[PATCH v2 0/2] Minor SQPOLL thread fix and improvement

[PATCH v2 0/2] Minor SQPOLL thread fix and improvement

[Olivier Langlois]

I have been investigated a deadlock situation in my io_uring usage where
the SQPOLL thread was going to sleep and my user threads were waiting
inside io_uring_enter() for completions.

https://github.com/axboe/liburing/issues/367

This patch serie is the result of my investigation.

Olivier Langlois (2):
  io_uring: Fix race condition when sqp thread goes to sleep
  io_uring: Create define to modify a SQPOLL parameter

 fs/io_uring.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

-- 
2.32.0

[PATCH v2 1/2] io_uring: Fix race condition when sqp thread goes to sleep

[Olivier Langlois]

If an asynchronous completion happens before the task is preparing
itself to wait and set its state to TASK_INTERRUPTIBLE, the completion
will not wake up the sqp thread.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
---
 fs/io_uring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index fc8637f591a6..7c545fa66f31 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6902,7 +6902,7 @@ static int io_sq_thread(void *data)
 		}
 
 		prepare_to_wait(&sqd->wait, &wait, TASK_INTERRUPTIBLE);
-		if (!io_sqd_events_pending(sqd)) {
+		if (!io_sqd_events_pending(sqd) && !io_run_task_work()) {
 			needs_sched = true;
 			list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
 				io_ring_set_wakeup_flag(ctx);
-- 
2.32.0

[PATCH v2 2/2] io_uring: Create define to modify a SQPOLL parameter

[Olivier Langlois]

The magic number used to cap the number of entries extracted from an
io_uring instance SQ before moving to the other instances is an
interesting parameter to experiment with.

A define has been created to make it easy to change its value from a
single location.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
---
 fs/io_uring.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 7c545fa66f31..e7997f9bf879 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -89,6 +89,7 @@
 
 #define IORING_MAX_ENTRIES	32768
 #define IORING_MAX_CQ_ENTRIES	(2 * IORING_MAX_ENTRIES)
+#define IORING_SQPOLL_CAP_ENTRIES_VALUE 8
 
 /*
  * Shift of 9 is 512 entries, or exactly one page on 64-bit archs
@@ -6797,8 +6798,8 @@ static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
 
 	to_submit = io_sqring_entries(ctx);
 	/* if we're handling multiple rings, cap submit size for fairness */
-	if (cap_entries && to_submit > 8)
-		to_submit = 8;
+	if (cap_entries && to_submit > IORING_SQPOLL_CAP_ENTRIES_VALUE)
+		to_submit = IORING_SQPOLL_CAP_ENTRIES_VALUE;
 
 	if (!list_empty(&ctx->iopoll_list) || to_submit) {
 		unsigned nr_events = 0;
-- 
2.32.0

Re: [PATCH v2 1/2] io_uring: Fix race condition when sqp thread goes to sleep

[Pavel Begunkov]

On 6/23/21 7:50 PM, Olivier Langlois wrote:
> If an asynchronous completion happens before the task is preparing
> itself to wait and set its state to TASK_INTERRUPTIBLE, the completion
> will not wake up the sqp thread.

Looks good, the bug should be pretty old.

Cc: stable@vger.kernel.org
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

> Signed-off-by: Olivier Langlois <olivier@trillion01.com>
> ---
>  fs/io_uring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index fc8637f591a6..7c545fa66f31 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6902,7 +6902,7 @@ static int io_sq_thread(void *data)
>  		}
>  
>  		prepare_to_wait(&sqd->wait, &wait, TASK_INTERRUPTIBLE);
> -		if (!io_sqd_events_pending(sqd)) {
> +		if (!io_sqd_events_pending(sqd) && !io_run_task_work()) {
>  			needs_sched = true;
>  			list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
>  				io_ring_set_wakeup_flag(ctx);
> 

-- 
Pavel Begunkov

Re: [PATCH v2 2/2] io_uring: Create define to modify a SQPOLL parameter

[Pavel Begunkov]

On 6/23/21 7:50 PM, Olivier Langlois wrote:
> The magic number used to cap the number of entries extracted from an
> io_uring instance SQ before moving to the other instances is an
> interesting parameter to experiment with.
> 
> A define has been created to make it easy to change its value from a
> single location.

