linux-kernel - Re: Re: [PATCH] net: remove local_bh

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Message-ID: <CANn89i+S0hD9FPNYatQz-Mn1PJ5WKfgpJ2T=LOgiA2+kA2K6vw@mail.gmail.com>
Date: Tue, 2 Sep 2025 03:34:14 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Lange Tang <lange_tang@....com>
Cc: Jakub Kicinski <kuba@...nel.org>, "davem@...emloft.net" <davem@...emloft.net>, 
	"pabeni@...hat.com" <pabeni@...hat.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Tang Longjun <tanglongjun@...inos.cn>
Subject: Re: Re: [PATCH] net: remove local_bh_enable during busy poll

On Tue, Sep 2, 2025 at 1:53 AM Lange Tang <lange_tang@....com> wrote:
>
> Thanks for your reply!
>
> I've done some testing, pps=350000，net.core.busy_read=50.

We can not let __napi_busy_loop() run for thousands of usec while blocking BH,
if net.core.busy_read=5000

To me, your patch is a no go.

Something is wrong in virtqueue_napi_complete() I think.


>
> Before apply this patch: unhandled ≈ 6400/s
> After apply this patch: unhandled < 10/s
>
> As you said, the driver needs to discern spurious interrupts in above describing situation, which I strongly agree with. and I also think that it's necessary to remove local_bh_enable during busy polling, as it causes interrupts to be enabled during the busy poll.
>
> I think fix this issue requires two patches, in addition to this patch, another patch is needed from the driver side to discern spurious interrupts.
>
>
>
>
>
>
> At 2025-09-02 04:23:30, "Jakub Kicinski" <kuba@...nel.org> wrote:
> >On Fri, 29 Aug 2025 11:04:56 +0800 Longjun Tang wrote:
> >> When CONFIG_NET_RX_BUSY_POLL==Y and net.core.busy_read > 0,
> >> the __napi_busy_loop function calls napi_poll to perform busy polling,
> >> such as in the case of virtio_net's virnet_poll. If interrupts are enabled
> >> during the busy polling process, it is possible that data has already been
> >> received and that last_used_idx is updated before the interrupt is handled.
> >> This can lead to the vring_interrupt returning IRQ_NONE in response to the
> >> interrupt because used_idx == last_used_idx, which is considered a spurious
> >> interrupt.Once certain conditions are met, this interrupt can be disabled.
> >
> >I'm not sure this patch completely fixes the issue you're describing.
> >It just makes it less likely to happen. Really, it feels like the onus
> >for fixing this is on the driver that can't discern its own IRQ sources.
> >--
> >pw-bot: cr