> -----Original Message----- > From: Intel-wired-lan <[email protected]> On Behalf Of > Jason Xing > Sent: Saturday, July 26, 2025 12:34 PM > To: Nguyen, Anthony L <[email protected]>; Kitszel, Przemyslaw > <[email protected]>; Zaremba, Larysa > <[email protected]>; [email protected]; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected]; Fijalkowski, Maciej > <[email protected]> > Cc: [email protected]; [email protected]; Jason Xing > <[email protected]> > Subject: [Intel-wired-lan] [PATCH v2 iwl-net] ixgbe: xsk: resolve the negative > overflow of budget in ixgbe_xmit_zc > > From: Jason Xing <[email protected]> > > 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 <[email protected]> > Reviewed-by: Larysa Zaremba <[email protected]> > --- > Link: https://lore.kernel.org/all/20250720091123.474-3- > [email protected]/ > 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
Tested-by: Priya Singh <[email protected]>
