Subject: Re: [PATCH net] sfc: use budget for TX completions

From: Martin Habets <[email-masked]>
Date: Wed, 14 Jun 2023 09:03:05 +0100

On Mon, Jun 12, 2023 at 04:42:54PM +0200, Íñigo Huguet wrote:
> When running workloads heavy unbalanced towards TX (high TX, low RX
> traffic), sfc driver can retain the CPU during too long times. Although
> in many cases this is not enough to be visible, it can affect
> performance and system responsiveness.
> 
> A way to reproduce it is to use a debug kernel and run some parallel
> netperf TX tests. In some systems, this will lead to this message being
> logged:
>   kernel:watchdog: BUG: soft lockup - CPU#12 stuck for 22s!
> 
> The reason is that sfc driver doesn't account any NAPI budget for the TX
> completion events work. With high-TX/low-RX traffic, this makes that the
> CPU is held for long time for NAPI poll.
> 
> Documentations says "drivers can process completions for any number of Tx
> packets but should only process up to budget number of Rx packets".
> However, many drivers do limit the amount of TX completions that they
> process in a single NAPI poll.

I think your work and what other drivers do shows that the documentation is
no longer correct. I haven't checked when that was written, but maybe it
was years ago when link speeds were lower.
Clearly for drivers that support higher link speeds this is an issue, so we
should update the documentation. Not sure what constitutes a high link speed,
with current CPUs for me it's anything >= 50G.

> In the same way, this patch adds a limit for the TX work in sfc. With
> the patch applied, the watchdog warning never appears.
> 
> Tested with netperf in different combinations: single process / parallel
> processes, TCP / UDP and different sizes of UDP messages. Repeated the
> tests before and after the patch, without any noticeable difference in
> network or CPU performance.
> 
> Test hardware:
> Intel(R) Xeon(R) CPU E5-1620 v4 @ 3.50GHz (4 cores, 2 threads/core)
> Solarflare Communications XtremeScale X2522-25G Network Adapter
> 
> Reported-by: Fei Liu <[email-masked]>
> Signed-off-by: Íñigo Huguet <[email-masked]>
> ---
>  drivers/net/ethernet/sfc/ef10.c      | 25 ++++++++++++++++++-------
>  drivers/net/ethernet/sfc/ef100_nic.c |  7 ++++++-
>  drivers/net/ethernet/sfc/ef100_tx.c  |  4 ++--
>  drivers/net/ethernet/sfc/ef100_tx.h  |  2 +-
>  drivers/net/ethernet/sfc/tx_common.c |  4 +++-
>  drivers/net/ethernet/sfc/tx_common.h |  2 +-
>  6 files changed, 31 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/ethernet/sfc/ef10.c b/drivers/net/ethernet/sfc/ef10.c
> index d30459dbfe8f..b63e47af6365 100644
> --- a/drivers/net/ethernet/sfc/ef10.c
> +++ b/drivers/net/ethernet/sfc/ef10.c
> @@ -2950,7 +2950,7 @@ static u32 efx_ef10_extract_event_ts(efx_qword_t *event)
>  	return tstamp;
>  }
>  
> -static void
> +static int
>  efx_ef10_handle_tx_event(struct efx_channel *channel, efx_qword_t *event)
>  {
>  	struct efx_nic *efx = channel->efx;
> @@ -2958,13 +2958,14 @@ efx_ef10_handle_tx_event(struct efx_channel *channel, efx_qword_t *event)
>  	unsigned int tx_ev_desc_ptr;
>  	unsigned int tx_ev_q_label;
>  	unsigned int tx_ev_type;
> +	int work_done;
>  	u64 ts_part;
>  
>  	if (unlikely(READ_ONCE(efx->reset_pending)))
> -		return;
> +		return 0;
>  
>  	if (unlikely(EFX_QWORD_FIELD(*event, ESF_DZ_TX_DROP_EVENT)))
> -		return;
> +		return 0;
>  
>  	/* Get the transmit queue */
>  	tx_ev_q_label = EFX_QWORD_FIELD(*event, ESF_DZ_TX_QLABEL);
> @@ -2973,8 +2974,7 @@ efx_ef10_handle_tx_event(struct efx_channel *channel, efx_qword_t *event)
>  	if (!tx_queue->timestamping) {
>  		/* Transmit completion */
>  		tx_ev_desc_ptr = EFX_QWORD_FIELD(*event, ESF_DZ_TX_DESCR_INDX);
> -		efx_xmit_done(tx_queue, tx_ev_desc_ptr & tx_queue->ptr_mask);
> -		return;
> +		return efx_xmit_done(tx_queue, tx_ev_desc_ptr & tx_queue->ptr_mask);
>  	}
>  
>  	/* Transmit timestamps are only available for 8XXX series. They result
> @@ -3000,6 +3000,7 @@ efx_ef10_handle_tx_event(struct efx_channel *channel, efx_qword_t *event)
>  	 * fields in the event.
>  	 */
>  	tx_ev_type = EFX_QWORD_FIELD(*event, ESF_EZ_TX_SOFT1);
> +	work_done = 0;
>  
>  	switch (tx_ev_type) {
>  	case TX_TIMESTAMP_EVENT_TX_EV_COMPLETION:
> @@ -3016,6 +3017,7 @@ efx_ef10_handle_tx_event(struct efx_channel *channel, efx_qword_t *event)
>  		tx_queue->completed_timestamp_major = ts_part;
>  
>  		efx_xmit_done_single(tx_queue);
> +		work_done = 1;
>  		break;
>  
>  	default:
> @@ -3026,6 +3028,8 @@ efx_ef10_handle_tx_event(struct efx_channel *channel, efx_qword_t *event)
>  			  EFX_QWORD_VAL(*event));
>  		break;
>  	}
> +
> +	return work_done;
>  }
>  
>  static void
> @@ -3081,13 +3085,16 @@ static void efx_ef10_handle_driver_generated_event(struct efx_channel *channel,
>  	}
>  }
>  
> +#define EFX_NAPI_MAX_TX 512

