On Sun, Jul 20, 2025 at 05:11:20PM +0800, Jason Xing wrote: > From: Jason Xing <[email protected]> > > Resolve the budget underflow 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 underflow > never happens. > > Fixes: 8221c5eba8c1 ("ixgbe: add AF_XDP zero-copy Tx support") > Signed-off-by: Jason Xing <[email protected]>
Right, it is possible to decrement a zero in the current state of code. Should target iwl-net, otherwise fine. You could include info why do you add likely, as such change is questionable in something that goes into a stable tree, but should be fine, as the standard budget is 256, so we rarely would not hit the loop codition. Reviewed-by: Larysa Zaremba <[email protected]> > --- > 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 0ade15058d98..a463c5ac9c7c 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 > >
