Subject: Re: [resend PATCH 2/2] dim: pass dim_sample to net_dim() by reference

From: Louis Peens <[email-masked]>
Date: Fri, 1 Nov 2024 10:54:47 +0200

On Thu, Oct 31, 2024 at 10:19:55AM -0700, Caleb Sander wrote:
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> 
> On Thu, Oct 31, 2024 at 5:49 AM Louis Peens <[email-masked]> wrote:
> >
> > On Wed, Oct 30, 2024 at 06:23:26PM -0600, Caleb Sander Mateos wrote:
> > > net_dim() is currently passed a struct dim_sample argument by value.
> > > struct dim_sample is 24 bytes. Since this is greater 16 bytes, x86-64
> > > passes it on the stack. All callers have already initialized dim_sample
> > > on the stack, so passing it by value requires pushing a duplicated copy
> > > to the stack. Either witing to the stack and immediately reading it, or
> > > perhaps dereferencing addresses relative to the stack pointer in a chain
> > > of push instructions, seems to perform quite poorly.
> > >
> > > In a heavy TCP workload, mlx5e_handle_rx_dim() consumes 3% of CPU time,
> > > 94% of which is attributed to the first push instruction to copy
> > > dim_sample on the stack for the call to net_dim():
> > > // Call ktime_get()
> > >   0.26 |4ead2:   call   4ead7 <mlx5e_handle_rx_dim+0x47>
> > > // Pass the address of struct dim in %rdi
> > >        |4ead7:   lea    0x3d0(%rbx),%rdi
> > > // Set dim_sample.pkt_ctr
> > >        |4eade:   mov    %r13d,0x8(%rsp)
> > > // Set dim_sample.byte_ctr
> > >        |4eae3:   mov    %r12d,0xc(%rsp)
> > > // Set dim_sample.event_ctr
> > >   0.15 |4eae8:   mov    %bp,0x10(%rsp)
> > > // Duplicate dim_sample on the stack
> > >  94.16 |4eaed:   push   0x10(%rsp)
> > >   2.79 |4eaf1:   push   0x10(%rsp)
> > >   0.07 |4eaf5:   push   %rax
> > > // Call net_dim()
> > >   0.21 |4eaf6:   call   4eafb <mlx5e_handle_rx_dim+0x6b>
> > >
> > > To allow the caller to reuse the struct dim_sample already on the stack,
> > > pass the struct dim_sample by reference to net_dim().
> > >
> > > Signed-off-by: Caleb Sander Mateos <[email-masked]>
> > > ---
> > >  Documentation/networking/net_dim.rst                   |  2 +-
> > >  drivers/net/ethernet/amazon/ena/ena_netdev.c           |  2 +-
> > >  drivers/net/ethernet/broadcom/bcmsysport.c             |  2 +-
> > >  drivers/net/ethernet/broadcom/bnxt/bnxt.c              |  4 ++--
> > >  drivers/net/ethernet/broadcom/genet/bcmgenet.c         |  2 +-
> > >  drivers/net/ethernet/freescale/enetc/enetc.c           |  2 +-
> > >  drivers/net/ethernet/hisilicon/hns3/hns3_enet.c        |  4 ++--
> > >  drivers/net/ethernet/intel/ice/ice_txrx.c              |  4 ++--
> > >  drivers/net/ethernet/intel/idpf/idpf_txrx.c            |  4 ++--
> > >  drivers/net/ethernet/marvell/octeontx2/nic/otx2_txrx.c |  2 +-
> > >  drivers/net/ethernet/mediatek/mtk_eth_soc.c            |  4 ++--
> > >  drivers/net/ethernet/mellanox/mlx5/core/en_txrx.c      |  4 ++--
> > >  drivers/net/ethernet/netronome/nfp/nfd3/dp.c           |  4 ++--
> > >  drivers/net/ethernet/netronome/nfp/nfdk/dp.c           |  4 ++--
> > >  drivers/net/ethernet/pensando/ionic/ionic_txrx.c       |  2 +-
> > >  drivers/net/virtio_net.c                               |  2 +-
> > >  drivers/soc/fsl/dpio/dpio-service.c                    |  2 +-
> > >  include/linux/dim.h                                    |  2 +-
> > >  lib/dim/net_dim.c                                      | 10 +++++-----
> > >  19 files changed, 31 insertions(+), 31 deletions(-)
> > >
> > --- snip --
> >
> > > diff --git a/drivers/net/ethernet/netronome/nfp/nfd3/dp.c b/drivers/net/ethernet/netronome/nfp/nfd3/dp.c
