Re: [PATCH] io_uring: create percpu io sq thread when IORING_SETUP_SQ_AFF is flagged

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

There're still some left work to do, fox example, use srcu to protect multiple
ctxs iteration to reduce mutex lock competition, make percpu thread aware of
cpu hotplug, but I send it now for some early comments, thanks in advance!

Regards,
Xiaoguang Wang

> Currently we can create multiple io_uring instances which all have SQPOLL
> enabled and make them bound to same cpu core by setting sq_thread_cpu argument,
> but sometimes this isn't efficient. Imagine such extreme case, create two io
> uring instances, which both have SQPOLL enabled and are bound to same cpu core.
> One instance submits io per 500ms, another instance submits io continually,
> then obviously the 1st instance still always contend for cpu resource, which
> will impact 2nd instance.
> 
> I have constructed tests case to the evaluate impact:
>      1, a test progarm which creates one io_uring instance with SQPOLL
> enabled, and bound to cpu core 61.
>      2, sudo taskset -c 60 fio  -name=fiotest -filename=/dev/nvme0n1 -iodepth=128
> -thread -rw=read -ioengine=io_uring  -hipri=0 -sqthread_poll=1  -sqthread_poll_cpu=61
> -registerfiles=1 -direct=1 -bs=4k -size=10G -numjobs=1  -time_based -runtime=120
> Note both io_uring instance's sq_thread_cpu are bound to cpu core 61.
> 
> In current kernel, run above two jobs concurrently, and the 2nd job will get:
> Run status group 0 (all jobs):
>     READ: bw=1623MiB/s (1701MB/s), 1623MiB/s-1623MiB/s (1701MB/s-1701MB/s),
> io=190GiB (204GB), run=120010-120010msec
> 
> We can see that 1st instance has a huge impact to 2nd instance, I think it's
> because of unordered competition for same cpu resource, every instance gets
> 50% cpu usage.
> 
> To fix this issue, when io_uring instance uses IORING_SETUP_SQ_AFF to specify a
> cpu,  we create a percpu io sq_thread to handle multiple io_uring instances' io
> requests serially. With this patch, in same environment, we get a huge improvement:
> Run status group 0 (all jobs):
>     READ: bw=3231MiB/s (3388MB/s), 3231MiB/s-3231MiB/s (3388MB/s-3388MB/s),
> io=379GiB (407GB), run=120001-120001msec
> 
> Link: https://lore.kernel.org/io-uring/c94098d2-279e-a552-91ec-8a8f177d770a@linux.alibaba.com/T/#t
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
>   fs/io_uring.c | 257 +++++++++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 232 insertions(+), 25 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index ed0c22eb9808..e49ff1f67681 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -258,8 +258,13 @@ struct io_ring_ctx {
>   	/* IO offload */
>   	struct io_wq		*io_wq;
>   	struct task_struct	*sqo_thread;	/* if using sq thread polling */
> +	wait_queue_head_t	*sqo_wait;
> +	int			submit_status;
> +	int			sq_thread_cpu;
> +	struct list_head	node;
> +
>   	struct mm_struct	*sqo_mm;
> -	wait_queue_head_t	sqo_wait;
> +	wait_queue_head_t	__sqo_wait;
>   
>   	/*
>   	 * If used, fixed file set. Writers must ensure that ->refs is dead,
> @@ -330,6 +335,16 @@ struct io_ring_ctx {
>   	struct work_struct		exit_work;
>   };
>   
> +struct io_percpu_thread {
> +	struct list_head ctx_list;
> +	wait_queue_head_t sqo_percpu_wait;
> +	struct mutex lock;
> +	struct task_struct *sqo_thread;
> +	unsigned int sq_thread_idle;
> +};
> +
> +static struct io_percpu_thread __percpu *percpu_threads;
> +
>   /*
>    * First field must be the file pointer in all the
>    * iocb unions! See also 'struct kiocb' in <linux/fs.h>
