[PATCH v2 0/3] cancel_hash per entry lock

[PATCH v2 0/3] cancel_hash per entry lock

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Make per entry lock for cancel_hash array, this reduces usage of
completion_lock and contension between cancel_hash entries.

v1->v2:
 - Add per entry lock for poll/apoll task work code which was missed
   in v1
 - add an member in io_kiocb to track req's indice in cancel_hash

Hao Xu (3):
  io_uring: add hash_index and its logic to track req in cancel_hash
  io_uring: add an io_hash_bucket structure for smaller granularity lock
  io_uring: switch cancel_hash to use per list spinlock

 io_uring/cancel.c         | 15 +++++++--
 io_uring/cancel.h         |  6 ++++
 io_uring/fdinfo.c         |  9 ++++--
 io_uring/io_uring.c       |  8 +++--
 io_uring/io_uring_types.h |  3 +-
 io_uring/poll.c           | 64 +++++++++++++++++++++------------------
 6 files changed, 67 insertions(+), 38 deletions(-)


base-commit: d8271bf021438f468dab3cd84fe5279b5bbcead8
-- 
2.25.1

[PATCH 1/3] io_uring: add hash_index and its logic to track req in cancel_hash

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Add a new member hash_index in struct io_kiocb to track the req index
in cancel_hash array. This is needed in later patches.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 io_uring/io_uring_types.h | 1 +
 io_uring/poll.c           | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/io_uring/io_uring_types.h b/io_uring/io_uring_types.h
index 7c22cf35a7e2..2041ee83467d 100644
--- a/io_uring/io_uring_types.h
+++ b/io_uring/io_uring_types.h
@@ -474,6 +474,7 @@ struct io_kiocb {
 			u64		extra2;
 		};
 	};
+	unsigned int			hash_index;
 	/* internal polling, see IORING_FEAT_FAST_POLL */
 	struct async_poll		*apoll;
 	/* opcode allocated if it needs to store data for async defer */
diff --git a/io_uring/poll.c b/io_uring/poll.c
index 0df5eca93b16..95e28f32b49c 100644
--- a/io_uring/poll.c
+++ b/io_uring/poll.c
@@ -74,8 +74,10 @@ static void io_poll_req_insert(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	struct hlist_head *list;
+	u32 index = hash_long(req->cqe.user_data, ctx->cancel_hash_bits);
 
-	list = &ctx->cancel_hash[hash_long(req->cqe.user_data, ctx->cancel_hash_bits)];
+	req->hash_index = index;
+	list = &ctx->cancel_hash[index];
 	hlist_add_head(&req->hash_node, list);
 }
 
-- 
2.25.1

Re: [PATCH 1/3] io_uring: add hash_index and its logic to track req in cancel_hash

[Pavel Begunkov]

On 6/6/22 07:57, Hao Xu wrote:
> From: Hao Xu <howeyxu@tencent.com>
> 
> Add a new member hash_index in struct io_kiocb to track the req index
> in cancel_hash array. This is needed in later patches.
> 
> Signed-off-by: Hao Xu <howeyxu@tencent.com>
> ---
>   io_uring/io_uring_types.h | 1 +
>   io_uring/poll.c           | 4 +++-
>   2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/io_uring/io_uring_types.h b/io_uring/io_uring_types.h
> index 7c22cf35a7e2..2041ee83467d 100644
> --- a/io_uring/io_uring_types.h
> +++ b/io_uring/io_uring_types.h
> @@ -474,6 +474,7 @@ struct io_kiocb {
>   			u64		extra2;
>   		};
>   	};
> +	unsigned int			hash_index;

Didn't take a closer look, but can we make rid of it?
E.g. computing it again when ejecting a request from
the hash? or keep it in struct io_poll?

