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1e5ae35072
For an overview of the race created by this patch goto synchronization label. In napi busy-poll mode, the kernel invokes the napi handler of the device repeatedly to poll the NIC's receive queues. This process repeats until a timeout, specific for each connection, is up. By polling packets in busy-poll mode the user may gain lower latency and higher throughput (since the kernel no longer waits for interrupts to poll the queues) in expense of CPU usage. Upon completing a napi routine, the driver checks whether the routine was called by an interrupt handler. If so, the driver re-enables interrupts for the device. This is needed since an interrupt routine invocation disables future invocations until explicitly re-enabled. The driver avoids re-enabling the interrupts if they were not disabled in the first place (e.g. if driver in busy mode). Originally, the driver checked whether interrupt re-enabling is needed by reading the 'ena_napi->unmask_interrupt' variable. This atomic variable was set upon interrupt and cleared after re-enabling it. In the 4.10 Linux version, the 'napi_complete_done' call was changed so that it returns 'false' when device should not re-enable interrupts, and 'true' otherwise. The change includes reading the "NAPIF_STATE_IN_BUSY_POLL" flag to check if the napi call is in busy-poll mode, and if so, return 'false'. The driver was changed to re-enable interrupts according to this routine's return value. The Linux community rejected the use of the 'ena_napi->unmaunmask_interrupt' variable to determine whether unmasking is needed, and urged to use napi_napi_complete_done() return value solely. See https://lore.kernel.org/patchwork/patch/741149/ for more details As explained, a busy-poll session exists for a specified timeout value, after which it exits the busy-poll mode and re-enters it later. This leads to many invocations of the napi handler where napi_complete_done() false indicates that interrupts should be re-enabled. This creates a bug in which the interrupts are re-enabled unnecessarily. To reproduce this bug: 1) echo 50 | sudo tee /proc/sys/net/core/busy_poll 2) echo 50 | sudo tee /proc/sys/net/core/busy_read 3) Add counters that check whether 'ena_unmask_interrupt(tx_ring, rx_ring);' is called without disabling the interrupts in the first place (i.e. with calling the interrupt routine ena_intr_msix_io()) Steps 1+2 enable busy-poll as the default mode for new connections. The busy poll routine rearms the interrupts after every session by design, and so we need to add an extra check that the interrupts were masked in the first place. synchronization: This patch introduces a race between the interrupt handler ena_intr_msix_io() and the napi routine ena_io_poll(). Some macros and instruction were added to prevent this race from leaving the interrupts masked. The following specifies the different race scenarios in this patch: 1) interrupt handler and napi routine run sequentially i) interrupt handler is called, sets 'interrupts_masked' flag and successfully schedules the napi handler via softirq. In this scenario the napi routine might not see the flag change for several reasons: a) The flag is stored in a register by the compiler. For this case the WRITE_ONCE macro which prevents this. b) The compiler might reorder the instruction. For this the smp_wmb() instruction was used which implies a compiler memory barrier. c) On archs with weak consistency model (like ARM64) the napi routine might be scheduled and start running before the flag STORE instruction is committed to cache/memory. To ensure this doesn't happen, the smp_wmb() instruction was added. It ensures that the flag set instruction is committed before scheduling napi. ii) compiler reorders the flag's value check in the 'if' with the flag set in the napi routine. This scenario is prevented by smp_rmb() call after the flag check. 2) interrupt handler and napi routine run in parallel (can happen when busy poll routine invokes the napi handler) i) interrupt handler sets the flag in one core, while the napi routine reads it in another core. This scenario also is divided into two cases: a) napi_complete_done() doesn't finish running, in which case napi_sched() would just set NAPIF_STATE_MISSED and the napi routine would reschedule itself without changing the flag's value. b) napi_complete_done() finishes running. In this case the napi routine might override the flag's value. This doesn't present any rise since it later unmasks the interrupt vector. Signed-off-by: Shay Agroskin <shayagr@amazon.com> Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com> Cc: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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