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

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Message-ID:
 <PAXPR04MB85101DE84124D424264BB4FD886C2@PAXPR04MB8510.eurprd04.prod.outlook.com>
Date: Fri, 20 Sep 2024 14:05:14 +0000
From: Wei Fang <wei.fang@....com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.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>, Vladimir Oltean <vladimir.oltean@....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: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> Sent: 2024年9月20日 21:05
> To: Wei Fang <wei.fang@....com>
> Cc: davem@...emloft.net; edumazet@...gle.com; kuba@...nel.org;
> pabeni@...hat.com; Claudiu Manoil <claudiu.manoil@....com>; Vladimir
> Oltean <vladimir.oltean@....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()")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Wei Fang <wei.fang@....com>
> > ---
> >  drivers/net/ethernet/freescale/enetc/enetc.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c
> > b/drivers/net/ethernet/freescale/enetc/enetc.c
> > index 5830c046cb7d..3cff76923ab9 100644
> > --- a/drivers/net/ethernet/freescale/enetc/enetc.c
> > +++ b/drivers/net/ethernet/freescale/enetc/enetc.c
> > @@ -2769,6 +2769,7 @@ static int enetc_reconfigure_xdp_cb(struct
> enetc_ndev_priv *priv, void *ctx)
> >  	for (i = 0; i < priv->num_rx_rings; i++) {
> >  		struct enetc_bdr *rx_ring = priv->rx_ring[i];
> >
> > +		rx_ring->xdp.xdp_tx_in_flight = 0;
> 
> zero init is good but shouldn't you be draining these buffers when removing
> XDP resources at least? what happens with DMA mappings that are related to
> these cached buffers?
> 

All the buffers will be freed and DMA will be unmapped when XDP program is
installed. I am thinking that another solution may be better, which is mentioned
in another thread replying to Vladimir, so that xdp_tx_in_flight will naturally drop
to 0, and the TX-related statistics will be more accurate.