Re: [patch net-next 5/5] mlxsw: spectrum: qdiscs: Support stats for PRIO qdisc

[Jakub Kicinski <kubakici@wp.pl>]

On Fri, 12 Jan 2018 09:29:22 +0000 Nogah Frankel wrote:

> > -----Original Message-----
> > From: Yuval Mintz
> > Sent: Friday, January 12, 2018 10:47 AM
> > To: Jakub Kicinski <kubakici@wp.pl>
> > Cc: Jiri Pirko <jiri@resnulli.us>; netdev@vger.kernel.org; Nogah
> > Frankel <nogahf@mellanox.com>; davem@davemloft.net; Ido Schimmel
> > <idosch@mellanox.com>; mlxsw <mlxsw@mellanox.com>; jhs@mojatatu.com;
> > xiyou.wangcong@gmail.com
> > Subject: RE: [patch net-next 5/5] mlxsw: spectrum: qdiscs: Support
> > stats for PRIO qdisc 
> > > > > Hm.  You you need this just because you didn't add the backlog
> > > > > pointer to destroy?  AFAIK on destroy we are free to reset
> > > > > stats as well, thus simplifying your driver...  Let me know
> > > > > if I misunderstand.  
> 
> The problem of doing it in destroy is when one qdisc is replacing
> another. I want to be able to destroy the old qdisc to "make room"
> for the new one before I get the destroy command for the old qdisc
> (that will come just after the replace command for the new qdisc).
> If I am saying that the destroy changes the stats, I need to save
> some data about the old qdisc till I get the destroy command for it.

Agreed, maintaining a coherent destroy behavior would be problematic
when successful replace with a new qdisc (e.g. different handle) is
involved :(

Besides the momentary stats seem to be reset before destroy so not
touching them may be in fact more correct.  I need to look into the
propagation done in qdisc_tree_reduce_backlog(), it worries me.  If
we start stacking the qdiscs (e.g. red on top of prio) it could mess
with the root's backlog...