netdev - RE: [PATCH v20 net-next 1/6] sched: Struct definition and parsing of dualpi2 qdisc

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Message-ID:
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Date: Mon, 30 Jun 2025 15:51:11 +0000
From: "Chia-Yu Chang (Nokia)" <chia-yu.chang@...ia-bell-labs.com>
To: Jakub Kicinski <kuba@...nel.org>
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Subject: RE: [PATCH v20 net-next 1/6] sched: Struct definition and parsing of
 dualpi2 qdisc

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org> 
> Sent: Monday, June 30, 2025 5:14 PM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@...ia-bell-labs.com>
> Cc: alok.a.tiwari@...cle.com; pctammela@...atatu.com; horms@...nel.org; donald.hunter@...il.com; xandfury@...il.com; netdev@...r.kernel.org; dave.taht@...il.com; pabeni@...hat.com; jhs@...atatu.com; stephen@...workplumber.org; xiyou.wangcong@...il.com; jiri@...nulli.us; davem@...emloft.net; edumazet@...gle.com; andrew+netdev@...n.ch; ast@...erby.net; liuhangbin@...il.com; shuah@...nel.org; linux-kselftest@...r.kernel.org; ij@...nel.org; ncardwell@...gle.com; Koen De Schepper (Nokia) <koen.de_schepper@...ia-bell-labs.com>; g.white@...lelabs.com; ingemar.s.johansson@...csson.com; mirja.kuehlewind@...csson.com; cheshire@...le.com; rs.ietf@....at; Jason_Livingood@...cast.com; vidhi_goel@...le.com
> Subject: Re: [PATCH v20 net-next 1/6] sched: Struct definition and parsing of dualpi2 qdisc
> 
> 
> CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
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> 
> 
> On Sun, 29 Jun 2025 22:49:24 +0000 Chia-Yu Chang (Nokia) wrote:
> > > I don't get the reason for all these WRITE_ONCE()s.
> > > You lock the qdisc to make modifications, right?
> > > And the block under which I'm responding is performing two dependent writes, one to ->step_in_packets and the other to ->step_thresh a change which is definitely not atomic..
> >
> > Thanks again for other comments again, and I will take actions in the next version.
> >
> > As there is only one step marking in L-queue, so we still need two 
> > WRITE_ONCE even two attributes (one for threshold in packets and one 
> > in time) are used.
> >
> > When applying the step marking, we need to know either the computation 
> > is based on the sojourn time or queue length.
> 
> Let me ask again - why do you use WRITE_ONCE() at all if the modification takes the qdisc lock? Which reader are you afraid of racing with?

This still needs 2 WRITE_ONCE even "step_thresh" (NLA_U32) and "step_in_packtes" (NLA_FLAG) are replaced with "step_pkt_thresh" (NLA_U32) and "step_time_thresh" (NLA_U32) - which was proposed in my another email.

Because we need to know which kind of AQM is applied, either based on # of packets or sojourn time in the L-queue. 

And the corresponding reader will be at the dualpi2_dump.