How did you determine this value? Is it what other driver use?

Martin

> +
>  static int efx_ef10_ev_process(struct efx_channel *channel, int quota)
>  {
>  	struct efx_nic *efx = channel->efx;
>  	efx_qword_t event, *p_event;
>  	unsigned int read_ptr;
> -	int ev_code;
> +	int spent_tx = 0;
>  	int spent = 0;
> +	int ev_code;
>  
>  	if (quota <= 0)
>  		return spent;
> @@ -3126,7 +3133,11 @@ static int efx_ef10_ev_process(struct efx_channel *channel, int quota)
>  			}
>  			break;
>  		case ESE_DZ_EV_CODE_TX_EV:
> -			efx_ef10_handle_tx_event(channel, &event);
> +			spent_tx += efx_ef10_handle_tx_event(channel, &event);
> +			if (spent_tx >= EFX_NAPI_MAX_TX) {
> +				spent = quota;
> +				goto out;
> +			}
>  			break;
>  		case ESE_DZ_EV_CODE_DRIVER_EV:
>  			efx_ef10_handle_driver_event(channel, &event);
> diff --git a/drivers/net/ethernet/sfc/ef100_nic.c b/drivers/net/ethernet/sfc/ef100_nic.c
> index 4dc643b0d2db..7adde9639c8a 100644
> --- a/drivers/net/ethernet/sfc/ef100_nic.c
> +++ b/drivers/net/ethernet/sfc/ef100_nic.c
> @@ -253,6 +253,8 @@ static void ef100_ev_read_ack(struct efx_channel *channel)
>  		   efx_reg(channel->efx, ER_GZ_EVQ_INT_PRIME));
>  }
>  
> +#define EFX_NAPI_MAX_TX 512
> +
>  static int ef100_ev_process(struct efx_channel *channel, int quota)
>  {
>  	struct efx_nic *efx = channel->efx;
> @@ -260,6 +262,7 @@ static int ef100_ev_process(struct efx_channel *channel, int quota)
>  	bool evq_phase, old_evq_phase;
>  	unsigned int read_ptr;
>  	efx_qword_t *p_event;
> +	int spent_tx = 0;
>  	int spent = 0;
>  	bool ev_phase;
>  	int ev_type;
> @@ -295,7 +298,9 @@ static int ef100_ev_process(struct efx_channel *channel, int quota)
>  			efx_mcdi_process_event(channel, p_event);
>  			break;
>  		case ESE_GZ_EF100_EV_TX_COMPLETION:
> -			ef100_ev_tx(channel, p_event);
> +			spent_tx += ef100_ev_tx(channel, p_event);
> +			if (spent_tx >= EFX_NAPI_MAX_TX)
> +				spent = quota;
>  			break;
>  		case ESE_GZ_EF100_EV_DRIVER:
>  			netif_info(efx, drv, efx->net_dev,
> diff --git a/drivers/net/ethernet/sfc/ef100_tx.c b/drivers/net/ethernet/sfc/ef100_tx.c
> index 29ffaf35559d..849e5555bd12 100644
> --- a/drivers/net/ethernet/sfc/ef100_tx.c
> +++ b/drivers/net/ethernet/sfc/ef100_tx.c
> @@ -346,7 +346,7 @@ void ef100_tx_write(struct efx_tx_queue *tx_queue)
>  	ef100_tx_push_buffers(tx_queue);
>  }
>  
> -void ef100_ev_tx(struct efx_channel *channel, const efx_qword_t *p_event)
> +int ef100_ev_tx(struct efx_channel *channel, const efx_qword_t *p_event)
>  {
>  	unsigned int tx_done =