> @@ -926,9 +941,11 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>   		goto err;
>   
>   	ctx->flags = p->flags;
> -	init_waitqueue_head(&ctx->sqo_wait);
> +	init_waitqueue_head(&ctx->__sqo_wait);
> +	ctx->sqo_wait = &ctx->__sqo_wait;
>   	init_waitqueue_head(&ctx->cq_wait);
>   	INIT_LIST_HEAD(&ctx->cq_overflow_list);
> +	INIT_LIST_HEAD(&ctx->node);
>   	init_completion(&ctx->completions[0]);
>   	init_completion(&ctx->completions[1]);
>   	idr_init(&ctx->io_buffer_idr);
> @@ -1157,8 +1174,8 @@ static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
>   {
>   	if (waitqueue_active(&ctx->wait))
>   		wake_up(&ctx->wait);
> -	if (waitqueue_active(&ctx->sqo_wait))
> -		wake_up(&ctx->sqo_wait);
> +	if (waitqueue_active(ctx->sqo_wait))
> +		wake_up(ctx->sqo_wait);
>   	if (io_should_trigger_evfd(ctx))
>   		eventfd_signal(ctx->cq_ev_fd, 1);
>   }
> @@ -1980,8 +1997,8 @@ static void io_iopoll_req_issued(struct io_kiocb *req)
>   		list_add_tail(&req->list, &ctx->poll_list);
>   
>   	if ((ctx->flags & IORING_SETUP_SQPOLL) &&
> -	    wq_has_sleeper(&ctx->sqo_wait))
> -		wake_up(&ctx->sqo_wait);
> +	    wq_has_sleeper(ctx->sqo_wait))
> +		wake_up(ctx->sqo_wait);
>   }
>   
>   static void io_file_put(struct io_submit_state *state)
> @@ -5994,7 +6011,7 @@ static int io_sq_thread(void *data)
>   				continue;
>   			}
>   
> -			prepare_to_wait(&ctx->sqo_wait, &wait,
> +			prepare_to_wait(ctx->sqo_wait, &wait,
>   						TASK_INTERRUPTIBLE);
>   
>   			/*
> @@ -6006,7 +6023,7 @@ static int io_sq_thread(void *data)
>   			 */
>   			if ((ctx->flags & IORING_SETUP_IOPOLL) &&
>   			    !list_empty_careful(&ctx->poll_list)) {
> -				finish_wait(&ctx->sqo_wait, &wait);
> +				finish_wait(ctx->sqo_wait, &wait);
>   				continue;
>   			}
>   
> @@ -6018,23 +6035,23 @@ static int io_sq_thread(void *data)
>   			to_submit = io_sqring_entries(ctx);
>   			if (!to_submit || ret == -EBUSY) {
>   				if (kthread_should_park()) {
> -					finish_wait(&ctx->sqo_wait, &wait);
> +					finish_wait(ctx->sqo_wait, &wait);
>   					break;
>   				}
>   				if (current->task_works) {
>   					task_work_run();
> -					finish_wait(&ctx->sqo_wait, &wait);
> +					finish_wait(ctx->sqo_wait, &wait);
>   					continue;
>   				}
>   				if (signal_pending(current))
>   					flush_signals(current);
>   				schedule();
> -				finish_wait(&ctx->sqo_wait, &wait);
> +				finish_wait(ctx->sqo_wait, &wait);
>   
>   				ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
>   				continue;
>   			}
> -			finish_wait(&ctx->sqo_wait, &wait);
> +			finish_wait(ctx->sqo_wait, &wait);
>   
>   			ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
>   		}
> @@ -6058,6 +6075,133 @@ static int io_sq_thread(void *data)
>   	return 0;
>   }
>   
> +static int process_ctx(struct io_ring_ctx *ctx)
> +{
> +	unsigned int to_submit;
> +	int ret = 0;
> +
> +	if (!list_empty(&ctx->poll_list)) {
> +		unsigned nr_events = 0;
> +
> +		mutex_lock(&ctx->uring_lock);
> +		if (!list_empty(&ctx->poll_list))
> +			io_iopoll_getevents(ctx, &nr_events, 0);
> +		mutex_unlock(&ctx->uring_lock);
> +	}
> +
> +	to_submit = io_sqring_entries(ctx);
> +	if (to_submit) {
> +		mutex_lock(&ctx->uring_lock);
> +		if (likely(!percpu_ref_is_dying(&ctx->refs)))
> +			ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
> +		mutex_unlock(&ctx->uring_lock);
> +	}
> +
> +	if (current->task_works)
> +		task_work_run();
> +
> +	io_sq_thread_drop_mm(ctx);
> +	return ret;
> +}
> +
> +static int io_sq_percpu_thread(void *data)
> +{
> +	struct io_percpu_thread *t = data;
