netdev - Re: [Intel-wired-lan] [PATCH v2 iwl-net] ixgbe: xsk: resolve the negative overflow of budget in ixgbe_xmit

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Message-ID: <14d36e48-251d-4dbf-aafe-57259003e064@intel.com>
Date: Mon, 11 Aug 2025 14:36:25 -0700
From: Tony Nguyen <anthony.l.nguyen@...el.com>
To: Jason Xing <kerneljasonxing@...il.com>, <przemyslaw.kitszel@...el.com>,
	<larysa.zaremba@...el.com>, <andrew+netdev@...n.ch>, <davem@...emloft.net>,
	<edumazet@...gle.com>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<bjorn@...nel.org>, <maciej.fijalkowski@...el.com>
CC: <intel-wired-lan@...ts.osuosl.org>, <netdev@...r.kernel.org>, Jason Xing
	<kernelxing@...cent.com>
Subject: Re: [Intel-wired-lan] [PATCH v2 iwl-net] ixgbe: xsk: resolve the
 negative overflow of budget in ixgbe_xmit_zc



On 8/10/2025 5:22 AM, Jason Xing wrote:
> On Sat, Jul 26, 2025 at 3:04 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>>
>> From: Jason Xing <kernelxing@...cent.com>
>>
>> Resolve the budget negative overflow which leads to returning true in
>> ixgbe_xmit_zc even when the budget of descs are thoroughly consumed.
>>
>> Before this patch, when the budget is decreased to zero and finishes
>> sending the last allowed desc in ixgbe_xmit_zc, it will always turn back
>> and enter into the while() statement to see if it should keep processing
>> packets, but in the meantime it unexpectedly decreases the value again to
>> 'unsigned int (0--)', namely, UINT_MAX. Finally, the ixgbe_xmit_zc returns
>> true, showing 'we complete cleaning the budget'. That also means
>> 'clean_complete = true' in ixgbe_poll.
>>
>> The true theory behind this is if that budget number of descs are consumed,
>> it implies that we might have more descs to be done. So we should return
>> false in ixgbe_xmit_zc to tell napi poll to find another chance to start
>> polling to handle the rest of descs. On the contrary, returning true here
>> means job done and we know we finish all the possible descs this time and
>> we don't intend to start a new napi poll.
>>
>> It is apparently against our expectations. Please also see how
>> ixgbe_clean_tx_irq() handles the problem: it uses do..while() statement
>> to make sure the budget can be decreased to zero at most and the negative
>> overflow never happens.
>>
>> The patch adds 'likely' because we rarely would not hit the loop codition
>> since the standard budget is 256.
>>
>> Fixes: 8221c5eba8c1 ("ixgbe: add AF_XDP zero-copy Tx support")
>> Signed-off-by: Jason Xing <kernelxing@...cent.com>
>> Reviewed-by: Larysa Zaremba <larysa.zaremba@...el.com>
> 
> Hi Tony,
> 
> Any update here? Thanks! I'm asking because I'm ready to send an afxdp
> patch series based on the patch :)

Hi Jason,

I have this slated to be part of my next net series. I do already have 
this patch applied/staged on the Intel tree [1] (dev-queue branch). You 
can base it on that and send the changes now; I'll ensure that this 
patch is merged before sending the other series.

Thanks,
Tony

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tnguy/next-queue.git/

> Thanks,
> jason