netdev - Re: [net-next 01/14] net/mlx5e: RX, Add a prefetch command for small L1_CACHE

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Date:   Tue, 23 Apr 2019 16:42:26 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "brouer@...hat.com" <brouer@...hat.com>,
        "alexander.duyck@...il.com" <alexander.duyck@...il.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        Tariq Toukan <tariqt@...lanox.com>, "bsd@...com" <bsd@...com>
Subject: Re: [net-next 01/14] net/mlx5e: RX, Add a prefetch command for small
 L1_CACHE_BYTES

On Tue, 2019-04-23 at 08:21 -0700, Alexander Duyck wrote:
> On Tue, Apr 23, 2019 at 6:23 AM Jesper Dangaard Brouer
> <brouer@...hat.com> wrote:
> > On Mon, 22 Apr 2019 19:46:47 -0700
> > Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:
> > 
> > > On Mon, 22 Apr 2019 15:32:53 -0700, Saeed Mahameed wrote:
> > > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h
> > > > b/drivers/net/ethernet/mellanox/mlx5/core/en.h
> > > > index 51e109fdeec1..6147be23a9b9 100644
> > > > --- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
> > > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
> > > > @@ -50,6 +50,7 @@
> > > >  #include <net/xdp.h>
> > > >  #include <linux/net_dim.h>
> > > >  #include <linux/bits.h>
> > > > +#include <linux/prefetch.h>
> > > >  #include "wq.h"
> > > >  #include "mlx5_core.h"
> > > >  #include "en_stats.h"
> > > > @@ -986,6 +987,22 @@ static inline void mlx5e_cq_arm(struct
> > > > mlx5e_cq *cq)
> > > >     mlx5_cq_arm(mcq, MLX5_CQ_DB_REQ_NOT, mcq->uar->map, cq-
> > > > >wq.cc);
> > > >  }
> > > > 
> > > > +static inline void mlx5e_prefetch(void *p)
> > > > +{
> > > > +   prefetch(p);
> > > > +#if L1_CACHE_BYTES < 128
> > > > +   prefetch(p + L1_CACHE_BYTES);
> > > > +#endif
> > > > +}
> > > > +
> > > > +static inline void mlx5e_prefetchw(void *p)
> > > > +{
> > > > +   prefetchw(p);
> > > > +#if L1_CACHE_BYTES < 128
> > > > +   prefetchw(p + L1_CACHE_BYTES);
> > > > +#endif
> > > > +}
> > > 
> > > All Intel drivers do the exact same thing, perhaps it's time to
> > > add a
> > > helper fot this?
> > > 
> > > net_prefetch_headers()
> > > 
> > > or some such?
> > 
> > I wonder if Tariq measured any effect from doing this?
> > 
> > Because Intel CPUs will usually already prefetch the next cache-
> > line,
> > as described in [1], you can even read (and modify) this MSR 0x1A4
> > e.g. via tools in [2].  Maybe Intel guys added it before this was
> > done
> > in HW, and never cleaned it up?
> > 
> > [1] 
> > https://software.intel.com/en-us/articles/disclosure-of-hw-prefetcher-control-on-some-intel-processors
> 
> The issue is the adjacent cache line prefetcher can be on or off and
> a
> network driver shouldn't really be going through and twiddling those
> sort of bits. In some cases having it on can result in more memory
> being consumed then is needed. The reason why I enabled the
> additional
> cacheline prefetch for the Intel NICs is because most TCP packets are
> at a minimum 68 bytes for just the headers so there was an advantage
> for TCP traffic to make certain we prefetched at least enough for us
> to process the headers.
> 

So if L2 adjacent cache line prefetcher bit is enabled then this
additional prefetch step is redundant ? what is the performance cost in
this case ?

> As far as Jakub comment about combining the functions I would be okay
> with that. We just need to make it a static inline function available
> to all the network drivers.
> 

Agreed, will drop this patch for now and Tariq will address, in next
version.

> Thanks.
> 
> - Alex