Re: [PATCH] io_uring: only wake up sq thread while current task is in io worker context

[Jens Axboe <axboe@kernel.dk>]

On 11/12/20 4:53 AM, Xiaoguang Wang wrote:
> When IORING_SETUP_SQPOLL is enabled, io_uring will always handle sqes
> in sq thread task context, so in io_iopoll_req_issued(), if we're not
> in io worker context, we don't need to check whether should wake up
> sq thread. io_iopoll_req_issued() calls wq_has_sleeper(), which has
> smp_mb() memory barrier, perf shows obvious overhead:
>   Samples: 481K of event 'cycles', Event count (approx.): 299807382878
>   Overhead  Comma  Shared Object     Symbol
>      3.69%  :9630  [kernel.vmlinux]  [k] io_issue_sqe
> 
> With this patch, perf shows:
>   Samples: 482K of event 'cycles', Event count (approx.): 299929547283
>   Overhead  Comma  Shared Object     Symbol
>      0.70%  :4015  [kernel.vmlinux]  [k] io_issue_sqe
> 
> It shows some obvious improvements.

Looks good to me, but:
  
> @@ -2761,7 +2761,7 @@ static void io_iopoll_req_issued(struct io_kiocb *req)
>  	else
>  		list_add_tail(&req->inflight_entry, &ctx->iopoll_list);
>  
> -	if ((ctx->flags & IORING_SETUP_SQPOLL) &&
> +	if (in_async && (ctx->flags & IORING_SETUP_SQPOLL) &&
>  	    wq_has_sleeper(&ctx->sq_data->wait))
>  		wake_up(&ctx->sq_data->wait);
>  }

This really needs a comment as to why we don't have to check and wake
from this path.

-- 
Jens Axboe