linux-kernel - RE: [PATCH net 3/3] net: enetc: reset xdp_tx_in

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Message-ID:
 <PAXPR04MB8510727BD7B77261491B4D31886C2@PAXPR04MB8510.eurprd04.prod.outlook.com>
Date: Fri, 20 Sep 2024 03:12:06 +0000
From: Wei Fang <wei.fang@....com>
To: Vladimir Oltean <vladimir.oltean@....com>
CC: "davem@...emloft.net" <davem@...emloft.net>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>, Claudiu Manoil
	<claudiu.manoil@....com>, "ast@...nel.org" <ast@...nel.org>,
	"daniel@...earbox.net" <daniel@...earbox.net>, "hawk@...nel.org"
	<hawk@...nel.org>, "john.fastabend@...il.com" <john.fastabend@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, "bpf@...r.kernel.org"
	<bpf@...r.kernel.org>, "stable@...r.kernel.org" <stable@...r.kernel.org>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>
Subject: RE: [PATCH net 3/3] net: enetc: reset xdp_tx_in_flight when updating
 bpf program

> -----Original Message-----
> From: Vladimir Oltean <vladimir.oltean@....com>
> Sent: 2024年9月19日 21:22
> To: Wei Fang <wei.fang@....com>
> Cc: davem@...emloft.net; edumazet@...gle.com; kuba@...nel.org;
> pabeni@...hat.com; Claudiu Manoil <claudiu.manoil@....com>;
> ast@...nel.org; daniel@...earbox.net; hawk@...nel.org;
> john.fastabend@...il.com; linux-kernel@...r.kernel.org;
> netdev@...r.kernel.org; bpf@...r.kernel.org; stable@...r.kernel.org;
> imx@...ts.linux.dev
> Subject: Re: [PATCH net 3/3] net: enetc: reset xdp_tx_in_flight when updating
> bpf program
> 
> On Thu, Sep 19, 2024 at 04:41:04PM +0800, Wei Fang wrote:
> > When running "xdp-bench tx eno0" to test the XDP_TX feature of ENETC
> > on LS1028A, it was found that if the command was re-run multiple
> > times, Rx could not receive the frames, and the result of xdo-bench
> > showed that the rx rate was 0.
> >
> > root@...028ardb:~# ./xdp-bench tx eno0 Hairpinning (XDP_TX) packets on
> > eno0 (ifindex 3; driver fsl_enetc)
> > Summary                      2046 rx/s                  0
> err,drop/s
> > Summary                         0 rx/s                  0
> err,drop/s
> > Summary                         0 rx/s                  0
> err,drop/s
> > Summary                         0 rx/s                  0
> err,drop/s
> >
> > By observing the Rx PIR and CIR registers, we found that CIR is always
> > equal to 0x7FF and PIR is always 0x7FE, which means that the Rx ring
> > is full and can no longer accommodate other Rx frames. Therefore, it
> > is obvious that the RX BD ring has not been cleaned up.
> >
> > Further analysis of the code revealed that the Rx BD ring will only be
> > cleaned if the "cleaned_cnt > xdp_tx_in_flight" condition is met.
> > Therefore, some debug logs were added to the driver and the current
> > values of cleaned_cnt and xdp_tx_in_flight were printed when the Rx BD
> > ring was full. The logs are as follows.
> >
> > [  178.762419] [XDP TX] >> cleaned_cnt:1728, xdp_tx_in_flight:2140 [
> > 178.771387] [XDP TX] >> cleaned_cnt:1941, xdp_tx_in_flight:2110 [
> > 178.776058] [XDP TX] >> cleaned_cnt:1792, xdp_tx_in_flight:2110
> >
> > From the results, we can see that the maximum value of
> > xdp_tx_in_flight has reached 2140. However, the size of the Rx BD ring
> > is only 2048. This is incredible, so checked the code again and found
> > that the driver did not reset xdp_tx_in_flight when installing or
> > uninstalling bpf program, resulting in xdp_tx_in_flight still
> > retaining the value after the last command was run.
> >
> > Fixes: c33bfaf91c4c ("net: enetc: set up XDP program under
> > enetc_reconfigure()")
> 
> This does not explain why enetc_recycle_xdp_tx_buff(), which decreases
> xdp_tx_in_flight, does not get called?
> 
> In patch 2/3 you wrote:
> 
> | Tx BD rings are disabled first in enetc_stop() and then wait for
> | empty. This operation is not safe while the Tx BD ring is actively
> | transmitting frames, and will cause the ring to not be empty and
> | hardware exception. As described in the block guide of LS1028A NETC,
> | software should only disable an active ring after all pending ring
> | entries have been consumed (i.e. when PI = CI).
> | Disabling a transmit ring that is actively processing BDs risks a
> | HW-SW race hazard whereby a hardware resource becomes assigned to work
> | on one or more ring entries only to have those entries be removed due
> | to the ring becoming disabled. So the correct behavior is that the
> | software stops putting frames on the Tx BD rings (this is what
> | ENETC_TX_DOWN does), then waits for the Tx BD rings to be empty, and
> | finally disables the Tx BD rings.
> 
> I'm surprised that after fixing that, this change would still be needed, rather
> than xdp_tx_in_flight naturally dropping down to 0 when stopping NAPI. Why
> doesn't that happen, and what happens to the pending XDP_TX buffers?

The reason is that interrupt is disabled (disable_irq() is called in enetc_stop()) so
enetc_recycle_xdp_tx_buff() will not be called. Actually all XDP_TX frames are
sent out and XDP_TX buffers will be freed by enetc_free_rxtx_rings(). So there is
no noticeable impact.

Another solution is that move disable_irq() to the end of enetc_stop(), so that
the IRQ is still active until the Tx is finished. In this case, the xdp_tx_in_flight will
naturally drop down to 0 as you expect.