> > > index d215efc6cad0..f1c6c47564b1 100644
> > > --- a/drivers/net/ethernet/netronome/nfp/nfd3/dp.c
> > > +++ b/drivers/net/ethernet/netronome/nfp/nfd3/dp.c
> > > @@ -1177,11 +1177,11 @@ int nfp_nfd3_poll(struct napi_struct *napi, int budget)
> > >                       pkts = r_vec->rx_pkts;
> > >                       bytes = r_vec->rx_bytes;
> > >               } while (u64_stats_fetch_retry(&r_vec->rx_sync, start));
> > >
> > >               dim_update_sample(r_vec->event_ctr, pkts, bytes, &dim_sample);
> > > -             net_dim(&r_vec->rx_dim, dim_sample);
> > > +             net_dim(&r_vec->rx_dim, &dim_sample);
> > >       }
> > >
> > >       if (r_vec->nfp_net->tx_coalesce_adapt_on && r_vec->tx_ring) {
> > >               struct dim_sample dim_sample = {};
> > >               unsigned int start;
> > > @@ -1192,11 +1192,11 @@ int nfp_nfd3_poll(struct napi_struct *napi, int budget)
> > >                       pkts = r_vec->tx_pkts;
> > >                       bytes = r_vec->tx_bytes;
> > >               } while (u64_stats_fetch_retry(&r_vec->tx_sync, start));
> > >
> > >               dim_update_sample(r_vec->event_ctr, pkts, bytes, &dim_sample);
> > > -             net_dim(&r_vec->tx_dim, dim_sample);
> > > +             net_dim(&r_vec->tx_dim, &dim_sample);
> > >       }
> > >
> > >       return pkts_polled;
> > >  }
> > >
> > > diff --git a/drivers/net/ethernet/netronome/nfp/nfdk/dp.c b/drivers/net/ethernet/netronome/nfp/nfdk/dp.c
> > > index dae5af7d1845..ebeb6ab4465c 100644
> > > --- a/drivers/net/ethernet/netronome/nfp/nfdk/dp.c
> > > +++ b/drivers/net/ethernet/netronome/nfp/nfdk/dp.c
> > > @@ -1287,11 +1287,11 @@ int nfp_nfdk_poll(struct napi_struct *napi, int budget)
> > >                       pkts = r_vec->rx_pkts;
> > >                       bytes = r_vec->rx_bytes;
> > >               } while (u64_stats_fetch_retry(&r_vec->rx_sync, start));
> > >
> > >               dim_update_sample(r_vec->event_ctr, pkts, bytes, &dim_sample);
> > > -             net_dim(&r_vec->rx_dim, dim_sample);
> > > +             net_dim(&r_vec->rx_dim, &dim_sample);
> > >       }
> > >
> > >       if (r_vec->nfp_net->tx_coalesce_adapt_on && r_vec->tx_ring) {
> > >               struct dim_sample dim_sample = {};
> > >               unsigned int start;
> > > @@ -1302,11 +1302,11 @@ int nfp_nfdk_poll(struct napi_struct *napi, int budget)
> > >                       pkts = r_vec->tx_pkts;
> > >                       bytes = r_vec->tx_bytes;
> > >               } while (u64_stats_fetch_retry(&r_vec->tx_sync, start));
> > >
> > >               dim_update_sample(r_vec->event_ctr, pkts, bytes, &dim_sample);
> > > -             net_dim(&r_vec->tx_dim, dim_sample);
> > > +             net_dim(&r_vec->tx_dim, &dim_sample);
> > >       }
> > >
> > >       return pkts_polled;
> > >  }
> > --- snip ---
> >
> > Hi Caleb. Looks like a fair enough update to me in general, but I am not an
> > expert on 'dim'. For the corresponding nfp driver changes feel free to add:
> >
> > Signed-off-by: Louis Peens <[email-masked]>
> 
> Hi Louis,
> Thanks for the review. Did you mean "Reviewed-by"? If there was a
> change you were suggesting, I missed it.
Hi - sorry, I do still manage to mix up when to use signed-off-by and
reviewed-by. I did not suggest any changes no, and since the main focus of the
patch is not the nfp driver I can see in hindsight that Reviewed-by: may make
more sense. So updated:

Reviewed-by: Louis Peens <[email-masked]>
> Best,
> Caleb