io_uring_prep_timeout_update on linked timeouts

io_uring_prep_timeout_update on linked timeouts

[Victor Stewart]

we're able to update timeouts with io_uring_prep_timeout_update
without having to cancel
and resubmit, has it ever been considered adding this ability to
linked timeouts?

maybe io_uring_prep_timeout_update could be extended to seamlessly
operate on linked
timeouts as well without any api change? i assume this isn't possible
currently because
there are no tests displaying it in liburing.

Re: io_uring_prep_timeout_update on linked timeouts

[Victor Stewart]

On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>
> we're able to update timeouts with io_uring_prep_timeout_update
> without having to cancel
> and resubmit, has it ever been considered adding this ability to
> linked timeouts?

whoops turns out this does work. just tested it.

well i guess it's documented now!

Re: io_uring_prep_timeout_update on linked timeouts

[Victor Stewart]

On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>
> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
> >
> > we're able to update timeouts with io_uring_prep_timeout_update
> > without having to cancel
> > and resubmit, has it ever been considered adding this ability to
> > linked timeouts?
>
> whoops turns out this does work. just tested it.

doesn't work actually. missed that because of a bit of misdirection.
returns -ENOENT.

the problem with the current way of cancelling then resubmitting
a new a timeout linked op (let's use poll here) is you have 3 situations:

1) the poll triggers and you get some positive value. all good.

2) the linked timeout triggers and cancels the poll, so the poll
operation returns -ECANCELED.

3) you cancel the existing poll op, and submit a new one with
the updated linked timeout. now the original poll op returns
-ECANCELED.

so solely from looking at the return value of the poll op in 2) and 3)
there is no way to disambiguate them. of course the linked timeout
operation result will allow you to do so, but you'd have to persist state
across cqe processings. you can also track the cancellations and know
to skip the explicitly cancelled ops' cqes (which is what i chose).

there's also the problem of efficiency. you can imagine in a QUIC
server where you're constantly updating that poll timeout in response
to idle timeout and ACK scheduling, this extra work mounts.

so i think the ability to update linked timeouts via
io_uring_prep_timeout_update would be fantastic.

V

Re: io_uring_prep_timeout_update on linked timeouts

[Jens Axboe]

On 8/26/21 7:40 PM, Victor Stewart wrote:
> On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>>
>> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>>>
>>> we're able to update timeouts with io_uring_prep_timeout_update
>>> without having to cancel
>>> and resubmit, has it ever been considered adding this ability to
>>> linked timeouts?
>>
>> whoops turns out this does work. just tested it.
> 
> doesn't work actually. missed that because of a bit of misdirection.
> returns -ENOENT.
> 
> the problem with the current way of cancelling then resubmitting
> a new a timeout linked op (let's use poll here) is you have 3 situations:
> 
> 1) the poll triggers and you get some positive value. all good.
> 
> 2) the linked timeout triggers and cancels the poll, so the poll
> operation returns -ECANCELED.
> 
> 3) you cancel the existing poll op, and submit a new one with
> the updated linked timeout. now the original poll op returns
> -ECANCELED.
> 
> so solely from looking at the return value of the poll op in 2) and 3)
> there is no way to disambiguate them. of course the linked timeout
> operation result will allow you to do so, but you'd have to persist state
> across cqe processings. you can also track the cancellations and know
> to skip the explicitly cancelled ops' cqes (which is what i chose).
> 
> there's also the problem of efficiency. you can imagine in a QUIC
> server where you're constantly updating that poll timeout in response
> to idle timeout and ACK scheduling, this extra work mounts.
> 
> so i think the ability to update linked timeouts via
> io_uring_prep_timeout_update would be fantastic.

Hmm, I'll need to dig a bit, but whether it's a linked timeout or not
should not matter. It's a timeout, it's queued and updated the same way.
And we even check this in some of the liburing tests.

Do you have a test case that doesn't work for you? Always easier to
reason about a test case.