>   	/* internal polling, see IORING_FEAT_FAST_POLL */
>   	struct async_poll		*apoll;
>   	/* opcode allocated if it needs to store data for async defer */
> diff --git a/io_uring/poll.c b/io_uring/poll.c
> index 0df5eca93b16..95e28f32b49c 100644
> --- a/io_uring/poll.c
> +++ b/io_uring/poll.c
> @@ -74,8 +74,10 @@ static void io_poll_req_insert(struct io_kiocb *req)
>   {
>   	struct io_ring_ctx *ctx = req->ctx;
>   	struct hlist_head *list;
> +	u32 index = hash_long(req->cqe.user_data, ctx->cancel_hash_bits);
>   
> -	list = &ctx->cancel_hash[hash_long(req->cqe.user_data, ctx->cancel_hash_bits)];
> +	req->hash_index = index;
> +	list = &ctx->cancel_hash[index];
>   	hlist_add_head(&req->hash_node, list);
>   }
>   

-- 
Pavel Begunkov

Re: [PATCH 1/3] io_uring: add hash_index and its logic to track req in cancel_hash

[Hao Xu]

On 6/6/22 19:59, Pavel Begunkov wrote:
> On 6/6/22 07:57, Hao Xu wrote:
>> From: Hao Xu <howeyxu@tencent.com>
>>
>> Add a new member hash_index in struct io_kiocb to track the req index
>> in cancel_hash array. This is needed in later patches.
>>
>> Signed-off-by: Hao Xu <howeyxu@tencent.com>
>> ---
>>   io_uring/io_uring_types.h | 1 +
>>   io_uring/poll.c           | 4 +++-
>>   2 files changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/io_uring/io_uring_types.h b/io_uring/io_uring_types.h
>> index 7c22cf35a7e2..2041ee83467d 100644
>> --- a/io_uring/io_uring_types.h
>> +++ b/io_uring/io_uring_types.h
>> @@ -474,6 +474,7 @@ struct io_kiocb {
>>               u64        extra2;
>>           };
>>       };
>> +    unsigned int            hash_index;
> 
> Didn't take a closer look, but can we make rid of it?
> E.g. computing it again when ejecting a request from
> the hash? or keep it in struct io_poll?

Good point, I prefer moving it to io_poll to computing it again since
this patchset is to try to make it faster.

[PATCH 2/3] io_uring: add an io_hash_bucket structure for smaller granularity lock

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Add a new io_hash_bucket structure so that each bucket in cancel_hash
has separate spinlock. This is a prep patch for later use.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 io_uring/cancel.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/io_uring/cancel.h b/io_uring/cancel.h
index 4f35d8696325..b9218310611c 100644
--- a/io_uring/cancel.h
+++ b/io_uring/cancel.h
@@ -4,3 +4,8 @@ int io_async_cancel_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
 int io_async_cancel(struct io_kiocb *req, unsigned int issue_flags);
 
 int io_try_cancel(struct io_kiocb *req, struct io_cancel_data *cd);
+
+struct io_hash_bucket {
+	spinlock_t		lock;
+	struct hlist_head	list;
+};
-- 
2.25.1

Re: [PATCH 2/3] io_uring: add an io_hash_bucket structure for smaller granularity lock

[Pavel Begunkov]

On 6/6/22 07:57, Hao Xu wrote:
> From: Hao Xu <howeyxu@tencent.com>
> 
> Add a new io_hash_bucket structure so that each bucket in cancel_hash
> has separate spinlock. This is a prep patch for later use.
> 
> Signed-off-by: Hao Xu <howeyxu@tencent.com>
> ---
>   io_uring/cancel.h | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/io_uring/cancel.h b/io_uring/cancel.h
> index 4f35d8696325..b9218310611c 100644
> --- a/io_uring/cancel.h
> +++ b/io_uring/cancel.h
> @@ -4,3 +4,8 @@ int io_async_cancel_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
>   int io_async_cancel(struct io_kiocb *req, unsigned int issue_flags);
>   
>   int io_try_cancel(struct io_kiocb *req, struct io_cancel_data *cd);
> +
> +struct io_hash_bucket {
> +	spinlock_t		lock;
> +	struct hlist_head	list;
> +};

please, in future just merge such patches into the next one,
separately it doesn't do anything meaningful, the struct is
not used here and IMHO only makes reviewing harder.

