Date:   Fri, 25 Feb 2022 10:32:57 -0500
From:   Olivier Langlois <olivier@...llion01.com>
To:     Hao Xu <haoxu@...ux.alibaba.com>, Jens Axboe <axboe@...nel.dk>
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        io-uring <io-uring@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] io_uring: Add support for napi_busy_poll

On Fri, 2022-02-25 at 00:32 -0500, Olivier Langlois wrote:
> 
> > > 
> > > +#ifdef CONFIG_NET_RX_BUSY_POLL
> > > +static void io_adjust_busy_loop_timeout(struct timespec64 *ts,
> > > +                                       struct io_wait_queue
> > > *iowq)
> > > +{
> > > +       unsigned busy_poll_to = READ_ONCE(sysctl_net_busy_poll);
> > > +       struct timespec64 pollto = ns_to_timespec64(1000 *
> > > (s64)busy_poll_to);
> > > +
> > > +       if (timespec64_compare(ts, &pollto) > 0) {
> > > +               *ts = timespec64_sub(*ts, pollto);
> > > +               iowq->busy_poll_to = busy_poll_to;
> > > +       } else {
> > > +               iowq->busy_poll_to = timespec64_to_ns(ts) / 1000;
> > 
> > How about timespec64_tons(ts) >> 10, since we don't need accurate
> > number.
> 
> Fantastic suggestion! The kernel test robot did also detect an issue
> with that statement. I did discover do_div() in the meantime but what
> you suggest is better, IMHO...

After having seen Jens patch (io_uring: don't convert to jiffies for
waiting on timeouts), I think that I'll stick with do_div().

I have a hard time considering removing timing accuracy when effort is
made to make the same function more accurate...
> 
> 
> > > +                !io_busy_loop_end(iowq, start_time));
> > > +}
> > > +#endif /* CONFIG_NET_RX_BUSY_POLL */
> > > +
> > >   /*
> > >    * Wait until events become available, if we don't already have
> > > some. The
> > >    * application must reap them itself, as they reside on the
> > > shared cq ring.
> > > @@ -7729,12 +7906,20 @@ static int io_cqring_wait(struct
> > > io_ring_ctx *ctx, int min_events,
> > >                 if (!io_run_task_work())
> > >                         break;
> > >         } while (1);
> > > -
> > > +#ifdef CONFIG_NET_RX_BUSY_POLL
> > > +       iowq.busy_poll_to = 0;
> > > +#endif
> > >         if (uts) {
> > >                 struct timespec64 ts;
> > >   
> > >                 if (get_timespec64(&ts, uts))
> > >                         return -EFAULT;
> > > +#ifdef CONFIG_NET_RX_BUSY_POLL
> > > +               if (!(ctx->flags & IORING_SETUP_SQPOLL) &&
> > > +                   !list_empty(&ctx->napi_list)) {
> > > +                       io_adjust_busy_loop_timeout(&ts, &iowq);
> > > +               }
> > > +#endif
> > >                 timeout = timespec64_to_jiffies(&ts);
> > >         }
> > >   
> > > @@ -7759,6 +7944,10 @@ static int io_cqring_wait(struct
> > > io_ring_ctx
> > > *ctx, int min_events,
> > >         iowq.cq_tail = READ_ONCE(ctx->rings->cq.head) +
> > > min_events;
> > >   
> > >         trace_io_uring_cqring_wait(ctx, min_events);
> > > +#ifdef CONFIG_NET_RX_BUSY_POLL
> > > +       if (iowq.busy_poll_to)
> > > +               io_blocking_napi_busy_loop(ctx, &iowq);
> > 
> > We may not need locks for the napi_list, the reason is we don't
> > need
> > to
> > poll an accurate list, the busy polling/NAPI itself is kind of
> > speculation. So the deletion is not an emergency.
> > To say the least, we can probably delay the deletion to some safe
> > place
> > like the original task's task work though this may cause other
> > problems...
> 
> There are 2 concerns here.
> 
> 1. Iterating a list while another thread modify it is not thread-safe
> unless you use a lock.
> 
> If we offer napi_busy_poll() without sqpoll with the modification in
> io_cqring_wait(), this is a real possibility. A thread could call
> io_uring_enter(IORING_ENTER_GETEVENTS) while another thread calls
> io_uring_enter() to submit new sqes that could trigger a call to
> io_add_napi().
> 
> If napi_busy_poll() is only offered through sqpoll thread, this
> becomes
> a non-issue since the only thread accessing/modifying the napi_list
> field is the sqpoll thread.
> 
> Providing the patch benchmark result with v2 could help deciding what
> to do with this choice.
> 
> 2. You are correct when you say that deletion is not an emergency. 
> 
> However, the design guideline that I did follow when writing the
> patch
> is that napi_busy_poll support should not impact users not using this
> feature. Doing the deletion where that patch is doing it fullfill
> this
> goal.
> 
> Comparing a timeout value with the jiffies variable is very cheap and
> will only be performed when napi_busy_poll is used.
> 
> The other option would be to add a refcount to each napi_entry and
> decrement it if needed everytime a request is discarded. Doing that
> that check for every requests that io_uring discards on completion, I
> am very confident that this would negatively impact various
> performance
> benchmarks that Jens routinely perform...
> 
Another fact to consider, it is that I expect the content of napi_list
to be extremely stable. Regular entry deletion should not be a thing.

postponing the deletion using task work is not an option too. How would
io_busy_loop_end() discern between a pending list entry deletion and
any other task work making the busy looping stop?

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