-- 
Jens Axboe

Re: io_uring_prep_timeout_update on linked timeouts

[Pavel Begunkov]

On 8/28/21 4:22 AM, Jens Axboe wrote:
> On 8/26/21 7:40 PM, Victor Stewart wrote:
>> On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>>>
>>> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>>>>
>>>> we're able to update timeouts with io_uring_prep_timeout_update
>>>> without having to cancel
>>>> and resubmit, has it ever been considered adding this ability to
>>>> linked timeouts?
>>>
>>> whoops turns out this does work. just tested it.
>>
>> doesn't work actually. missed that because of a bit of misdirection.
>> returns -ENOENT.
>>
>> the problem with the current way of cancelling then resubmitting
>> a new a timeout linked op (let's use poll here) is you have 3 situations:
>>
>> 1) the poll triggers and you get some positive value. all good.
>>
>> 2) the linked timeout triggers and cancels the poll, so the poll
>> operation returns -ECANCELED.
>>
>> 3) you cancel the existing poll op, and submit a new one with
>> the updated linked timeout. now the original poll op returns
>> -ECANCELED.
>>
>> so solely from looking at the return value of the poll op in 2) and 3)
>> there is no way to disambiguate them. of course the linked timeout
>> operation result will allow you to do so, but you'd have to persist state
>> across cqe processings. you can also track the cancellations and know
>> to skip the explicitly cancelled ops' cqes (which is what i chose).
>>
>> there's also the problem of efficiency. you can imagine in a QUIC
>> server where you're constantly updating that poll timeout in response
>> to idle timeout and ACK scheduling, this extra work mounts.
>>
>> so i think the ability to update linked timeouts via
>> io_uring_prep_timeout_update would be fantastic.
> 
> Hmm, I'll need to dig a bit, but whether it's a linked timeout or not
> should not matter. It's a timeout, it's queued and updated the same way.
> And we even check this in some of the liburing tests.

We don't keep linked timeouts in ->timeout_list, so it's not
supported and has never been. Should be doable, but we need
to be careful synchronising with the link's head.

> Do you have a test case that doesn't work for you? Always easier to
> reason about a test case.
> 

-- 
Pavel Begunkov

Re: io_uring_prep_timeout_update on linked timeouts

[Jens Axboe]

On 8/28/21 7:39 AM, Pavel Begunkov wrote:
> On 8/28/21 4:22 AM, Jens Axboe wrote:
>> On 8/26/21 7:40 PM, Victor Stewart wrote:
>>> On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>>>>
>>>> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>>>>>
>>>>> we're able to update timeouts with io_uring_prep_timeout_update
>>>>> without having to cancel
>>>>> and resubmit, has it ever been considered adding this ability to
>>>>> linked timeouts?
>>>>
>>>> whoops turns out this does work. just tested it.
>>>
>>> doesn't work actually. missed that because of a bit of misdirection.
>>> returns -ENOENT.
>>>
>>> the problem with the current way of cancelling then resubmitting
>>> a new a timeout linked op (let's use poll here) is you have 3 situations:
>>>
>>> 1) the poll triggers and you get some positive value. all good.
>>>
>>> 2) the linked timeout triggers and cancels the poll, so the poll
>>> operation returns -ECANCELED.
>>>
>>> 3) you cancel the existing poll op, and submit a new one with
>>> the updated linked timeout. now the original poll op returns
>>> -ECANCELED.
>>>
>>> so solely from looking at the return value of the poll op in 2) and 3)
>>> there is no way to disambiguate them. of course the linked timeout
>>> operation result will allow you to do so, but you'd have to persist state
>>> across cqe processings. you can also track the cancellations and know
>>> to skip the explicitly cancelled ops' cqes (which is what i chose).
>>>
>>> there's also the problem of efficiency. you can imagine in a QUIC
>>> server where you're constantly updating that poll timeout in response
>>> to idle timeout and ACK scheduling, this extra work mounts.
>>>
>>> so i think the ability to update linked timeouts via
>>> io_uring_prep_timeout_update would be fantastic.
>>
>> Hmm, I'll need to dig a bit, but whether it's a linked timeout or not
>> should not matter. It's a timeout, it's queued and updated the same way.
>> And we even check this in some of the liburing tests.
> 
> We don't keep linked timeouts in ->timeout_list, so it's not
> supported and has never been. Should be doable, but we need
> to be careful synchronising with the link's head.