It's better to send fixes separately from other improvements,
because the process a bit different for them, go into different
branches and so on.

Jens, any chance you can pick as is (at least 1/2)?

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
 
> Signed-off-by: Olivier Langlois <olivier@trillion01.com>
> ---
>  fs/io_uring.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 7c545fa66f31..e7997f9bf879 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -89,6 +89,7 @@
>  
>  #define IORING_MAX_ENTRIES	32768
>  #define IORING_MAX_CQ_ENTRIES	(2 * IORING_MAX_ENTRIES)
> +#define IORING_SQPOLL_CAP_ENTRIES_VALUE 8
>  
>  /*
>   * Shift of 9 is 512 entries, or exactly one page on 64-bit archs
> @@ -6797,8 +6798,8 @@ static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
>  
>  	to_submit = io_sqring_entries(ctx);
>  	/* if we're handling multiple rings, cap submit size for fairness */
> -	if (cap_entries && to_submit > 8)
> -		to_submit = 8;
> +	if (cap_entries && to_submit > IORING_SQPOLL_CAP_ENTRIES_VALUE)
> +		to_submit = IORING_SQPOLL_CAP_ENTRIES_VALUE;
>  
>  	if (!list_empty(&ctx->iopoll_list) || to_submit) {
>  		unsigned nr_events = 0;
> 

-- 
Pavel Begunkov

Re: [PATCH v2 2/2] io_uring: Create define to modify a SQPOLL parameter

[Jens Axboe]

On 6/23/21 4:14 PM, Pavel Begunkov wrote:
> On 6/23/21 7:50 PM, Olivier Langlois wrote:
>> The magic number used to cap the number of entries extracted from an
>> io_uring instance SQ before moving to the other instances is an
>> interesting parameter to experiment with.
>>
>> A define has been created to make it easy to change its value from a
>> single location.
> 
> It's better to send fixes separately from other improvements,
> because the process a bit different for them, go into different
> branches and so on.

It's not a huge problem even if they go to different branches,
for these I'd be more comfortable doing 5.14 anyway and that
makes it even less of a concern.

-- 
Jens Axboe

Re: [PATCH v2 0/2] Minor SQPOLL thread fix and improvement

[Jens Axboe]

On 6/23/21 12:50 PM, Olivier Langlois wrote:
> I have been investigated a deadlock situation in my io_uring usage where
> the SQPOLL thread was going to sleep and my user threads were waiting
> inside io_uring_enter() for completions.
> 
> https://github.com/axboe/liburing/issues/367

Applied, thanks.

-- 
Jens Axboe

Re: [PATCH v2 2/2] io_uring: Create define to modify a SQPOLL parameter

[Pavel Begunkov]

On 6/23/21 11:24 PM, Jens Axboe wrote:
> On 6/23/21 4:14 PM, Pavel Begunkov wrote:
>> On 6/23/21 7:50 PM, Olivier Langlois wrote:
>>> The magic number used to cap the number of entries extracted from an
>>> io_uring instance SQ before moving to the other instances is an
>>> interesting parameter to experiment with.
>>>
>>> A define has been created to make it easy to change its value from a
>>> single location.
>>
>> It's better to send fixes separately from other improvements,
>> because the process a bit different for them, go into different
>> branches and so on.
> 
> It's not a huge problem even if they go to different branches,
> for these I'd be more comfortable doing 5.14 anyway and that
> makes it even less of a concern.

Ok, good to know. I was finding splitting more convenient
as a default option, easier with b4, more confidence that
they apply to the right branch and so on

-- 
Pavel Begunkov