netdev - RE: [PATCH] net: renesas: ravb: Fix a stuck issue when a lot of frames are received

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Date:   Mon, 17 May 2021 08:23:07 +0000
From:   Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:     Sergei Shtylyov <sergei.shtylyov@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
Subject: RE: [PATCH] net: renesas: ravb: Fix a stuck issue when a lot of
 frames are received

Hi again,

> From: Yoshihiro Shimoda, Sent: Monday, May 10, 2021 7:30 PM
> 
> > From: Sergei Shtylyov, Sent: Sunday, May 9, 2021 7:22 PM
> >
> > On 08.05.2021 23:47, Sergei Shtylyov wrote:
> >
> > >     Posting a review of the already commited (over my head) patch. It would have
> > > been appropriate if the patch looked OK but it's not. :-/
> > >
> > >> When a lot of frames were received in the short term, the driver
> > >> caused a stuck of receiving until a new frame was received. For example,
> > >> the following command from other device could cause this issue.
> > >>
> > >>      $ sudo ping -f -l 1000 -c 1000 <this driver's ipaddress>
> > >
> > >     -l is essential here, right?
> 
> Yes.
> 
> > >     Have you tried testing sh_eth sriver like that, BTW?
> >
> >     It's driver! :-)
> 
> I have not tried testing sh_eth driver yet. I'll test it after I got an actual board.

I could reproduce a similar issue on R-Car Gen2 with sh_eth driver if the RX ring size is 1024 like
the ravb driver. Also, I confirmed a similar patch for sh_eth driver could fix the issue.
So, I'll send a patch later.

Best regards,
Yoshihiro Shimoda

> > >> The previous code always cleared the interrupt flag of RX but checks
> > >> the interrupt flags in ravb_poll(). So, ravb_poll() could not call
> > >> ravb_rx() in the next time until a new RX frame was received if
> > >> ravb_rx() returned true. To fix the issue, always calls ravb_rx()
> > >> regardless the interrupt flags condition.
> > >
> > >     That bacially defeats the purpose of IIUC...
> >                                            ^ NAPI,
> >
> >     I was sure I typed NAPI here, yet it got lost in the edits. :-)
> 
> I could not understand "that" (calling ravb_rx() regardless the interrupt
> flags condition) defeats the purpose of NAPI. According to an article on
> the Linux Foundation wiki [1], one of the purpose of NAPI is "Interrupt mitigation".
> In poll(), the interrupts are already disabled, and ravb_rx() will check the
> descriptor's status. So, this patch keeps the "Interrupt mitigation" IIUC.
> 
> [1]
> https://wiki.linuxfoundation.org/networking/napi
> 
> Best regards,
> Yoshihiro Shimoda