From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jakub Kicinski <kubakici@wp.pl>
Subject: Re: [patch net-next 5/5] mlxsw: spectrum: qdiscs: Support stats for
 PRIO qdisc
Date: Fri, 12 Jan 2018 01:40:09 -0800
Message-ID: <20180112014009.09524ba4@laptop>
References: <20180111102102.4310-1-jiri@resnulli.us>
        <20180111102102.4310-6-jiri@resnulli.us>
        <20180111160726.57a7ae5b@cakuba.netronome.com>
        <VI1PR05MB35201776F2FCD9F7821B1962BF170@VI1PR05MB3520.eurprd05.prod.outlook.com>
        <20180112003230.2f00e36a@laptop>
        <VI1PR05MB3520B6EB9352F60F4639B6FEBF170@VI1PR05MB3520.eurprd05.prod.outlook.com>
        <DB5PR05MB1895FDC0D523983C76B74AA5AC170@DB5PR05MB1895.eurprd05.prod.outlook.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Cc: Yuval Mintz <yuvalm@mellanox.com>, Jiri Pirko <jiri@resnulli.us>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "Ido Schimmel" <idosch@mellanox.com>, mlxsw <mlxsw@mellanox.com>,
        "jhs@mojatatu.com" <jhs@mojatatu.com>,
        "xiyou.wangcong@gmail.com" <xiyou.wangcong@gmail.com>
To: Nogah Frankel <nogahf@mellanox.com>
Return-path: <netdev-owner@vger.kernel.org>
Received: from mx4.wp.pl ([212.77.101.11]:12937 "EHLO mx4.wp.pl"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1754419AbeALJkS (ORCPT <rfc822;netdev@vger.kernel.org>);
        Fri, 12 Jan 2018 04:40:18 -0500
In-Reply-To: <DB5PR05MB1895FDC0D523983C76B74AA5AC170@DB5PR05MB1895.eurprd05.prod.outlook.com>
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

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...