Re: [PATCH 3/8] io_uring: add a limited tw list for irq completion work

Pavel Begunkov
2021-09-28T11:29:56Z

On 9/27/21 7:17 AM, Hao Xu wrote:
> Now we have a lot of task_work users, some are just to complete a req
> and generate a cqe. Let's put the work to a new tw list which has a
> higher priority, so that it can be handled quickly and thus to reduce
> avg req latency. an explanatory case:
>
> origin timeline:
> submit_sqe-->irq-->add completion task_work
> -->run heavy work0~n-->run completion task_work
> now timeline:
> submit_sqe-->irq-->add completion task_work
> -->run completion task_work-->run heavy work0~n
>
> One thing to watch out is sometimes irq completion TWs comes
> overwhelmingly, which makes the new tw list grows fast, and TWs in
> the old list are starved. So we have to limit the length of the new
> tw list. A practical value is 1/3:
> len of new tw list < 1/3 * (len of new + old tw list)
>
> In this way, the new tw list has a limited length and normal task get
> there chance to run.
>
> Tested this patch(and the following ones) by manually replace
> __io_queue_sqe() to io_req_task_complete() to construct 'heavy' task
> works. Then test with fio:
>
> ioengine=io_uring
> thread=1
> bs=4k
> direct=1
> rw=randread
> time_based=1
> runtime=600
> randrepeat=0
> group_reporting=1
> filename=/dev/nvme0n1
>
> Tried various iodepth.
> The peak IOPS for this patch is 314K, while the old one is 249K.
> For avg latency, difference shows when iodepth grow:
> depth and avg latency(usec):
> depth new old
> 1 22.80 23.77
> 2 23.48 24.54
> 4 24.26 25.57
> 8 29.21 32.89
> 16 53.61 63.50
> 32 106.29 131.34
> 64 217.21 256.33
> 128 421.59 513.87
> 256 815.15 1050.99
>
> 95%, 99% etc more data in cover letter.
>
> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
> fs/io_uring.c | 44 +++++++++++++++++++++++++++++++-------------
> 1 file changed, 31 insertions(+), 13 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 8317c360f7a4..9272b2cfcfb7 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -461,6 +461,7 @@ struct io_ring_ctx {
> };
> };
>
> +#define MAX_EMERGENCY_TW_RATIO 3
> struct io_uring_task {
> /* submission side */
> int cached_refs;
> @@ -475,6 +476,9 @@ struct io_uring_task {
> spinlock_t task_lock;
> struct io_wq_work_list task_list;
> struct callback_head task_work;
> + struct io_wq_work_list prior_task_list;
> + unsigned int nr;
> + unsigned int prior_nr;
> bool task_running;
> };
>
> @@ -2132,12 +2136,16 @@ static void tctx_task_work(struct callback_head *cb)
> while (1) {
> struct io_wq_work_node *node;
>
> - if (!tctx->task_list.first && locked)
> + if (!tctx->prior_task_list.first &&
> + !tctx->task_list.first && locked)
> io_submit_flush_completions(ctx);
>
> spin_lock_irq(&tctx->task_lock);
> - node = tctx->task_list.first;
> + wq_list_merge(&tctx->prior_task_list, &tctx->task_list);
> + node = tctx->prior_task_list.first;

I find all this accounting expensive, sure I'll see it for my BPF tests.

How about
1) remove MAX_EMERGENCY_TW_RATIO and all the counters,
prior_nr and others.

2) rely solely on list merging

So, when it enters an iteration of the loop it finds a set of requests
to run, it first executes all priority ones of that set and then the
rest (just by the fact that you merged the lists and execute all from
them).

It solves the problem of total starvation of non-prio requests, e.g.
if new completions coming as fast as you complete previous ones. One
downside is that prio requests coming while we execute a previous
batch will be executed only after a previous batch of non-prio
requests, I don't think it's much of a problem but interesting to
see numbers.

> INIT_WQ_LIST(&tctx->task_list);
> + INIT_WQ_LIST(&tctx->prior_task_list);
> + tctx->nr = tctx->prior_nr = 0;
> if (!node)
> tctx->task_running = false;
> spin_unlock_irq(&tctx->task_lock);
> @@ -2166,7 +2174,7 @@ static void tctx_task_work(struct callback_head *cb)
> ctx_flush_and_put(ctx, &locked);
> }
>
> -static void io_req_task_work_add(struct io_kiocb *req)
> +static void io_req_task_work_add(struct io_kiocb *req, bool emergency)

It think "priority" instead of "emergency" will be more accurate

> {
> struct task_struct *tsk = req->task;
> struct io_uring_task *tctx = tsk->io_uring;
> @@ -2178,7 +2186,13 @@ static void io_req_task_work_add(struct io_kiocb *req)
> WARN_ON_ONCE(!tctx);
>
> spin_lock_irqsave(&tctx->task_lock, flags);
> - wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
> + if (emergency && tctx->prior_nr * MAX_EMERGENCY_TW_RATIO < tctx->nr) {
> + wq_list_add_tail(&req->io_task_work.node, &tctx->prior_task_list);
> + tctx->prior_nr++;
> + } else {
> + wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
> + }
> + tctx->nr++;
> running = tctx->task_running;
> if (!running)
> tctx->task_running = true;

--
Pavel Begunkov