Yeah shoot you are right, I guess that explains the ENOENT. Would be
nice to add, though. Synchronization should not be that different from
dealing with regular timeouts.

-- 
Jens Axboe

Re: io_uring_prep_timeout_update on linked timeouts

[Pavel Begunkov]

On 8/28/21 2:43 PM, Jens Axboe wrote:
> On 8/28/21 7:39 AM, Pavel Begunkov wrote:
>> On 8/28/21 4:22 AM, Jens Axboe wrote:
>>> On 8/26/21 7:40 PM, Victor Stewart wrote:
>>>> On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>>>>>
>>>>> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>>>>>>
>>>>>> we're able to update timeouts with io_uring_prep_timeout_update
>>>>>> without having to cancel
>>>>>> and resubmit, has it ever been considered adding this ability to
>>>>>> linked timeouts?
>>>>>
>>>>> whoops turns out this does work. just tested it.
>>>>
>>>> doesn't work actually. missed that because of a bit of misdirection.
>>>> returns -ENOENT.
>>>>
>>>> the problem with the current way of cancelling then resubmitting
>>>> a new a timeout linked op (let's use poll here) is you have 3 situations:
>>>>
>>>> 1) the poll triggers and you get some positive value. all good.
>>>>
>>>> 2) the linked timeout triggers and cancels the poll, so the poll
>>>> operation returns -ECANCELED.
>>>>
>>>> 3) you cancel the existing poll op, and submit a new one with
>>>> the updated linked timeout. now the original poll op returns
>>>> -ECANCELED.
>>>>
>>>> so solely from looking at the return value of the poll op in 2) and 3)
>>>> there is no way to disambiguate them. of course the linked timeout
>>>> operation result will allow you to do so, but you'd have to persist state
>>>> across cqe processings. you can also track the cancellations and know
>>>> to skip the explicitly cancelled ops' cqes (which is what i chose).
>>>>
>>>> there's also the problem of efficiency. you can imagine in a QUIC
>>>> server where you're constantly updating that poll timeout in response
>>>> to idle timeout and ACK scheduling, this extra work mounts.
>>>>
>>>> so i think the ability to update linked timeouts via
>>>> io_uring_prep_timeout_update would be fantastic.
>>>
>>> Hmm, I'll need to dig a bit, but whether it's a linked timeout or not
>>> should not matter. It's a timeout, it's queued and updated the same way.
>>> And we even check this in some of the liburing tests.
>>
>> We don't keep linked timeouts in ->timeout_list, so it's not
>> supported and has never been. Should be doable, but we need
>> to be careful synchronising with the link's head.
> 
> Yeah shoot you are right, I guess that explains the ENOENT. Would be
> nice to add, though. Synchronization should not be that different from
> dealing with regular timeouts.

_Not tested_, but something like below should do. will get it
done properly later, but even better if we already have a test
case. Victor?


From: Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH 1/2] io_uring: keep ltimeouts in a list

A preparation patch. Keep all queued linked timeout in a list, so they
may be found and updated.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index bf6551ea2c00..aad230b4cc2c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -375,6 +375,7 @@ struct io_ring_ctx {
 
 		struct io_submit_state	submit_state;
 		struct list_head	timeout_list;
+		struct list_head	ltimeout_list;
 		struct list_head	cq_overflow_list;
 		struct xarray		io_buffers;
 		struct xarray		personalities;
@@ -1277,6 +1278,7 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	INIT_LIST_HEAD(&ctx->iopoll_list);
 	INIT_LIST_HEAD(&ctx->defer_list);
 	INIT_LIST_HEAD(&ctx->timeout_list);
+	INIT_LIST_HEAD(&ctx->ltimeout_list);
 	spin_lock_init(&ctx->rsrc_ref_lock);
 	INIT_LIST_HEAD(&ctx->rsrc_ref_list);
 	INIT_DELAYED_WORK(&ctx->rsrc_put_work, io_rsrc_put_work);
@@ -1966,6 +1968,7 @@ static bool io_kill_linked_timeout(struct io_kiocb *req)
 		io_remove_next_linked(req);
 		link->timeout.head = NULL;
 		if (hrtimer_try_to_cancel(&io->timer) != -1) {
+			list_del(&link->timeout.list);
 			io_cqring_fill_event(link->ctx, link->user_data,
 					     -ECANCELED, 0);
 			io_put_req_deferred(link);
@@ -5830,6 +5833,7 @@ static int io_timeout_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	if (hweight32(flags & IORING_TIMEOUT_CLOCK_MASK) > 1)
 		return -EINVAL;
 
