[PATCH] io_uring: be smarter about waking multiple CQ ring waiters

[PATCH] io_uring: be smarter about waking multiple CQ ring waiters

[Jens Axboe]

Currently we only wake the first waiter, even if we have enough entries
posted to satisfy multiple waiters. Improve that situation so that
every waiter knows how much the CQ tail has to advance before they can
be safely woken up.

With this change, if we have N waiters each asking for 1 event and we get
4 completions, then we wake up 4 waiters. If we have N waiters asking
for 2 completions and we get 4 completions, then we wake up the first
two. Previously, only the first waiter would've been woken up.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/fs/io_uring.c b/fs/io_uring.c
index bf548af0426c..04df4fa3c75e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1435,11 +1435,13 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
 
 static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
 {
-	/* see waitqueue_active() comment */
-	smp_mb();
-
-	if (waitqueue_active(&ctx->cq_wait))
-		wake_up(&ctx->cq_wait);
+	/*
+	 * wake_up_all() may seem excessive, but io_wake_function() and
+	 * io_should_wake() handle the termination of the loop and only
+	 * wake as many waiters as we need to.
+	 */
+	if (wq_has_sleeper(&ctx->cq_wait))
+		wake_up_all(&ctx->cq_wait);
 	if (ctx->sq_data && waitqueue_active(&ctx->sq_data->wait))
 		wake_up(&ctx->sq_data->wait);
 	if (io_should_trigger_evfd(ctx))
@@ -6968,20 +6970,21 @@ static int io_sq_thread(void *data)
 struct io_wait_queue {
 	struct wait_queue_entry wq;
 	struct io_ring_ctx *ctx;
-	unsigned to_wait;
+	unsigned cq_tail;
 	unsigned nr_timeouts;
 };
 
 static inline bool io_should_wake(struct io_wait_queue *iowq)
 {
 	struct io_ring_ctx *ctx = iowq->ctx;
+	unsigned tail = ctx->cached_cq_tail + atomic_read(&ctx->cq_timeouts);
 
 	/*
 	 * Wake up if we have enough events, or if a timeout occurred since we
 	 * started waiting. For timeouts, we always want to return to userspace,
 	 * regardless of event count.
 	 */
-	return io_cqring_events(ctx) >= iowq->to_wait ||
+	return tail >= iowq->cq_tail ||
 			atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
 }
 
@@ -7045,7 +7048,6 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 			.entry		= LIST_HEAD_INIT(iowq.wq.entry),
 		},
 		.ctx		= ctx,
-		.to_wait	= min_events,
 	};
 	struct io_rings *rings = ctx->rings;
 	signed long timeout = MAX_SCHEDULE_TIMEOUT;
@@ -7081,6 +7083,8 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 	}
 
 	iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
+	iowq.cq_tail = READ_ONCE(ctx->rings->cq.head) + min_events +
+			iowq.nr_timeouts;
 	trace_io_uring_cqring_wait(ctx, min_events);
 	do {
 		/* if we can't even flush overflow, don't wait for more */

-- 
Jens Axboe

Re: [PATCH] io_uring: be smarter about waking multiple CQ ring waiters

[Pavel Begunkov]

On 8/6/21 9:19 PM, Jens Axboe wrote:
> Currently we only wake the first waiter, even if we have enough entries
> posted to satisfy multiple waiters. Improve that situation so that
> every waiter knows how much the CQ tail has to advance before they can
> be safely woken up.
> 
> With this change, if we have N waiters each asking for 1 event and we get
> 4 completions, then we wake up 4 waiters. If we have N waiters asking
> for 2 completions and we get 4 completions, then we wake up the first
> two. Previously, only the first waiter would've been woken up.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index bf548af0426c..04df4fa3c75e 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1435,11 +1435,13 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
>  
>  static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
>  {
> -	/* see waitqueue_active() comment */
> -	smp_mb();
> -
> -	if (waitqueue_active(&ctx->cq_wait))
> -		wake_up(&ctx->cq_wait);
> +	/*
> +	 * wake_up_all() may seem excessive, but io_wake_function() and
> +	 * io_should_wake() handle the termination of the loop and only
> +	 * wake as many waiters as we need to.
> +	 */
> +	if (wq_has_sleeper(&ctx->cq_wait))
> +		wake_up_all(&ctx->cq_wait);
>  	if (ctx->sq_data && waitqueue_active(&ctx->sq_data->wait))
>  		wake_up(&ctx->sq_data->wait);
>  	if (io_should_trigger_evfd(ctx))
> @@ -6968,20 +6970,21 @@ static int io_sq_thread(void *data)
>  struct io_wait_queue {
>  	struct wait_queue_entry wq;
>  	struct io_ring_ctx *ctx;
> -	unsigned to_wait;
> +	unsigned cq_tail;
>  	unsigned nr_timeouts;
>  };
>  
>  static inline bool io_should_wake(struct io_wait_queue *iowq)
>  {
>  	struct io_ring_ctx *ctx = iowq->ctx;
> +	unsigned tail = ctx->cached_cq_tail + atomic_read(&ctx->cq_timeouts);

