netdev - Re: [net-next 08/16] net/mlx5e: Add devlink fdb_large

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Message-ID: <20200302112236.0474d821@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 2 Mar 2020 11:22:36 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jianbo Liu <jianbol@...lanox.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Vlad Buslov <vladbu@...lanox.com>,
        Roi Dayan <roid@...lanox.com>, Jiri Pirko <jiri@...lanox.com>
Subject: Re: [net-next 08/16] net/mlx5e: Add devlink fdb_large_groups
 parameter

On Mon, 2 Mar 2020 02:04:20 +0000 Jianbo Liu wrote:
> The 02/28/2020 11:10, Jakub Kicinski wrote:
> > On Thu, 27 Feb 2020 16:44:38 -0800 Saeed Mahameed wrote:  
> > > From: Jianbo Liu <jianbol@...lanox.com>
> > > 
> > > Add a devlink parameter to control the number of large groups in a
> > > autogrouped flow table. The default value is 15, and the range is between 1
> > > and 1024.
> > > 
> > > The size of each large group can be calculated according to the following
> > > formula: size = 4M / (fdb_large_groups + 1).
> > > 
> > > Examples:
> > > - Set the number of large groups to 20.
> > >     $ devlink dev param set pci/0000:82:00.0 name fdb_large_groups \
> > >       cmode driverinit value 20
> > > 
> > >   Then run devlink reload command to apply the new value.
> > >     $ devlink dev reload pci/0000:82:00.0
> > > 
> > > - Read the number of large groups in flow table.
> > >     $ devlink dev param show pci/0000:82:00.0 name fdb_large_groups
> > >     pci/0000:82:00.0:
> > >       name fdb_large_groups type driver-specific
> > >         values:
> > >           cmode driverinit value 20
> > > 
> > > Signed-off-by: Jianbo Liu <jianbol@...lanox.com>
> > > Reviewed-by: Vlad Buslov <vladbu@...lanox.com>
> > > Reviewed-by: Roi Dayan <roid@...lanox.com>
> > > Acked-by: Jiri Pirko <jiri@...lanox.com>
> > > Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>  
> > 
> > Slicing memory up sounds like something that should be supported via
> > the devlink-resource API, not by params and non-obvious calculations :(  
> 
> No, it's not to configure memory resource. It is to control how many
> large groups in FW FDB. The calculations to to tell how many rules in each
> large group.

I don't know what a "large group" is. Please update the documentation
so it's not an utter pleonasm.

What's a large group? How does twiddling with this knob change the
behavior of the system? Who and in what conditions may be interested 
in adjusting it?