netdev - Re: [RFC net-next 1/2] page_pool: allow caching from safely localized NAPI

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Date:   Tue, 4 Apr 2023 06:21:37 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net,
        netdev@...r.kernel.org, pabeni@...hat.com, hawk@...nel.org,
        ilias.apalodimas@...aro.org
Subject: Re: [RFC net-next 1/2] page_pool: allow caching from safely localized NAPI

On Tue, Apr 4, 2023 at 2:53 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:

> Interesting.
> I wonder if we can make this more generic by adding the skb to per napi
> list instead of sd->defer_list, so that we can always use NAPI kicking to
> flush skb as net_tx_action() done for sd->completion_queue instead of
> softirq kicking?

We do not have direct skb  -> napi association yet, but using an
expensive hash lookup.

I had the intent of adding per-cpu caches in this infrastructure,
to not acquire the remote-cpu defer_lock for one skb at a time.
(This is I think causing some regressions for small packets, with no frags)

>
> And it seems we know which napi binds to a specific socket through
> busypoll mechanism, we can reuse that to release a skb to the napi
> bound to that socket?

busypoll is not often used, and we usually burn (spinning) cycles there,
not sure we want to optimize it ?

>
> >
> > The main case we'll miss out on is when application runs on the same
> > CPU as NAPI. In that case we don't use the deferred skb free path.
> > We could disable softirq one that path, too... maybe?
> >