-- 
Pavel Begunkov

[PATCH 3/3] io_uring: switch cancel_hash to use per list spinlock

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Use per list lock for cancel_hash, this removes some completion lock
invocation and remove contension between different cancel_hash entries

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 io_uring/cancel.c         | 15 ++++++++--
 io_uring/cancel.h         |  1 +
 io_uring/fdinfo.c         |  9 ++++--
 io_uring/io_uring.c       |  8 +++--
 io_uring/io_uring_types.h |  2 +-
 io_uring/poll.c           | 62 +++++++++++++++++++++------------------
 6 files changed, 59 insertions(+), 38 deletions(-)

diff --git a/io_uring/cancel.c b/io_uring/cancel.c
index 83cceb52d82d..c3e5b8058b0d 100644
--- a/io_uring/cancel.c
+++ b/io_uring/cancel.c
@@ -6,6 +6,7 @@
 #include <linux/mm.h>
 #include <linux/slab.h>
 #include <linux/namei.h>
+#include <linux/list.h>
 #include <linux/io_uring.h>
 
 #include <uapi/linux/io_uring.h>
@@ -93,14 +94,14 @@ int io_try_cancel(struct io_kiocb *req, struct io_cancel_data *cd)
 	if (!ret)
 		return 0;
 
-	spin_lock(&ctx->completion_lock);
 	ret = io_poll_cancel(ctx, cd);
 	if (ret != -ENOENT)
 		goto out;
+	spin_lock(&ctx->completion_lock);
 	if (!(cd->flags & IORING_ASYNC_CANCEL_FD))
 		ret = io_timeout_cancel(ctx, cd);
-out:
 	spin_unlock(&ctx->completion_lock);
+out:
 	return ret;
 }
 
@@ -192,3 +193,13 @@ int io_async_cancel(struct io_kiocb *req, unsigned int issue_flags)
 	io_req_set_res(req, ret, 0);
 	return IOU_OK;
 }
+
+inline void init_hash_table(struct io_hash_bucket *hash_table, unsigned size)
+{
+	unsigned int i;
+
+	for (i = 0; i < size; i++) {
+		spin_lock_init(&hash_table[i].lock);
+		INIT_HLIST_HEAD(&hash_table[i].list);
+	}
+}
diff --git a/io_uring/cancel.h b/io_uring/cancel.h
index b9218310611c..f682e9811e68 100644
--- a/io_uring/cancel.h
+++ b/io_uring/cancel.h
@@ -4,6 +4,7 @@ int io_async_cancel_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
 int io_async_cancel(struct io_kiocb *req, unsigned int issue_flags);
 
 int io_try_cancel(struct io_kiocb *req, struct io_cancel_data *cd);
+inline void init_hash_table(struct io_hash_bucket *hash_table, unsigned size);
 
 struct io_hash_bucket {
 	spinlock_t		lock;
diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
index fcedde4b4b1e..f941c73f5502 100644
--- a/io_uring/fdinfo.c
+++ b/io_uring/fdinfo.c
@@ -13,6 +13,7 @@
 #include "io_uring.h"
 #include "sqpoll.h"
 #include "fdinfo.h"
+#include "cancel.h"
 
 #ifdef CONFIG_PROC_FS
 static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
@@ -157,17 +158,19 @@ static __cold void __io_uring_show_fdinfo(struct io_ring_ctx *ctx,
 		mutex_unlock(&ctx->uring_lock);
 
 	seq_puts(m, "PollList:\n");
-	spin_lock(&ctx->completion_lock);
 	for (i = 0; i < (1U << ctx->cancel_hash_bits); i++) {
-		struct hlist_head *list = &ctx->cancel_hash[i];
+		struct io_hash_bucket *hb = &ctx->cancel_hash[i];
 		struct io_kiocb *req;
 
-		hlist_for_each_entry(req, list, hash_node)
+		spin_lock(&hb->lock);
+		hlist_for_each_entry(req, &hb->list, hash_node)
 			seq_printf(m, "  op=%d, task_works=%d\n", req->opcode,
 					task_work_pending(req->task));
+		spin_unlock(&hb->lock);
 	}
 
 	seq_puts(m, "CqOverflowList:\n");
+	spin_lock(&ctx->completion_lock);
 	list_for_each_entry(ocqe, &ctx->cq_overflow_list, list) {
 		struct io_uring_cqe *cqe = &ocqe->cqe;
 
