linux-kernel - Re: [PATCH v2 net 3/3] net: enetc: disable IRQ after Rx and Tx BD rings are disabled

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Message-ID: <20240930220249.dio23fh7mqw4pojn@skbuf>
Date: Tue, 1 Oct 2024 01:02:49 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Wei Fang <wei.fang@....com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, 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,
	rkannoth@...vell.com, maciej.fijalkowski@...el.com,
	sbhatta@...vell.com
Subject: Re: [PATCH v2 net 3/3] net: enetc: disable IRQ after Rx and Tx BD
 rings are disabled

On Sun, Sep 29, 2024 at 10:45:06AM +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, we
> can conclude that the problem is caused by the Rx BD ring not being
> 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 max value of xdp_tx_in_flight
> has reached 2140. However, the size of the Rx BD ring is only 2048.
> This is incredible, so we checked the code again and found that
> xdp_tx_in_flight did not drop to 0 when the bpf program was uninstalled
> and it was not reset when the bfp program was installed again. The
> root cause is that the IRQ is disabled too early in enetc_stop(),
> resulting in enetc_recycle_xdp_tx_buff() not being called, therefore,
> xdp_tx_in_flight is not cleared.
> 
> Fixes: ff58fda09096 ("net: enetc: prioritize ability to go down over packet processing")
> Cc: stable@...r.kernel.org
> Signed-off-by: Wei Fang <wei.fang@....com>
> ---
> v2 changes:
> 1. Modify the titile and rephrase the commit meesage.
> 2. Use the new solution as described in the title
> ---

I gave this another test under a bit different set of circumstances this time,
and I'm confident that there are still problems, which I haven't identified
though (yet).

With 64 byte frames at 2.5 Gbps, I see this going on:

$ xdp-bench tx eno0 &
$ while :; do taskset $((1 << 0)) hwstamp_ctl -i eno0 -r 1 && sleep 1 && taskset $((1 << 0)) hwstamp_ctl -i eno0 -r 0 && sleep 1; done
current settings:
tx_type 0
rx_filter 0
new settings:
tx_type 0
rx_filter 1
Summary                 1,556,952 rx/s                  0 err,drop/s
Summary                         0 rx/s                  0 err,drop/s
Summary                         0 rx/s                  0 err,drop/s
current settings:
tx_type 0
rx_filter 1
Summary                         0 rx/s                  0 err,drop/s
[  883.780346] fsl_enetc 0000:00:00.0 eno0: timeout for tx ring #6 clear (its RX ring has 2072 XDP_TX frames in flight)
new settings:
tx_type 0
rx_filter 0
Summary                     1,027 rx/s                  0 err,drop/s
current settings:
tx_type 0
rx_filter 0
Summary                         0 rx/s                  0 err,drop/s

which looks like the symptoms that the patch tries to solve.

My previous testing was with 390 byte frames, and this did not happen.

Please do not merge this.