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

From: Martin Habets <[email-masked]>
Date: Wed, 14 Jun 2023 09:10:55 +0100

On Tue, Jun 13, 2023 at 04:42:25PM +0200, Íñigo Huguet wrote:
> On Mon, Jun 12, 2023 at 6:05 PM Maciej Fijalkowski
> <[email-masked]> wrote:
> >
> > On Mon, Jun 12, 2023 at 04:42:54PM +0200, Íñigo Huguet wrote:
> >
> > Hey Inigo, some trivial things below.
> >
> > > 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.
> > >
> > > 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]>
> >
> > You're missing Fixes: tag.
> 
> Thanks for the reminder!
> 
> >
> > > ---
> > >  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
> > > +
> > >  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
> >
> > couldn't this go to tx_common.h ? you're defining this two times.
> 
> Not sure about this. I didn't like any place to put it, so I ended up
> putting it here. As it is an implementation detail, I didn't like the
> idea of putting it in a header file that contains the driver's API.
> 
> I would better like to hear the opinion from the sfc maintainers, but
> I don't mind changing it because I'm neither happy with the chosen
> location.

I think we should add it in include/linux/netdevice.h, close to
NAPI_POLL_WEIGHT. That way all drivers can use it.
Do we need to add this TX poll weight to struct napi_struct and
extend netif_napi_add_weight()?
That way all drivers could use the value from napi_struct in stead of using
a hard-coded define. And at some point we can adjust it.

Martin

> 
> >
> > > +
> > >  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
> > >
> > >
> >
> 
> 
> -- 
> Íñigo Huguet