[xdp-hints] Re: [PATCH bpf-next v6 11/18] ice: put XDP meta sources assignment under a static key condition

[Larysa Zaremba <larysa.zaremba@intel.com>]

On Sat, Oct 28, 2023 at 09:55:52PM +0200, Maciej Fijalkowski wrote:
> On Mon, Oct 23, 2023 at 11:35:46AM +0200, Larysa Zaremba wrote:
> > On Fri, Oct 20, 2023 at 06:32:13PM +0200, Maciej Fijalkowski wrote:
> > > On Thu, Oct 12, 2023 at 07:05:17PM +0200, Larysa Zaremba wrote:
> > > > Usage of XDP hints requires putting additional information after the
> > > > xdp_buff. In basic case, only the descriptor has to be copied on a
> > > > per-packet basis, because xdp_buff permanently resides before per-ring
> > > > metadata (cached time and VLAN protocol ID).
> > > > 
> > > > However, in ZC mode, xdp_buffs come from a pool, so memory after such
> > > > buffer does not contain any reliable information, so everything has to be
> > > > copied, damaging the performance.
> > > > 
> > > > Introduce a static key to enable meta sources assignment only when attached
> > > > XDP program is device-bound.
> > > > 
> > > > This patch eliminates a 6% performance drop in ZC mode, which was a result
> > > > of addition of XDP hints to the driver.
> > > > 
> > > > Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> > > > ---
> > > >  drivers/net/ethernet/intel/ice/ice.h      |  1 +
> > > >  drivers/net/ethernet/intel/ice/ice_main.c | 14 ++++++++++++++
> > > >  drivers/net/ethernet/intel/ice/ice_txrx.c |  3 ++-
> > > >  drivers/net/ethernet/intel/ice/ice_xsk.c  |  3 +++
> > > >  4 files changed, 20 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/net/ethernet/intel/ice/ice.h b/drivers/net/ethernet/intel/ice/ice.h
> > > > index 3d0f15f8b2b8..76d22be878a4 100644
> > > > --- a/drivers/net/ethernet/intel/ice/ice.h
> > > > +++ b/drivers/net/ethernet/intel/ice/ice.h
> > > > @@ -210,6 +210,7 @@ enum ice_feature {
> > > >  };
> > > >  
> > > >  DECLARE_STATIC_KEY_FALSE(ice_xdp_locking_key);
> > > > +DECLARE_STATIC_KEY_FALSE(ice_xdp_meta_key);
> > > >  
> > > >  struct ice_channel {
> > > >  	struct list_head list;
> > > > diff --git a/drivers/net/ethernet/intel/ice/ice_main.c b/drivers/net/ethernet/intel/ice/ice_main.c
> > > > index 47e8920e1727..ee0df86d34b7 100644
> > > > --- a/drivers/net/ethernet/intel/ice/ice_main.c
> > > > +++ b/drivers/net/ethernet/intel/ice/ice_main.c
> > > > @@ -48,6 +48,9 @@ MODULE_PARM_DESC(debug, "netif level (0=none,...,16=all)");
> > > >  DEFINE_STATIC_KEY_FALSE(ice_xdp_locking_key);
> > > >  EXPORT_SYMBOL(ice_xdp_locking_key);
> > > >  
> > > > +DEFINE_STATIC_KEY_FALSE(ice_xdp_meta_key);
> > > > +EXPORT_SYMBOL(ice_xdp_meta_key);
> > > > +
> > > >  /**
> > > >   * ice_hw_to_dev - Get device pointer from the hardware structure
> > > >   * @hw: pointer to the device HW structure
> > > > @@ -2634,6 +2637,11 @@ static int ice_xdp_alloc_setup_rings(struct ice_vsi *vsi)
> > > >  	return -ENOMEM;
> > > >  }
> > > >  
> > > > +static bool ice_xdp_prog_has_meta(struct bpf_prog *prog)
> > > > +{
> > > > +	return prog && prog->aux->dev_bound;
> > > > +}
> > > > +
> > > >  /**
> > > >   * ice_vsi_assign_bpf_prog - set or clear bpf prog pointer on VSI
> > > >   * @vsi: VSI to set the bpf prog on
> > > > @@ -2644,10 +2652,16 @@ static void ice_vsi_assign_bpf_prog(struct ice_vsi *vsi, struct bpf_prog *prog)
> > > >  	struct bpf_prog *old_prog;
> > > >  	int i;
> > > >  
> > > > +	if (ice_xdp_prog_has_meta(prog))
> > > > +		static_branch_inc(&ice_xdp_meta_key);
> > > 
> > > i thought boolean key would be enough but inc/dec should serve properly
> > > for example prog hotswap cases.
> > >
> > 
> > My thought process on using counting instead of boolean was: there can be 
> > several PFs that use the same driver, so therefore we need to keep track of how 
> > many od them use hints. 
> 
> Very good point. This implies that if PF0 has hints-enabled prog loaded,
> PF1 with non-hints prog will "suffer" from it.
> 
> Sorry for such a long delays in responses but I was having a hard time
> making up my mind about it. In the end I have come up to some conclusions.
> I know the timing for sending this response is not ideal, but I need to
> get this off my chest and bring discussion back to life:)
> 
> IMHO having static keys to eliminate ZC overhead does not scale. I assume
> every other driver would have to follow that.
> 
> XSK pool allows us to avoid initializing various things per each packet.
> Instead, taking xdp_rxq_info as an example, each xdp_buff from pool has
> xdp_rxq_info assigned at init time. With this in mind, we should have some
> mechanism to set hints-specific things in xdp_buff_xsk::cb, at init time
> as well. Such mechanism should not require us to expose driver's private
> xdp_buff hints containers (such as ice_pkt_ctx) to XSK pool.
> 
> Right now you moved phctime down to ice_pkt_ctx and to me that's the main
> reason we have to copy ice_pkt_ctx to each xdp_buff on ZC. What if we keep
> the cached_phctime at original offset in ring but ice_pkt_ctx would get a
> pointer to that?
> 
> This would allow us to init the pointer in each xdp_buff from XSK pool at
> init time. I have come up with a way to program that via so called XSK
> meta descriptors. Each desc would have data to write onto cb, offset
> within cb and amount of bytes to write/copy.
> 
> I'll share the diff below but note that I didn't measure how much lower
> the performance is degraded. My icelake machine where I used to measure
> performance-sensitive code got broke. For now we can't escape initing
> eop_desc per each xdp_buff, but I moved it to alloc side, as we mangle
> descs there anyway.
> 
> I think mlx5 could benefit from that approach as well with initing the rq
> ptr at init time.
> 
> Diff does mostly these things:
> - move cached_phctime to old place in ice_rx_ring and add ptr to that in
>   ice_pkt_ctx
> - introduce xsk_pool_set_meta()
> - use it from ice side.
> 
> I consider this as a discussion trigger rather than ready code. Any
> feedback will be appreciated.
> 
> ---------------------------------8<---------------------------------
> 
> diff --git a/drivers/net/ethernet/intel/ice/ice_base.c b/drivers/net/ethernet/intel/ice/ice_base.c
> index 7fa43827a3f0..c192e84bee55 100644
> --- a/drivers/net/ethernet/intel/ice/ice_base.c
> +++ b/drivers/net/ethernet/intel/ice/ice_base.c
> @@ -519,6 +519,23 @@ static int ice_setup_rx_ctx(struct ice_rx_ring *ring)
>  	return 0;
>  }
>  