> +	struct io_ring_ctx *ctx, *tmp;
> +	mm_segment_t old_fs;
> +	const struct cred *saved_creds, *cur_creds, *old_creds;
> +	unsigned long timeout;
> +	DEFINE_WAIT(wait);
> +	int iters = 0;
> +
> +	timeout = jiffies + t->sq_thread_idle;
> +	old_fs = get_fs();
> +	set_fs(USER_DS);
> +	saved_creds = cur_creds = NULL;
> +	while (!kthread_should_park()) {
> +		bool continue_run;
> +		bool needs_wait;
> +		unsigned int to_submit;
> +
> +		mutex_lock(&t->lock);
> +again:
> +		continue_run = false;
> +		list_for_each_entry_safe(ctx, tmp, &t->ctx_list, node) {
> +			if (cur_creds != ctx->creds) {
> +				old_creds = override_creds(ctx->creds);
> +				cur_creds = ctx->creds;
> +				if (saved_creds)
> +					put_cred(old_creds);
> +				else
> +					saved_creds = old_creds;
> +			}
> +			ctx->submit_status = process_ctx(ctx);
> +
> +			to_submit = io_sqring_entries(ctx);
> +			if (!continue_run &&
> +			    ((to_submit && ctx->submit_status != -EBUSY) ||
> +			    !list_empty(&ctx->poll_list)))
> +				continue_run = true;
> +		}
> +		if (continue_run && (++iters & 7)) {
> +			timeout = jiffies + t->sq_thread_idle;
> +			goto again;
> +		}
> +		mutex_unlock(&t->lock);
> +		if (continue_run) {
> +			timeout = jiffies + t->sq_thread_idle;
> +			continue;
> +		}
> +		if (!time_after(jiffies, timeout)) {
> +			cond_resched();
> +			continue;
> +		}
> +
> +		needs_wait = true;
> +		prepare_to_wait(&t->sqo_percpu_wait, &wait, TASK_INTERRUPTIBLE);
> +		mutex_lock(&t->lock);
> +		list_for_each_entry_safe(ctx, tmp, &t->ctx_list, node) {
> +			if ((ctx->flags & IORING_SETUP_IOPOLL) &&
> +			    !list_empty_careful(&ctx->poll_list)) {
> +				needs_wait = false;
> +				break;
> +			}
> +			to_submit = io_sqring_entries(ctx);
> +			if (to_submit && ctx->submit_status != -EBUSY) {
> +				needs_wait = false;
> +				break;
> +			}
> +		}
> +		if (needs_wait) {
> +			list_for_each_entry_safe(ctx, tmp, &t->ctx_list, node)
> +				ctx->rings->sq_flags |= IORING_SQ_NEED_WAKEUP;
> +			smp_mb();
> +
> +		}
> +		mutex_unlock(&t->lock);
> +
> +		if (needs_wait) {
> +			schedule();
> +			mutex_lock(&t->lock);
> +			list_for_each_entry_safe(ctx, tmp,
> +						 &t->ctx_list, node)
> +				ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
> +			mutex_unlock(&t->lock);
> +			finish_wait(&t->sqo_percpu_wait, &wait);
> +		} else
> +			finish_wait(&t->sqo_percpu_wait, &wait);
> +		timeout = jiffies + t->sq_thread_idle;
> +		cond_resched();
> +	}
> +
> +	if (current->task_works)
> +		task_work_run();
> +	set_fs(old_fs);
> +	revert_creds(saved_creds);
> +	kthread_parkme();
> +	return 0;
> +}
> +
>   struct io_wait_queue {
>   	struct wait_queue_entry wq;
>   	struct io_ring_ctx *ctx;
> @@ -6219,18 +6363,23 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
>   	return 0;
>   }
>   
> +static void destroy_io_percpu_thread(struct io_ring_ctx *ctx, int cpu);
> +
>   static void io_sq_thread_stop(struct io_ring_ctx *ctx)
>   {
>   	if (ctx->sqo_thread) {
> -		wait_for_completion(&ctx->completions[1]);
> -		/*
> -		 * The park is a bit of a work-around, without it we get
> -		 * warning spews on shutdown with SQPOLL set and affinity
> -		 * set to a single CPU.
> -		 */
> -		kthread_park(ctx->sqo_thread);
> -		kthread_stop(ctx->sqo_thread);
> -		ctx->sqo_thread = NULL;
> +		if (!(ctx->flags & IORING_SETUP_SQ_AFF)) {
> +			wait_for_completion(&ctx->completions[1]);
> +			/*
> +			 * The park is a bit of a work-around, without it we get
> +			 * warning spews on shutdown with SQPOLL set and affinity
> +			 * set to a single CPU.