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 1572ebe3cff1..b67ab76b9e56 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -725,11 +725,13 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	if (hash_bits <= 0)
 		hash_bits = 1;
 	ctx->cancel_hash_bits = hash_bits;
-	ctx->cancel_hash = kmalloc((1U << hash_bits) * sizeof(struct hlist_head),
-					GFP_KERNEL);
+	ctx->cancel_hash =
+		kmalloc((1U << hash_bits) * sizeof(struct io_hash_bucket),
+			GFP_KERNEL);
 	if (!ctx->cancel_hash)
 		goto err;
-	__hash_init(ctx->cancel_hash, 1U << hash_bits);
+
+	init_hash_table(ctx->cancel_hash, 1U << hash_bits);
 
 	ctx->dummy_ubuf = kzalloc(sizeof(*ctx->dummy_ubuf), GFP_KERNEL);
 	if (!ctx->dummy_ubuf)
diff --git a/io_uring/io_uring_types.h b/io_uring/io_uring_types.h
index 2041ee83467d..59231f7345ac 100644
--- a/io_uring/io_uring_types.h
+++ b/io_uring/io_uring_types.h
@@ -230,7 +230,7 @@ struct io_ring_ctx {
 		 * manipulate the list, hence no extra locking is needed there.
 		 */
 		struct io_wq_work_list	iopoll_list;
-		struct hlist_head	*cancel_hash;
+		struct io_hash_bucket	*cancel_hash;
 		unsigned		cancel_hash_bits;
 		bool			poll_multi_queue;
 
diff --git a/io_uring/poll.c b/io_uring/poll.c
index 95e28f32b49c..d40fad768d58 100644
--- a/io_uring/poll.c
+++ b/io_uring/poll.c
@@ -19,6 +19,7 @@
 #include "opdef.h"
 #include "kbuf.h"
 #include "poll.h"
+#include "cancel.h"
 
 struct io_poll_update {
 	struct file			*file;
@@ -73,12 +74,22 @@ static struct io_poll *io_poll_get_single(struct io_kiocb *req)
 static void io_poll_req_insert(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;
-	struct hlist_head *list;
 	u32 index = hash_long(req->cqe.user_data, ctx->cancel_hash_bits);
+	struct io_hash_bucket *hb = &ctx->cancel_hash[index];
 
 	req->hash_index = index;
-	list = &ctx->cancel_hash[index];
-	hlist_add_head(&req->hash_node, list);
+	spin_lock(&hb->lock);
+	hlist_add_head(&req->hash_node, &hb->list);
+	spin_unlock(&hb->lock);
+}
+
+static void io_poll_req_delete(struct io_kiocb *req, struct io_ring_ctx *ctx)
+{
+	spinlock_t *lock = &ctx->cancel_hash[req->hash_index].lock;
+
+	spin_lock(lock);
+	hash_del(&req->hash_node);
+	spin_unlock(lock);
 }
 
 static void io_init_poll_iocb(struct io_poll *poll, __poll_t events,
@@ -222,8 +233,8 @@ static void io_poll_task_func(struct io_kiocb *req, bool *locked)
 	}
 
 	io_poll_remove_entries(req);
+	io_poll_req_delete(req, ctx);
 	spin_lock(&ctx->completion_lock);
-	hash_del(&req->hash_node);
 	req->cqe.flags = 0;
 	__io_req_complete_post(req);
 	io_commit_cqring(ctx);
@@ -233,7 +244,6 @@ static void io_poll_task_func(struct io_kiocb *req, bool *locked)
 
 static void io_apoll_task_func(struct io_kiocb *req, bool *locked)
 {
-	struct io_ring_ctx *ctx = req->ctx;
 	int ret;
 
 	ret = io_poll_check_events(req, locked);
@@ -241,9 +251,7 @@ static void io_apoll_task_func(struct io_kiocb *req, bool *locked)
 		return;
 
 	io_poll_remove_entries(req);
-	spin_lock(&ctx->completion_lock);
-	hash_del(&req->hash_node);
-	spin_unlock(&ctx->completion_lock);
+	io_poll_req_delete(req, req->ctx);
 
 	if (!ret)
 		io_req_task_submit(req, locked);
@@ -437,9 +445,7 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
 		return 0;
 	}
 
-	spin_lock(&ctx->completion_lock);
 	io_poll_req_insert(req);
-	spin_unlock(&ctx->completion_lock);
 
 	if (mask && (poll->events & EPOLLET)) {
 		/* can't multishot if failed, just queue the event we've got */
@@ -536,32 +542,32 @@ __cold bool io_poll_remove_all(struct io_ring_ctx *ctx, struct task_struct *tsk,
 	bool found = false;
 	int i;
 
-	spin_lock(&ctx->completion_lock);
 	for (i = 0; i < (1U << ctx->cancel_hash_bits); i++) {
-		struct hlist_head *list;
+		struct io_hash_bucket *hb = &ctx->cancel_hash[i];
 
