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: <IA3PR11MB89865D26E0D34111782E9BC8E55AA@IA3PR11MB8986.namprd11.prod.outlook.com>
Date: Mon, 28 Jul 2025 08:06:15 +0000
From: "Loktionov, Aleksandr" <aleksandr.loktionov@...el.com>
To: Jason Xing <kerneljasonxing@...il.com>, "Nguyen, Anthony L"
	<anthony.l.nguyen@...el.com>, "Kitszel, Przemyslaw"
	<przemyslaw.kitszel@...el.com>, "Zaremba, Larysa" <larysa.zaremba@...el.com>,
	"andrew+netdev@...n.ch" <andrew+netdev@...n.ch>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"bjorn@...nel.org" <bjorn@...nel.org>, "Fijalkowski, Maciej"
	<maciej.fijalkowski@...el.com>
CC: "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
	"netdev@...r.kernel.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



> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@...osl.org> On Behalf
> Of Jason Xing
> Sent: Saturday, July 26, 2025 9:04 AM
> To: Nguyen, Anthony L <anthony.l.nguyen@...el.com>; Kitszel,
> Przemyslaw <przemyslaw.kitszel@...el.com>; Zaremba, Larysa
> <larysa.zaremba@...el.com>; andrew+netdev@...n.ch;
> davem@...emloft.net; edumazet@...gle.com; kuba@...nel.org;
> pabeni@...hat.com; bjorn@...nel.org; Fijalkowski, Maciej
> <maciej.fijalkowski@...el.com>
> Cc: intel-wired-lan@...ts.osuosl.org; netdev@...r.kernel.org; Jason
> Xing <kernelxing@...cent.com>
> Subject: [Intel-wired-lan] [PATCH v2 iwl-net] ixgbe: xsk: resolve the
> negative overflow of budget in ixgbe_xmit_zc
> 
> 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>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@...el.com>

> ---
> Link: https://lore.kernel.org/all/20250720091123.474-3-
> kerneljasonxing@...il.com/
> 1. use 'negative overflow' instead of 'underflow' (Willem) 2. add
> reviewed-by tag (Larysa) 3. target iwl-net branch (Larysa) 4. add the
> reason why the patch adds likely() (Larysa)
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> index ac58964b2f08..7b941505a9d0 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> @@ -398,7 +398,7 @@ static bool ixgbe_xmit_zc(struct ixgbe_ring
> *xdp_ring, unsigned int budget)
>  	dma_addr_t dma;
>  	u32 cmd_type;
> 
> -	while (budget-- > 0) {
> +	while (likely(budget)) {
>  		if (unlikely(!ixgbe_desc_unused(xdp_ring))) {
>  			work_done = false;
>  			break;
> @@ -433,6 +433,8 @@ static bool ixgbe_xmit_zc(struct ixgbe_ring
> *xdp_ring, unsigned int budget)
>  		xdp_ring->next_to_use++;
>  		if (xdp_ring->next_to_use == xdp_ring->count)
>  			xdp_ring->next_to_use = 0;
> +
> +		budget--;
>  	}
> 
>  	if (tx_desc) {
> --
> 2.41.3