linux-kernel - Re: [PATCH v4 2/2] io_uring: Add support for napi_busy

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Date:   Wed, 02 Mar 2022 00:12:32 -0500
From:   Olivier Langlois <olivier@...llion01.com>
To:     Hao Xu <haoxu@...ux.alibaba.com>, Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>
Cc:     io-uring <io-uring@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] io_uring: Add support for napi_busy_poll

On Wed, 2022-03-02 at 02:31 +0800, Hao Xu wrote:
> > +static void io_blocking_napi_busy_loop(struct list_head
> > *napi_list,
> > +                                      struct io_wait_queue *iowq)
> > +{
> > +       unsigned long start_time =
> > +               list_is_singular(napi_list) ? 0 :
> > +               busy_loop_current_time();
> > +
> > +       do {
> > +               if (list_is_singular(napi_list)) {
> > +                       struct napi_entry *ne =
> > +                               list_first_entry(napi_list,
> > +                                                struct napi_entry,
> > list);
> > +
> > +                       napi_busy_loop(ne->napi_id,
> > io_busy_loop_end, iowq,
> > +                                      true, BUSY_POLL_BUDGET);
> > +                       io_check_napi_entry_timeout(ne);
> > +                       break;
> > +               }
> > +       } while (io_napi_busy_loop(napi_list) &&
> > +                !io_busy_loop_end(iowq, start_time));
> > +}
> > +
> 
> How about:
> 
> if (list is singular) {
> 
>      do something;
> 
>      return;
> 
> }
> 
> while (!io_busy_loop_end() && io_napi_busy_loop())
> 
>      ;
> 
> 
> Btw, start_time seems not used in singular branch.

Hao,

it takes me few readings before being able to figure out the idea
behind your suggestions. Sorry about that!

So, if I get it correctly, you are proposing extract out the singular
block out of the while loop...

IMHO, this is not a good idea because you could start iterating the
do/while loop with a multiple entries list that ends up becoming a
singular list after one or few iterations.

Check what io_napi_busy_loop() is doing...

It does not look like that but a lot thoughts have been put into
writing io_blocking_napi_busy_loop()...