netdev - Re: [PATCH net] net: fix raising a softirq on the current cpu with rps enabled

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Date:   Mon, 27 Mar 2023 10:25:31 +0800
From:   Jason Xing <kerneljasonxing@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net] net: fix raising a softirq on the current cpu with
 rps enabled

On Mon, Mar 27, 2023 at 1:35 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> >
> > Forgive me. Really I need some coffee. I made a mistake. This line
> > above should be:
> >
> > +               if (!test_bit(NAPI_STATE_SCHED, &mysd->backlog.state))
> >
> > But the whole thing doesn't feel right. I need a few days to dig into
> > this part until Eric can help me with more of it.
> >
>
> I am still traveling, and this is weekend time :/

Thanks for your time, Eric, really appreciate it.

>
> It should not be too hard to read net/core/dev.c and remember that not
> _all_ drivers (or some core networking functions) use the NAPI model.
>
> eg look at netif_rx() and ask yourself why your patch is buggy.

Yes, it is. In my last email I sent yesterday I encountered one issue
which exactly happened when I started hundreds of iperf processes
transmitting data to loopback.
It got stuck :( So I realized it is the non-napi case that triggers
such a problem.

>
> Just look at callers of enqueue_to_backlog() and ask yourself if all
> of them are called from net_rx_action()
>
> [The answer is no, just in case you wonder]
>
> In order to add your optimization, more work is needed, like adding
> new parameters so that we do not miss critical
> __raise_softirq_irqoff(NET_RX_SOFTIRQ) when _needed_.

Thanks, I need to do more work/study on it.

>
> We keep going circles around softirq deficiencies, I feel you are
> trying to fix a second-order 'issue'.

Right, going circles gives me a headache.

>
> Real cause is elsewhere, look at recent patches from Jakub.

After you pointed out, I searched and found there is indeed one patchset in 2022

The tile like this:

[PATCH 0/3] softirq: uncontroversial change

Thanks,
Jason


>
> Thanks.