Seems, adding cq_timeouts can be dropped from here and iowq.cq_tail

>  
>  	/*
>  	 * Wake up if we have enough events, or if a timeout occurred since we
>  	 * started waiting. For timeouts, we always want to return to userspace,
>  	 * regardless of event count.
>  	 */
> -	return io_cqring_events(ctx) >= iowq->to_wait ||

Don't we miss smp_rmb() previously provided my io_cqring_events()?

> +	return tail >= iowq->cq_tail ||

tails might overflow

>  			atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
>  }
>  
> @@ -7045,7 +7048,6 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
>  			.entry		= LIST_HEAD_INIT(iowq.wq.entry),
>  		},
>  		.ctx		= ctx,
> -		.to_wait	= min_events,
>  	};
>  	struct io_rings *rings = ctx->rings;
>  	signed long timeout = MAX_SCHEDULE_TIMEOUT;
> @@ -7081,6 +7083,8 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
>  	}
>  
>  	iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
> +	iowq.cq_tail = READ_ONCE(ctx->rings->cq.head) + min_events +
> +			iowq.nr_timeouts;
>  	trace_io_uring_cqring_wait(ctx, min_events);
>  	do {
>  		/* if we can't even flush overflow, don't wait for more */
> 

-- 
Pavel Begunkov

Re: [PATCH] io_uring: be smarter about waking multiple CQ ring waiters

[Jens Axboe]

On 8/9/21 7:42 PM, Pavel Begunkov wrote:
> On 8/6/21 9:19 PM, Jens Axboe wrote:
>> Currently we only wake the first waiter, even if we have enough entries
>> posted to satisfy multiple waiters. Improve that situation so that
>> every waiter knows how much the CQ tail has to advance before they can
>> be safely woken up.
>>
>> With this change, if we have N waiters each asking for 1 event and we get
>> 4 completions, then we wake up 4 waiters. If we have N waiters asking
>> for 2 completions and we get 4 completions, then we wake up the first
>> two. Previously, only the first waiter would've been woken up.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index bf548af0426c..04df4fa3c75e 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -1435,11 +1435,13 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
>>  
>>  static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
>>  {
>> -	/* see waitqueue_active() comment */
>> -	smp_mb();
>> -
>> -	if (waitqueue_active(&ctx->cq_wait))
>> -		wake_up(&ctx->cq_wait);
>> +	/*
>> +	 * wake_up_all() may seem excessive, but io_wake_function() and
>> +	 * io_should_wake() handle the termination of the loop and only
>> +	 * wake as many waiters as we need to.
>> +	 */
>> +	if (wq_has_sleeper(&ctx->cq_wait))
>> +		wake_up_all(&ctx->cq_wait);
>>  	if (ctx->sq_data && waitqueue_active(&ctx->sq_data->wait))
>>  		wake_up(&ctx->sq_data->wait);
>>  	if (io_should_trigger_evfd(ctx))
>> @@ -6968,20 +6970,21 @@ static int io_sq_thread(void *data)
>>  struct io_wait_queue {
>>  	struct wait_queue_entry wq;
>>  	struct io_ring_ctx *ctx;
>> -	unsigned to_wait;
>> +	unsigned cq_tail;
>>  	unsigned nr_timeouts;
>>  };
>>  
>>  static inline bool io_should_wake(struct io_wait_queue *iowq)
>>  {
>>  	struct io_ring_ctx *ctx = iowq->ctx;
>> +	unsigned tail = ctx->cached_cq_tail + atomic_read(&ctx->cq_timeouts);
> 
> Seems, adding cq_timeouts can be dropped from here and iowq.cq_tail

Good point, we can drop it at both ends.

