Re: [PATCH V2 net-next 1/7] net: ena: avoid unnecessary rearming of interrupt vector when busy-polling

[Eric Dumazet <eric.dumazet@gmail.com>]

On 7/12/20 3:36 PM, akiyano@amazon.com wrote:
> From: Arthur Kiyanovski <akiyano@amazon.com>
> 
> 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

Yeah, and I see you did not bother to CC me on this new submission.

> 
> 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.
> 
> Signed-off-by: Shay Agroskin <shayagr@amazon.com>
> Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
> ---
>  drivers/net/ethernet/amazon/ena/ena_netdev.c | 7 ++++++-
>  drivers/net/ethernet/amazon/ena/ena_netdev.h | 1 +
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.c b/drivers/net/ethernet/amazon/ena/ena_netdev.c
> index 91be3ffa1c5c..90c0fe15cd23 100644
> --- a/drivers/net/ethernet/amazon/ena/ena_netdev.c
> +++ b/drivers/net/ethernet/amazon/ena/ena_netdev.c
> @@ -1913,7 +1913,9 @@ static int ena_io_poll(struct napi_struct *napi, int budget)
>  		/* Update numa and unmask the interrupt only when schedule
>  		 * from the interrupt context (vs from sk_busy_loop)
>  		 */
> -		if (napi_complete_done(napi, rx_work_done)) {
> +		if (napi_complete_done(napi, rx_work_done) &&
> +		    READ_ONCE(ena_napi->interrupts_masked)) {
> +			WRITE_ONCE(ena_napi->interrupts_masked, false);
>  			/* We apply adaptive moderation on Rx path only.
>  			 * Tx uses static interrupt moderation.
>  			 */
> @@ -1961,6 +1963,9 @@ static irqreturn_t ena_intr_msix_io(int irq, void *data)
>  
>  	ena_napi->first_interrupt = true;
>  
> +	WRITE_ONCE(ena_napi->interrupts_masked, true);
> +	smp_wmb(); /* write interrupts_masked before calling napi */

It is not clear where is the paired smp_wmb() 

> +
>  	napi_schedule_irqoff(&ena_napi->napi);
>  
>  	return IRQ_HANDLED;
> diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.h b/drivers/net/ethernet/amazon/ena/ena_netdev.h
> index ba030d260940..89304b403995 100644
> --- a/drivers/net/ethernet/amazon/ena/ena_netdev.h
> +++ b/drivers/net/ethernet/amazon/ena/ena_netdev.h
> @@ -167,6 +167,7 @@ struct ena_napi {
>  	struct ena_ring *rx_ring;
>  	struct ena_ring *xdp_ring;
>  	bool first_interrupt;
> +	bool interrupts_masked;
>  	u32 qid;
>  	struct dim dim;
>  };
> 

Note that writing/reading over bool fields from hard irq context without proper sync is not generally allowed.

Compiler could perform RMW over plain 32bit words.

Sometimes we accept the races, but in this context this might be bad.