> +			 */
> +			kthread_park(ctx->sqo_thread);
> +			kthread_stop(ctx->sqo_thread);
> +			ctx->sqo_thread = NULL;
> +		} else
> +			destroy_io_percpu_thread(ctx, ctx->sq_thread_cpu);
>   	}
>   }
>   
> @@ -6841,6 +6990,52 @@ static int io_init_wq_offload(struct io_ring_ctx *ctx,
>   	return ret;
>   }
>   
> +static void create_io_percpu_thread(struct io_ring_ctx *ctx, int cpu)
> +{
> +	struct io_percpu_thread *t;
> +
> +	t = per_cpu_ptr(percpu_threads, cpu);
> +	mutex_lock(&t->lock);
> +	if (!t->sqo_thread) {
> +		t->sqo_thread = kthread_create_on_cpu(io_sq_percpu_thread, t,
> +					cpu, "io_uring_percpu-sq");
> +		if (IS_ERR(t->sqo_thread)) {
> +			ctx->sqo_thread = t->sqo_thread;
> +			t->sqo_thread = NULL;
> +			mutex_unlock(&t->lock);
> +			return;
> +		}
> +	}
> +
> +	if (t->sq_thread_idle < ctx->sq_thread_idle)
> +		t->sq_thread_idle = ctx->sq_thread_idle;
> +	ctx->sqo_wait = &t->sqo_percpu_wait;
> +	ctx->sq_thread_cpu = cpu;
> +	list_add_tail(&ctx->node, &t->ctx_list);
> +	ctx->sqo_thread = t->sqo_thread;
> +	mutex_unlock(&t->lock);
> +}
> +
> +static void destroy_io_percpu_thread(struct io_ring_ctx *ctx, int cpu)
> +{
> +	struct io_percpu_thread *t;
> +	struct task_struct *sqo_thread = NULL;
> +
> +	t = per_cpu_ptr(percpu_threads, cpu);
> +	mutex_lock(&t->lock);
> +	list_del(&ctx->node);
> +	if (list_empty(&t->ctx_list)) {
> +		sqo_thread = t->sqo_thread;
> +		t->sqo_thread = NULL;
> +	}
> +	mutex_unlock(&t->lock);
> +
> +	if (sqo_thread) {
> +		kthread_park(sqo_thread);
> +		kthread_stop(sqo_thread);
> +	}
> +}
> +
>   static int io_sq_offload_start(struct io_ring_ctx *ctx,
>   			       struct io_uring_params *p)
>   {
> @@ -6867,9 +7062,7 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
>   			if (!cpu_online(cpu))
>   				goto err;
>   
> -			ctx->sqo_thread = kthread_create_on_cpu(io_sq_thread,
> -							ctx, cpu,
> -							"io_uring-sq");
> +			create_io_percpu_thread(ctx, cpu);
>   		} else {
>   			ctx->sqo_thread = kthread_create(io_sq_thread, ctx,
>   							"io_uring-sq");
> @@ -7516,7 +7709,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
>   		if (!list_empty_careful(&ctx->cq_overflow_list))
>   			io_cqring_overflow_flush(ctx, false);
>   		if (flags & IORING_ENTER_SQ_WAKEUP)
> -			wake_up(&ctx->sqo_wait);
> +			wake_up(ctx->sqo_wait);
>   		submitted = to_submit;
>   	} else if (to_submit) {
>   		mutex_lock(&ctx->uring_lock);
> @@ -8102,6 +8295,8 @@ SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
>   
>   static int __init io_uring_init(void)
>   {
> +	int cpu;
> +
>   #define __BUILD_BUG_VERIFY_ELEMENT(stype, eoffset, etype, ename) do { \
>   	BUILD_BUG_ON(offsetof(stype, ename) != eoffset); \
>   	BUILD_BUG_ON(sizeof(etype) != sizeof_field(stype, ename)); \
> @@ -8141,6 +8336,18 @@ static int __init io_uring_init(void)
>   	BUILD_BUG_ON(ARRAY_SIZE(io_op_defs) != IORING_OP_LAST);
>   	BUILD_BUG_ON(__REQ_F_LAST_BIT >= 8 * sizeof(int));
>   	req_cachep = KMEM_CACHE(io_kiocb, SLAB_HWCACHE_ALIGN | SLAB_PANIC);
> +
> +	percpu_threads = alloc_percpu(struct io_percpu_thread);
> +	for_each_possible_cpu(cpu) {
> +		struct io_percpu_thread *t;
> +
> +		t = per_cpu_ptr(percpu_threads, cpu);
> +		INIT_LIST_HEAD(&t->ctx_list);
> +		init_waitqueue_head(&t->sqo_percpu_wait);
> +		mutex_init(&t->lock);
> +		t->sqo_thread = NULL;
> +		t->sq_thread_idle = 0;
> +	}
>   	return 0;
>   };
>   __initcall(io_uring_init);
>