+	INIT_LIST_HEAD(&req->timeout.list);
 	req->timeout.off = off;
 	if (unlikely(off && !req->ctx->off_timeout_used))
 		req->ctx->off_timeout_used = true;
@@ -6585,6 +6589,7 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 		if (!req_ref_inc_not_zero(prev))
 			prev = NULL;
 	}
+	list_del(&req->timeout.list);
 	req->timeout.prev = prev;
 	spin_unlock_irqrestore(&ctx->timeout_lock, flags);
 
@@ -6608,6 +6613,7 @@ static void io_queue_linked_timeout(struct io_kiocb *req)
 		data->timer.function = io_link_timeout_fn;
 		hrtimer_start(&data->timer, timespec64_to_ktime(data->ts),
 				data->mode);
+		list_add_tail(&req->timeout.list, &ctx->ltimeout_list);
 	}
 	spin_unlock_irq(&ctx->timeout_lock);
 	/* drop submission reference */
@@ -8900,6 +8906,7 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
 		sock_release(ctx->ring_sock);
 	}
 #endif
+	WARN_ON_ONCE(!list_empty(&ctx->ltimeout_list));
 
 	io_mem_free(ctx->rings);
 	io_mem_free(ctx->sq_sqes);
-- 
2.33.0


From: Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH 2/2] io_uring: allow updating linked timeouts

We allow updating normal timeouts, add support for adjusting timings of
linked timeouts as well.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c                 | 44 +++++++++++++++++++++++++++++++----
 include/uapi/linux/io_uring.h | 11 +++++----
 2 files changed, 46 insertions(+), 9 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index aad230b4cc2c..c9d672ba5eaf 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -552,6 +552,7 @@ struct io_timeout_rem {
 	/* timeout update */
 	struct timespec64		ts;
 	u32				flags;
+	bool				ltimeout;
 };
 
 struct io_rw {
@@ -1069,6 +1070,7 @@ static int io_req_prep_async(struct io_kiocb *req);
 
 static int io_install_fixed_file(struct io_kiocb *req, struct file *file,
 				 unsigned int issue_flags, u32 slot_index);
+static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer);
 
 static struct kmem_cache *req_cachep;
 
@@ -5732,6 +5734,31 @@ static clockid_t io_timeout_get_clock(struct io_timeout_data *data)
 	}
 }
 
+static int io_linked_timeout_update(struct io_ring_ctx *ctx, __u64 user_data,
+				    struct timespec64 *ts, enum hrtimer_mode mode)
+	__must_hold(&ctx->timeout_lock)
+{
+	struct io_timeout_data *io;
+	struct io_kiocb *req;
+	bool found = false;
+
+	list_for_each_entry(req, &ctx->ltimeout_list, timeout.list) {
+		found = user_data == req->user_data;
+		if (found)
+			break;
+	}
+	if (!found)
+		return -ENOENT;
+
+	io = req->async_data;
+	if (hrtimer_try_to_cancel(&io->timer) == -1)
+		return -EALREADY;
+	hrtimer_init(&io->timer, io_timeout_get_clock(io), mode);
+	io->timer.function = io_link_timeout_fn;
+	hrtimer_start(&io->timer, timespec64_to_ktime(*ts), mode);
+	return 0;
+}
+
 static int io_timeout_update(struct io_ring_ctx *ctx, __u64 user_data,
 			     struct timespec64 *ts, enum hrtimer_mode mode)
 	__must_hold(&ctx->timeout_lock)
@@ -5763,10 +5790,15 @@ static int io_timeout_remove_prep(struct io_kiocb *req,
 	if (sqe->ioprio || sqe->buf_index || sqe->len || sqe->splice_fd_in)
 		return -EINVAL;
 
