, &stime);
+	return ns_to_timespec64(stime);
+}
+
+static void io_sq_update_worktime(struct io_sq_data *sqd, struct timespec64 start)
+{
+	struct timespec64 ts;
+
+	ts = timespec64_sub(io_sq_cpu_time(current), start);
+	sqd->work_time += ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
+}
+
 static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
 {
 	unsigned int to_submit;
@@ -255,23 +272,12 @@ static bool io_sq_tw_pending(struct llist_node *retry_list)
 	return retry_list || !llist_empty(&tctx->task_list);
 }
 
-static void io_sq_update_worktime(struct io_sq_data *sqd, struct rusage *start)
-{
-	struct rusage end;
-
-	getrusage(current, RUSAGE_SELF, &end);
-	end.ru_stime.tv_sec -= start->ru_stime.tv_sec;
-	end.ru_stime.tv_usec -= start->ru_stime.tv_usec;
-
-	sqd->work_time += end.ru_stime.tv_usec + end.ru_stime.tv_sec * 1000000;
-}
-
 static int io_sq_thread(void *data)
 {
 	struct llist_node *retry_list = NULL;
 	struct io_sq_data *sqd = data;
 	struct io_ring_ctx *ctx;
-	struct rusage start;
+	struct timespec64 start;
 	unsigned long timeout = 0;
 	char buf[TASK_COMM_LEN] = {};
 	DEFINE_WAIT(wait);
@@ -317,7 +323,7 @@ static int io_sq_thread(void *data)
 		}
 
 		cap_entries = !list_is_singular(&sqd->ctx_list);
-		getrusage(current, RUSAGE_SELF, &start);
+		start = io_sq_cpu_time(current);
 		list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
 			int ret = __io_sq_thread(ctx, cap_entries);
 
@@ -333,7 +339,7 @@ static int io_sq_thread(void *data)
 
 		if (sqt_spin || !time_after(jiffies, timeout)) {
 			if (sqt_spin) {
-				io_sq_update_worktime(sqd, &start);
+				io_sq_update_worktime(sqd, start);
 				timeout = jiffies + sqd->sq_thread_idle;
 			}
 			if (unlikely(need_resched())) {
diff --git a/io_uring/sqpoll.h b/io_uring/sqpoll.h
index b83dcdec9765..84ed2b312e88 100644
--- a/io_uring/sqpoll.h
+++ b/io_uring/sqpoll.h
@@ -29,6 +29,7 @@ void io_sq_thread_unpark(struct io_sq_data *sqd);
 void io_put_sq_data(struct io_sq_data *sqd);
 void io_sqpoll_wait_sq(struct io_ring_ctx *ctx);
 int io_sqpoll_wq_cpu_affinity(struct io_ring_ctx *ctx, cpumask_var_t mask);
+struct timespec64 io_sq_cpu_time(struct task_struct *tsk);
 
 static inline struct task_struct *sqpoll_task_locked(struct io_sq_data *sqd)
 {
-- 
2.51.0