-		list = &ctx->cancel_hash[i];
-		hlist_for_each_entry_safe(req, tmp, list, hash_node) {
+		spin_lock(&hb->lock);
+		hlist_for_each_entry_safe(req, tmp, &hb->list, hash_node) {
 			if (io_match_task_safe(req, tsk, cancel_all)) {
 				hlist_del_init(&req->hash_node);
 				io_poll_cancel_req(req);
 				found = true;
 			}
 		}
+		spin_unlock(&hb->lock);
 	}
-	spin_unlock(&ctx->completion_lock);
 	return found;
 }
 
 static struct io_kiocb *io_poll_find(struct io_ring_ctx *ctx, bool poll_only,
 				     struct io_cancel_data *cd)
-	__must_hold(&ctx->completion_lock)
 {
-	struct hlist_head *list;
 	struct io_kiocb *req;
 
-	list = &ctx->cancel_hash[hash_long(cd->data, ctx->cancel_hash_bits)];
-	hlist_for_each_entry(req, list, hash_node) {
+	u32 index = hash_long(cd->data, ctx->cancel_hash_bits);
+	struct io_hash_bucket *hb = &ctx->cancel_hash[index];
+
+	spin_lock(&hb->lock);
+	hlist_for_each_entry(req, &hb->list, hash_node) {
 		if (cd->data != req->cqe.user_data)
 			continue;
 		if (poll_only && req->opcode != IORING_OP_POLL_ADD)
@@ -571,47 +577,48 @@ static struct io_kiocb *io_poll_find(struct io_ring_ctx *ctx, bool poll_only,
 				continue;
 			req->work.cancel_seq = cd->seq;
 		}
+		spin_unlock(&hb->lock);
 		return req;
 	}
+	spin_unlock(&hb->lock);
 	return NULL;
 }
 
 static struct io_kiocb *io_poll_file_find(struct io_ring_ctx *ctx,
 					  struct io_cancel_data *cd)
-	__must_hold(&ctx->completion_lock)
 {
 	struct io_kiocb *req;
 	int i;
 
 	for (i = 0; i < (1U << ctx->cancel_hash_bits); i++) {
-		struct hlist_head *list;
+		struct io_hash_bucket *hb = &ctx->cancel_hash[i];
 
-		list = &ctx->cancel_hash[i];
-		hlist_for_each_entry(req, list, hash_node) {
+		spin_lock(&hb->lock);
+		hlist_for_each_entry(req, &hb->list, hash_node) {
 			if (!(cd->flags & IORING_ASYNC_CANCEL_ANY) &&
 			    req->file != cd->file)
 				continue;
 			if (cd->seq == req->work.cancel_seq)
 				continue;
 			req->work.cancel_seq = cd->seq;
+			spin_unlock(&hb->lock);
 			return req;
 		}
+		spin_unlock(&hb->lock);
 	}
 	return NULL;
 }
 
 static bool io_poll_disarm(struct io_kiocb *req)
-	__must_hold(&ctx->completion_lock)
 {
 	if (!io_poll_get_ownership(req))
 		return false;
 	io_poll_remove_entries(req);
-	hash_del(&req->hash_node);
+	io_poll_req_delete(req, req->ctx);
 	return true;
 }
 
 int io_poll_cancel(struct io_ring_ctx *ctx, struct io_cancel_data *cd)
-	__must_hold(&ctx->completion_lock)
 {
 	struct io_kiocb *req;
 
@@ -720,14 +727,11 @@ int io_poll_remove(struct io_kiocb *req, unsigned int issue_flags)
 	int ret2, ret = 0;
 	bool locked;
 
-	spin_lock(&ctx->completion_lock);
 	preq = io_poll_find(ctx, true, &cd);
 	if (!preq || !io_poll_disarm(preq)) {
-		spin_unlock(&ctx->completion_lock);
 		ret = preq ? -EALREADY : -ENOENT;
 		goto out;
 	}
-	spin_unlock(&ctx->completion_lock);
 
 	if (poll_update->update_events || poll_update->update_user_data) {
 		/* only mask one event flags, keep behavior flags */
-- 
2.25.1

Re: [PATCH v2 0/3] cancel_hash per entry lock

[Hao Xu]

On 6/6/22 14:57, Hao Xu wrote:
> From: Hao Xu <howeyxu@tencent.com>
> 
> Make per entry lock for cancel_hash array, this reduces usage of
> completion_lock and contension between cancel_hash entries.
> 
> v1->v2:
>   - Add per entry lock for poll/apoll task work code which was missed
>     in v1
>   - add an member in io_kiocb to track req's indice in cancel_hash

Tried to test it with many poll_add IOSQQE_ASYNC requests but turned out
that there is little conpletion_lock contention, so no visible change in
data. But I still think this may be good for cancel_hash access in some
real cases where completion lock matters.