+	tr->ltimeout = false;
 	tr->addr = READ_ONCE(sqe->addr);
 	tr->flags = READ_ONCE(sqe->timeout_flags);
-	if (tr->flags & IORING_TIMEOUT_UPDATE) {
-		if (tr->flags & ~(IORING_TIMEOUT_UPDATE|IORING_TIMEOUT_ABS))
+	if (tr->flags & IORING_TIMEOUT_UPDATE_MASK) {
+		if (hweight32(tr->flags & IORING_TIMEOUT_CLOCK_MASK) > 1)
+			return -EINVAL;
+		if (tr->flags & IORING_LINK_TIMEOUT_UPDATE)
+			tr->ltimeout = true;
+		if (tr->flags & ~(IORING_TIMEOUT_UPDATE_MASK|IORING_TIMEOUT_ABS))
 			return -EINVAL;
 		if (get_timespec64(&tr->ts, u64_to_user_ptr(sqe->addr2)))
 			return -EFAULT;
@@ -5800,9 +5832,13 @@ static int io_timeout_remove(struct io_kiocb *req, unsigned int issue_flags)
 		spin_unlock_irq(&ctx->timeout_lock);
 		spin_unlock(&ctx->completion_lock);
 	} else {
+		enum hrtimer_mode mode = io_translate_timeout_mode(tr->flags);
+
 		spin_lock_irq(&ctx->timeout_lock);
-		ret = io_timeout_update(ctx, tr->addr, &tr->ts,
-					io_translate_timeout_mode(tr->flags));
+		if (tr->ltimeout)
+			ret = io_linked_timeout_update(ctx, tr->addr, &tr->ts, mode);
+		else
+			ret = io_timeout_update(ctx, tr->addr, &tr->ts, mode);
 		spin_unlock_irq(&ctx->timeout_lock);
 	}
 
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 4ea0b46e3da0..4ae753650513 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -149,12 +149,13 @@ enum {
 /*
  * sqe->timeout_flags
  */
-#define IORING_TIMEOUT_ABS	(1U << 0)
-#define IORING_TIMEOUT_UPDATE	(1U << 1)
-#define IORING_TIMEOUT_BOOTTIME	(1U << 2)
-#define IORING_TIMEOUT_REALTIME	(1U << 3)
+#define IORING_TIMEOUT_ABS		(1U << 0)
+#define IORING_TIMEOUT_UPDATE		(1U << 1)
+#define IORING_TIMEOUT_BOOTTIME		(1U << 2)
+#define IORING_TIMEOUT_REALTIME		(1U << 3)
+#define IORING_LINK_TIMEOUT_UPDATE	(1U << 4)
 #define IORING_TIMEOUT_CLOCK_MASK	(IORING_TIMEOUT_BOOTTIME | IORING_TIMEOUT_REALTIME)
-
+#define IORING_TIMEOUT_UPDATE_MASK	(IORING_TIMEOUT_UPDATE | IORING_LINK_TIMEOUT_UPDATE)
 /*
  * sqe->splice_flags
  * extends splice(2) flags
-- 
2.33.0

Re: io_uring_prep_timeout_update on linked timeouts

[Jens Axboe]

