| Message ID | 1573243090-2721-2-git-send-email-magnus.karlsson@intel.com |
|---|---|
| State | Accepted |
| Delegated to: | Jeff Kirsher |
| Headers | show |
| Series | [net,1/2] i40e: need_wakeup flag might not be set for Tx | expand |
> -----Original Message----- > From: Intel-wired-lan [mailto:intel-wired-lan-bounces@osuosl.org] On > Behalf Of Magnus Karlsson > Sent: Friday, November 8, 2019 11:58 AM > To: Karlsson, Magnus <magnus.karlsson@intel.com>; Topel, Bjorn > <bjorn.topel@intel.com>; intel-wired-lan@lists.osuosl.org > Cc: maciejromanfijalkowski@gmail.com; Fijalkowski, Maciej > <maciej.fijalkowski@intel.com>; netdev@vger.kernel.org > Subject: [Intel-wired-lan] [PATCH net 2/2] ixgbe: need_wakeup flag might > not be set for Tx > > The need_wakeup flag for Tx might not be set for AF_XDP sockets that are > only used to send packets. This happens if there is at least one outstanding > packet that has not been completed by the hardware and we get that > corresponding completion (which will not generate an interrupt since > interrupts are disabled in the napi poll loop) between the time we stopped > processing the Tx completions and interrupts are enabled again. In this case, > the need_wakeup flag will have been cleared at the end of the Tx > completion processing as we believe we will get an interrupt from the > outstanding completion at a later point in time. But if this completion > interrupt occurs before interrupts are enable, we lose it and should at that > point really have set the need_wakeup flag since there are no more > outstanding completions that can generate an interrupt to continue the > processing. When this happens, user space will see a Tx queue > need_wakeup of 0 and skip issuing a syscall, which means will never get into > the Tx processing again and we have a deadlock. > > This patch introduces a quick fix for this issue by just setting the > need_wakeup flag for Tx to 1 all the time. I am working on a proper fix for > this that will toggle the flag appropriately, but it is more challenging than I > anticipated and I am afraid that this patch will not be completed before the > merge window closes, therefore this easier fix for now. This fix has a > negative performance impact in the range of 0% to 4%. Towards the higher > end of the scale if you have driver and application on the same core and issue > a lot of packets, and towards no negative impact if you use two cores, lower > transmission speeds and/or a workload that also receives packets. > > Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com> > --- > drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 10 ++-------- > 1 file changed, 2 insertions(+), 8 deletions(-) Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c index 100ac89..d6feaac 100644 --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c @@ -622,8 +622,6 @@ static bool ixgbe_xmit_zc(struct ixgbe_ring *xdp_ring, unsigned int budget) if (tx_desc) { ixgbe_xdp_ring_update_tail(xdp_ring); xsk_umem_consume_tx_done(xdp_ring->xsk_umem); - if (xsk_umem_uses_need_wakeup(xdp_ring->xsk_umem)) - xsk_clear_tx_need_wakeup(xdp_ring->xsk_umem); } return !!budget && work_done; @@ -691,12 +689,8 @@ bool ixgbe_clean_xdp_tx_irq(struct ixgbe_q_vector *q_vector, if (xsk_frames) xsk_umem_complete_tx(umem, xsk_frames); - if (xsk_umem_uses_need_wakeup(tx_ring->xsk_umem)) { - if (tx_ring->next_to_clean == tx_ring->next_to_use) - xsk_set_tx_need_wakeup(tx_ring->xsk_umem); - else - xsk_clear_tx_need_wakeup(tx_ring->xsk_umem); - } + if (xsk_umem_uses_need_wakeup(tx_ring->xsk_umem)) + xsk_set_tx_need_wakeup(tx_ring->xsk_umem); return ixgbe_xmit_zc(tx_ring, q_vector->tx.work_limit); }
The need_wakeup flag for Tx might not be set for AF_XDP sockets that are only used to send packets. This happens if there is at least one outstanding packet that has not been completed by the hardware and we get that corresponding completion (which will not generate an interrupt since interrupts are disabled in the napi poll loop) between the time we stopped processing the Tx completions and interrupts are enabled again. In this case, the need_wakeup flag will have been cleared at the end of the Tx completion processing as we believe we will get an interrupt from the outstanding completion at a later point in time. But if this completion interrupt occurs before interrupts are enable, we lose it and should at that point really have set the need_wakeup flag since there are no more outstanding completions that can generate an interrupt to continue the processing. When this happens, user space will see a Tx queue need_wakeup of 0 and skip issuing a syscall, which means will never get into the Tx processing again and we have a deadlock. This patch introduces a quick fix for this issue by just setting the need_wakeup flag for Tx to 1 all the time. I am working on a proper fix for this that will toggle the flag appropriately, but it is more challenging than I anticipated and I am afraid that this patch will not be completed before the merge window closes, therefore this easier fix for now. This fix has a negative performance impact in the range of 0% to 4%. Towards the higher end of the scale if you have driver and application on the same core and issue a lot of packets, and towards no negative impact if you use two cores, lower transmission speeds and/or a workload that also receives packets. Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com> --- drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 10 ++-------- 1 file changed, 2 insertions(+), 8 deletions(-)