>>  	/*
>>  	 * Wake up if we have enough events, or if a timeout occurred since we
>>  	 * started waiting. For timeouts, we always want to return to userspace,
>>  	 * regardless of event count.
>>  	 */
>> -	return io_cqring_events(ctx) >= iowq->to_wait ||
> 
> Don't we miss smp_rmb() previously provided my io_cqring_events()?

For? We aren't reading any user modified pats.

> 
>> +	return tail >= iowq->cq_tail ||
> 
> tails might overflow

Indeed, I actually did fix this one before committing it.

-- 
Jens Axboe

Re: [PATCH] io_uring: be smarter about waking multiple CQ ring waiters

[Pavel Begunkov]

On 8/10/21 2:55 AM, Jens Axboe wrote:
> On 8/9/21 7:42 PM, Pavel Begunkov wrote:
>> On 8/6/21 9:19 PM, Jens Axboe wrote:
>>> Currently we only wake the first waiter, even if we have enough entries
>>> posted to satisfy multiple waiters. Improve that situation so that
>>> every waiter knows how much the CQ tail has to advance before they can
>>> be safely woken up.
>>>
>>> With this change, if we have N waiters each asking for 1 event and we get
>>> 4 completions, then we wake up 4 waiters. If we have N waiters asking
>>> for 2 completions and we get 4 completions, then we wake up the first
>>> two. Previously, only the first waiter would've been woken up.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>
>>> ---
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index bf548af0426c..04df4fa3c75e 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -1435,11 +1435,13 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
>>>  
>>>  static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
>>>  {
>>> -	/* see waitqueue_active() comment */
>>> -	smp_mb();
>>> -
>>> -	if (waitqueue_active(&ctx->cq_wait))
>>> -		wake_up(&ctx->cq_wait);
>>> +	/*
>>> +	 * wake_up_all() may seem excessive, but io_wake_function() and
>>> +	 * io_should_wake() handle the termination of the loop and only
>>> +	 * wake as many waiters as we need to.
>>> +	 */
>>> +	if (wq_has_sleeper(&ctx->cq_wait))
>>> +		wake_up_all(&ctx->cq_wait);
>>>  	if (ctx->sq_data && waitqueue_active(&ctx->sq_data->wait))
>>>  		wake_up(&ctx->sq_data->wait);
>>>  	if (io_should_trigger_evfd(ctx))
>>> @@ -6968,20 +6970,21 @@ static int io_sq_thread(void *data)
>>>  struct io_wait_queue {
>>>  	struct wait_queue_entry wq;
>>>  	struct io_ring_ctx *ctx;
>>> -	unsigned to_wait;
>>> +	unsigned cq_tail;
>>>  	unsigned nr_timeouts;
>>>  };
>>>  
>>>  static inline bool io_should_wake(struct io_wait_queue *iowq)
>>>  {
>>>  	struct io_ring_ctx *ctx = iowq->ctx;
>>> +	unsigned tail = ctx->cached_cq_tail + atomic_read(&ctx->cq_timeouts);
>>
>> Seems, adding cq_timeouts can be dropped from here and iowq.cq_tail
> 
> Good point, we can drop it at both ends.
> 
>>>  	/*
>>>  	 * Wake up if we have enough events, or if a timeout occurred since we
>>>  	 * started waiting. For timeouts, we always want to return to userspace,
>>>  	 * regardless of event count.
>>>  	 */
>>> -	return io_cqring_events(ctx) >= iowq->to_wait ||
>>
>> Don't we miss smp_rmb() previously provided my io_cqring_events()?
> 
> For? We aren't reading any user modified pats.

I was rather thinking about who provides the barrier for userspace,
but that should be indeed on the userspace, and the function is
called from arbitrary CPU/context anyway.

>>
>>> +	return tail >= iowq->cq_tail ||
>>
>> tails might overflow
> 
> Indeed, I actually did fix this one before committing it.

Great

-- 
Pavel Begunkov