On 8/28/21 3:38 PM, Pavel Begunkov wrote:
> On 8/28/21 2:43 PM, Jens Axboe wrote:
>> On 8/28/21 7:39 AM, Pavel Begunkov wrote:
>>> On 8/28/21 4:22 AM, Jens Axboe wrote:
>>>> On 8/26/21 7:40 PM, Victor Stewart wrote:
>>>>> On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>>>>>>
>>>>>> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>>>>>>>
>>>>>>> we're able to update timeouts with io_uring_prep_timeout_update
>>>>>>> without having to cancel
>>>>>>> and resubmit, has it ever been considered adding this ability to
>>>>>>> linked timeouts?
>>>>>>
>>>>>> whoops turns out this does work. just tested it.
>>>>>
>>>>> doesn't work actually. missed that because of a bit of misdirection.
>>>>> returns -ENOENT.
>>>>>
>>>>> the problem with the current way of cancelling then resubmitting
>>>>> a new a timeout linked op (let's use poll here) is you have 3 situations:
>>>>>
>>>>> 1) the poll triggers and you get some positive value. all good.
>>>>>
>>>>> 2) the linked timeout triggers and cancels the poll, so the poll
>>>>> operation returns -ECANCELED.
>>>>>
>>>>> 3) you cancel the existing poll op, and submit a new one with
>>>>> the updated linked timeout. now the original poll op returns
>>>>> -ECANCELED.
>>>>>
>>>>> so solely from looking at the return value of the poll op in 2) and 3)
>>>>> there is no way to disambiguate them. of course the linked timeout
>>>>> operation result will allow you to do so, but you'd have to persist state
>>>>> across cqe processings. you can also track the cancellations and know
>>>>> to skip the explicitly cancelled ops' cqes (which is what i chose).
>>>>>
>>>>> there's also the problem of efficiency. you can imagine in a QUIC
>>>>> server where you're constantly updating that poll timeout in response
>>>>> to idle timeout and ACK scheduling, this extra work mounts.
>>>>>
>>>>> so i think the ability to update linked timeouts via
>>>>> io_uring_prep_timeout_update would be fantastic.
>>>>
>>>> Hmm, I'll need to dig a bit, but whether it's a linked timeout or not
>>>> should not matter. It's a timeout, it's queued and updated the same way.
>>>> And we even check this in some of the liburing tests.
>>>
>>> We don't keep linked timeouts in ->timeout_list, so it's not
>>> supported and has never been. Should be doable, but we need
>>> to be careful synchronising with the link's head.
>>
>> Yeah shoot you are right, I guess that explains the ENOENT. Would be
>> nice to add, though. Synchronization should not be that different from
>> dealing with regular timeouts.
> 
> _Not tested_, but something like below should do. will get it
> done properly later, but even better if we already have a test
> case. Victor?

FWIW, I wrote a simple test case for it, and it seemed to work fine.
Nothing fancy, just a piped read that would never finish with a linked
timeout (1s), submit, then submit a ltimeout update that changes it to
2s instead. Test runs and update completes first with res == 0 as
expected, and 2s later the ltimeout completes with -EALREADY (because
the piped read went async) and the piped read gets canceled.

That seems to be as expected, and didn't trigger anything odd.

-- 
Jens Axboe

Re: io_uring_prep_timeout_update on linked timeouts

[Victor Stewart]

> > _Not tested_, but something like below should do. will get it
> > done properly later, but even better if we already have a test
> > case. Victor?

sorry been traveling! i can extract out a simple test today...

> FWIW, I wrote a simple test case for it, and it seemed to work fine.
> Nothing fancy, just a piped read that would never finish with a linked
> timeout (1s), submit, then submit a ltimeout update that changes it to
> 2s instead. Test runs and update completes first with res == 0 as
> expected, and 2s later the ltimeout completes with -EALREADY (because
> the piped read went async) and the piped read gets canceled.

...unless?

Re: io_uring_prep_timeout_update on linked timeouts

[Pavel Begunkov]

