netdev - Re: [RFC PATCH v3 0/3] io_uring: add napi busy polling support

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Date:   Wed, 16 Nov 2022 12:09:41 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Stefan Roesch <shr@...kernel.io>, kernel-team@...com,
        olivier@...llion01.com, netdev@...r.kernel.org,
        io-uring@...r.kernel.org
Subject: Re: [RFC PATCH v3 0/3] io_uring: add napi busy polling support

On Wed, 16 Nov 2022 11:44:38 -0700 Jens Axboe wrote:
> Thanks Jakub! Question on the need for patch 3, which I think came about
> because of comments from you. Can you expand on why we need both an
> enable and timeout setting? Are there cases where timeout == 0 and
> enabled == true make sense?

The enable is for the "prefer busy poll" mode, rather that just busy
polling.

The prefer busy poll mode disables interrupts and arms a (hopefully
long enough) fail safe timer, and expects user to come back and busy
poll before the timer fires. The timer length is set thru sysfs params
for NAPI/queue.

Because the Rx traffic is fully async and not in control of the local
app, this gives the local app the ability to postpone the Rx IRQ.
No interruptions means lower response latency. 
With the expectation that the app will read/"busy poll" next batch of
packets once its done servicing the previous batch.

We don't have to implement this bit from the start, "normal" busy poll
is already functional with patches 1 and 2.