Regards,
Hao

Re: [PATCH v2 0/3] cancel_hash per entry lock

[Pavel Begunkov]

On 6/6/22 08:06, Hao Xu wrote:
> On 6/6/22 14:57, Hao Xu wrote:
>> From: Hao Xu <howeyxu@tencent.com>
>>
>> Make per entry lock for cancel_hash array, this reduces usage of
>> completion_lock and contension between cancel_hash entries.
>>
>> v1->v2:
>>   - Add per entry lock for poll/apoll task work code which was missed
>>     in v1
>>   - add an member in io_kiocb to track req's indice in cancel_hash
> 
> Tried to test it with many poll_add IOSQQE_ASYNC requests but turned out
> that there is little conpletion_lock contention, so no visible change in
> data. But I still think this may be good for cancel_hash access in some
> real cases where completion lock matters.

Conceptually I don't mind it, but let me ask in what
circumstances you expect it to make a difference? And
what can we do to get favourable numbers? For instance,
how many CPUs io-wq was using?

-- 
Pavel Begunkov

Re: [PATCH v2 0/3] cancel_hash per entry lock

[Pavel Begunkov]

On 6/6/22 13:02, Pavel Begunkov wrote:
> On 6/6/22 08:06, Hao Xu wrote:
>> On 6/6/22 14:57, Hao Xu wrote:
>>> From: Hao Xu <howeyxu@tencent.com>
>>>
>>> Make per entry lock for cancel_hash array, this reduces usage of
>>> completion_lock and contension between cancel_hash entries.
>>>
>>> v1->v2:
>>>   - Add per entry lock for poll/apoll task work code which was missed
>>>     in v1
>>>   - add an member in io_kiocb to track req's indice in cancel_hash
>>
>> Tried to test it with many poll_add IOSQQE_ASYNC requests but turned out
>> that there is little conpletion_lock contention, so no visible change in
>> data. But I still think this may be good for cancel_hash access in some
>> real cases where completion lock matters.
> 
> Conceptually I don't mind it, but let me ask in what
> circumstances you expect it to make a difference? And
> what can we do to get favourable numbers? For instance,
> how many CPUs io-wq was using?

Btw, I couldn't find ____cacheline_aligned_in_smp anywhere,
which I expect around those new spinlocks to avoid them sharing
cache lines
  
-- 
Pavel Begunkov

Re: [PATCH v2 0/3] cancel_hash per entry lock

[Hao Xu]

On 6/6/22 20:02, Pavel Begunkov wrote:
> On 6/6/22 08:06, Hao Xu wrote:
>> On 6/6/22 14:57, Hao Xu wrote:
>>> From: Hao Xu <howeyxu@tencent.com>
>>>
>>> Make per entry lock for cancel_hash array, this reduces usage of
>>> completion_lock and contension between cancel_hash entries.
>>>
>>> v1->v2:
>>>   - Add per entry lock for poll/apoll task work code which was missed
>>>     in v1
>>>   - add an member in io_kiocb to track req's indice in cancel_hash
>>
>> Tried to test it with many poll_add IOSQQE_ASYNC requests but turned out
>> that there is little conpletion_lock contention, so no visible change in
>> data. But I still think this may be good for cancel_hash access in some
>> real cases where completion lock matters.
> 
> Conceptually I don't mind it, but let me ask in what
> circumstances you expect it to make a difference? And

I suppose there are cases where a bunch of users trying to access
cancel_hash[] at the same time when people use multiple threads to
submit sqes or they use IOSQE_ASYNC. And these io-workers or task works
run parallel on different CPUs.

> what can we do to get favourable numbers? For instance,
> how many CPUs io-wq was using?

It is not easy to construct manually since it is related with task
scheduling, like if we just issue many IOSQE_ASYNC polls in an
idle machine with many CPUs, there won't be much contention because of
different thread start time(thus they access cancel_hash at different
time
>