[...]

> > > 
> > > My only concern is that we might be hurting in a minor way hints path now,
> > > no?
> > 
> > I have thought "unlikely" refers to the default state the code is compiled with 
> > and after static key incrementation this should be patched to "likely". Isn't 
> > this how static keys work?
> 
> I was only referring to that it ends with compiler hint:
> #define unlikely_notrace(x)	__builtin_expect(!!(x), 0)
> 
> see include/linux/jump_label.h
> 

You are right, I have misunderstood the concept a little bit.

> > 
> > > 
> > > > +		ice_xdp_meta_set_desc(xdp, eop_desc);
> > > >  
> > > >  	act = bpf_prog_run_xdp(xdp_prog, xdp);
> > > >  	switch (act) {
> > > > diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > > > index 39775bb6cec1..f92d7d33fde6 100644
> > > > --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> > > > +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > > > @@ -773,6 +773,9 @@ static void ice_prepare_pkt_ctx_zc(struct xdp_buff *xdp,
> > > >  				   union ice_32b_rx_flex_desc *eop_desc,
> > > >  				   struct ice_rx_ring *rx_ring)
> > > >  {
> > > > +	if (!static_branch_unlikely(&ice_xdp_meta_key))
> > > > +		return;
> > > 
> > > wouldn't it be better to pull it out and avoid calling
> > > ice_prepare_pkt_ctx_zc() unnecessarily?
> > > 
> > > > +
> > > >  	XSK_CHECK_PRIV_TYPE(struct ice_xdp_buff);
> > > >  	((struct ice_xdp_buff *)xdp)->pkt_ctx = rx_ring->pkt_ctx;
> > > >  	ice_xdp_meta_set_desc(xdp, eop_desc);
> > > > -- 
> > > > 2.41.0
> > > >