linux-kernel - Re: [PATCH v1] io_uring: Add support for napi_busy

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Date:   Mon, 21 Feb 2022 14:29:59 -0500
From:   Olivier Langlois <olivier@...llion01.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        Hao Xu <haoxu@...ux.alibaba.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 Sun, 2022-02-20 at 12:38 -0700, Jens Axboe wrote:
> 
> OK, that's a pretty good improvement in both latency and
> deviation/consistency. Is this using SQPOLL, or is it using polling
> off
> cqring_wait from the task itself? Also something to consider for the
> test benchmark app, should be able to run both (which is usually just
> setting the SETUP_SQPOLL flag or not, if done right).
> 
> 
The answer to your question is complex. This is one of the external
factor that I was refering too.

1 thread is managing 49 TCP sockets. This thread io_uring context is
configured with SQPOLL. Upon receiving a packet of interest, it will
wake up thread #2 with an eventfd installed into a private non SQPOLL 
io_uring context and will send a request to a 50th TCP socket.

Both threads are now busy polling NAPI. One from the SQPOLL code and
the other with the io_cqring_wait() code.

If it was not enough, since I have discovered busy poll benefits and
that to reschedule a sleeping task takes about 5-10 uSecs, thread #1 is
also busy polling io_uring instead of blocking in io_uring_enter().

Thx for suggesting designing the benchmark to be able to test both
SQPOLL and non SQPOLL busy polling. This is something that I already in
mind.

I have completed 3 small improvements for the patch v2. I need to check
the kernel test bot and Hao comments to see if I have more to work on
but if all is good, I only need to complete the benchmark program. I
might able to send v2 later today.

Greetings,