netdev - Re: [RFC PATCH net-next 1/6] net: implement threaded-able napi poll loop support

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Message-Id: <2ab7cdc1-b9e1-48c7-89b2-a10cd5e19545@www.fastmail.com>
Date:   Fri, 25 Sep 2020 21:45:53 +0200
From:   "Hannes Frederic Sowa" <hannes@...essinduktion.org>
To:     "Wei Wang" <weiwan@...gle.com>,
        "David Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     "Jakub Kicinski" <kuba@...nel.org>,
        "Eric Dumazet" <edumazet@...gle.com>,
        "Paolo Abeni" <pabeni@...hat.com>, "Felix Fietkau" <nbd@....name>
Subject: Re: [RFC PATCH net-next 1/6] net: implement threaded-able napi poll loop support

Hello,

Happy to see this work being resurrected (in case it is useful). :)

On Mon, Sep 14, 2020, at 19:24, Wei Wang wrote:
>
> [...]
>
> +static void napi_thread_start(struct napi_struct *n)
> +{
> +	if (test_bit(NAPI_STATE_THREADED, &n->state) && !n->thread)
> +		n->thread = kthread_create(napi_threaded_poll, n, "%s-%d",
> +					   n->dev->name, n->napi_id);
> +}
> +

The format string is only based on variable strings. To ease a quick
grep for napi threads with ps I would propose to use "napi-%s-%d" or
something alike to distinguish all threads created that way.

Some other comments and questions:

Back then my plan was to get this somewhat integrated with the
`threadirqs` kernel boot option because triggering the softirq from
threaded context (if this option is set) seemed wrong to me. Maybe in
theory the existing interrupt thread could already be used in this case.
This would also allow for fine tuning the corresponding threads as in
this patch series.

Maybe the whole approach of threaded irqs plus the already existing
infrastructure could also be used for this series if it wouldn't be an
all or nothing opt-in based on the kernel cmd line parameter? napi would
then be able to just poll directly inline in the interrupt thread.

The difference for those kthreads and the extra threads created here
would be that fifo scheduling policy is set by default and they seem to
automatically get steered to the appropriate CPUs via the IRQTF_AFFINITY
mechanism. Maybe this approach is useful here as well?

I hadn't had a look at the code for a while thus my memories might be
wrong here.

Thanks,
Hannes