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

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Date:   Fri, 12 Jan 2018 01:40:09 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     Nogah Frankel <nogahf@...lanox.com>
Cc:     Yuval Mintz <yuvalm@...lanox.com>, Jiri Pirko <jiri@...nulli.us>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "Ido Schimmel" <idosch@...lanox.com>, mlxsw <mlxsw@...lanox.com>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
Subject: Re: [patch net-next 5/5] mlxsw: spectrum: qdiscs: Support stats for
 PRIO qdisc

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@...pl>
> > Cc: Jiri Pirko <jiri@...nulli.us>; netdev@...r.kernel.org; Nogah
> > Frankel <nogahf@...lanox.com>; davem@...emloft.net; Ido Schimmel
> > <idosch@...lanox.com>; mlxsw <mlxsw@...lanox.com>; jhs@...atatu.com;
> > xiyou.wangcong@...il.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...