>  		EFX_QWORD_FIELD(*p_event, ESF_GZ_EV_TXCMPL_NUM_DESC);
> @@ -357,7 +357,7 @@ void ef100_ev_tx(struct efx_channel *channel, const efx_qword_t *p_event)
>  	unsigned int tx_index = (tx_queue->read_count + tx_done - 1) &
>  				tx_queue->ptr_mask;
>  
> -	efx_xmit_done(tx_queue, tx_index);
> +	return efx_xmit_done(tx_queue, tx_index);
>  }
>  
>  /* Add a socket buffer to a TX queue
> diff --git a/drivers/net/ethernet/sfc/ef100_tx.h b/drivers/net/ethernet/sfc/ef100_tx.h
> index e9e11540fcde..d9a0819c5a72 100644
> --- a/drivers/net/ethernet/sfc/ef100_tx.h
> +++ b/drivers/net/ethernet/sfc/ef100_tx.h
> @@ -20,7 +20,7 @@ void ef100_tx_init(struct efx_tx_queue *tx_queue);
>  void ef100_tx_write(struct efx_tx_queue *tx_queue);
>  unsigned int ef100_tx_max_skb_descs(struct efx_nic *efx);
>  
> -void ef100_ev_tx(struct efx_channel *channel, const efx_qword_t *p_event);
> +int ef100_ev_tx(struct efx_channel *channel, const efx_qword_t *p_event);
>  
>  netdev_tx_t ef100_enqueue_skb(struct efx_tx_queue *tx_queue, struct sk_buff *skb);
>  int __ef100_enqueue_skb(struct efx_tx_queue *tx_queue, struct sk_buff *skb,
> diff --git a/drivers/net/ethernet/sfc/tx_common.c b/drivers/net/ethernet/sfc/tx_common.c
> index 67e789b96c43..755aa92bf823 100644
> --- a/drivers/net/ethernet/sfc/tx_common.c
> +++ b/drivers/net/ethernet/sfc/tx_common.c
> @@ -249,7 +249,7 @@ void efx_xmit_done_check_empty(struct efx_tx_queue *tx_queue)
>  	}
>  }
>  
> -void efx_xmit_done(struct efx_tx_queue *tx_queue, unsigned int index)
> +int efx_xmit_done(struct efx_tx_queue *tx_queue, unsigned int index)
>  {
>  	unsigned int fill_level, pkts_compl = 0, bytes_compl = 0;
>  	unsigned int efv_pkts_compl = 0;
> @@ -279,6 +279,8 @@ void efx_xmit_done(struct efx_tx_queue *tx_queue, unsigned int index)
>  	}
>  
>  	efx_xmit_done_check_empty(tx_queue);
> +
> +	return pkts_compl + efv_pkts_compl;
>  }
>  
>  /* Remove buffers put into a tx_queue for the current packet.
> diff --git a/drivers/net/ethernet/sfc/tx_common.h b/drivers/net/ethernet/sfc/tx_common.h
> index d87aecbc7bf1..1e9f42938aac 100644
> --- a/drivers/net/ethernet/sfc/tx_common.h
> +++ b/drivers/net/ethernet/sfc/tx_common.h
> @@ -28,7 +28,7 @@ static inline bool efx_tx_buffer_in_use(struct efx_tx_buffer *buffer)
>  }
>  
>  void efx_xmit_done_check_empty(struct efx_tx_queue *tx_queue);
> -void efx_xmit_done(struct efx_tx_queue *tx_queue, unsigned int index);
> +int efx_xmit_done(struct efx_tx_queue *tx_queue, unsigned int index);
>  
>  void efx_enqueue_unwind(struct efx_tx_queue *tx_queue,
>  			unsigned int insert_count);
> -- 
> 2.40.1