On 8/29/21 3:40 AM, Jens Axboe wrote:
> On 8/28/21 3:38 PM, Pavel Begunkov wrote:
>> On 8/28/21 2:43 PM, Jens Axboe wrote:
>>> On 8/28/21 7:39 AM, Pavel Begunkov wrote:
>>>> On 8/28/21 4:22 AM, Jens Axboe wrote:
>>>>> On 8/26/21 7:40 PM, Victor Stewart wrote:
>>>>>> On Wed, Aug 25, 2021 at 2:27 AM Victor Stewart <v@nametag.social> wrote:
>>>>>>>
>>>>>>> On Tue, Aug 24, 2021 at 11:43 PM Victor Stewart <v@nametag.social> wrote:
>>>>>>>>
>>>>>>>> we're able to update timeouts with io_uring_prep_timeout_update
>>>>>>>> without having to cancel
>>>>>>>> and resubmit, has it ever been considered adding this ability to
>>>>>>>> linked timeouts?
>>>>>>>
>>>>>>> whoops turns out this does work. just tested it.
>>>>>>
>>>>>> doesn't work actually. missed that because of a bit of misdirection.
>>>>>> returns -ENOENT.
>>>>>>
>>>>>> the problem with the current way of cancelling then resubmitting
>>>>>> a new a timeout linked op (let's use poll here) is you have 3 situations:
>>>>>>
>>>>>> 1) the poll triggers and you get some positive value. all good.
>>>>>>
>>>>>> 2) the linked timeout triggers and cancels the poll, so the poll
>>>>>> operation returns -ECANCELED.
>>>>>>
>>>>>> 3) you cancel the existing poll op, and submit a new one with
>>>>>> the updated linked timeout. now the original poll op returns
>>>>>> -ECANCELED.
>>>>>>
>>>>>> so solely from looking at the return value of the poll op in 2) and 3)
>>>>>> there is no way to disambiguate them. of course the linked timeout
>>>>>> operation result will allow you to do so, but you'd have to persist state
>>>>>> across cqe processings. you can also track the cancellations and know
>>>>>> to skip the explicitly cancelled ops' cqes (which is what i chose).
>>>>>>
>>>>>> there's also the problem of efficiency. you can imagine in a QUIC
>>>>>> server where you're constantly updating that poll timeout in response
>>>>>> to idle timeout and ACK scheduling, this extra work mounts.
>>>>>>
>>>>>> so i think the ability to update linked timeouts via
>>>>>> io_uring_prep_timeout_update would be fantastic.
>>>>>
>>>>> Hmm, I'll need to dig a bit, but whether it's a linked timeout or not
>>>>> should not matter. It's a timeout, it's queued and updated the same way.
>>>>> And we even check this in some of the liburing tests.
>>>>
>>>> We don't keep linked timeouts in ->timeout_list, so it's not
>>>> supported and has never been. Should be doable, but we need
>>>> to be careful synchronising with the link's head.
>>>
>>> Yeah shoot you are right, I guess that explains the ENOENT. Would be
>>> nice to add, though. Synchronization should not be that different from
>>> dealing with regular timeouts.
>>
>> _Not tested_, but something like below should do. will get it
>> done properly later, but even better if we already have a test
>> case. Victor?
> 
> FWIW, I wrote a simple test case for it, and it seemed to work fine.
> Nothing fancy, just a piped read that would never finish with a linked
> timeout (1s), submit, then submit a ltimeout update that changes it to
> 2s instead. Test runs and update completes first with res == 0 as
> expected, and 2s later the ltimeout completes with -EALREADY (because
> the piped read went async) and the piped read gets canceled.
> 
> That seems to be as expected, and didn't trigger anything odd.

Perfect. Thanks, Jens

-- 
Pavel Begunkov

Re: io_uring_prep_timeout_update on linked timeouts

[Pavel Begunkov]

On 8/31/21 12:36 PM, Victor Stewart wrote:
>>> _Not tested_, but something like below should do. will get it
>>> done properly later, but even better if we already have a test
>>> case. Victor?
> 
> sorry been traveling! i can extract out a simple test today...
> 
>> FWIW, I wrote a simple test case for it, and it seemed to work fine.
>> Nothing fancy, just a piped read that would never finish with a linked
>> timeout (1s), submit, then submit a ltimeout update that changes it to
>> 2s instead. Test runs and update completes first with res == 0 as
>> expected, and 2s later the ltimeout completes with -EALREADY (because
>> the piped read went async) and the piped read gets canceled.
> 
> ...unless?

Extra tests are always appreciated, there is never too many of them :)
FWIW, don't see any new related added by Jens (yet?).

-- 
Pavel Begunkov