netdev - Re: [PATCH] sched: Optimize return value of qdisc

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Message-ID: <OFF9085DDC.F5960247-ON652572D7.001B794E-652572D7.001CA7E2@in.ibm.com>
Date:	Thu, 10 May 2007 10:42:59 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	hadi@...erus.ca
Cc:	David Miller <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	netdev@...r.kernel.org
Subject: Re: [PATCH] sched: Optimize return value of qdisc_restart

Hi Jamal,

J Hadi Salim <j.hadi123@...il.com> wrote on 05/09/2007 09:22:44 PM:

> > Since I am holding a lock in these two changed areas till I return
> > back to __qdisc_run (which clears the RUNNING bit) and then drop the
> > lock, there is no way packets can be on the queue while I falsely
> > return 0, or no packets on the queue while I falsely return -1.
>
> If you relinquish yourself from being a dequeuer by letting go of
> RUNNING then it is possible during that short window one of the other
> N-1 CPUs could have been enqueueing; that packet will never be dequeued
> unless a new packet shows up some X amount of time later.

But RUNNING is never relinquished till all the way back out to
__qdisc_run. Only the lock is dropped before calling xmit and
re-got after xmit is finished (all the while holding RUNNING).
After xmit both queue_lock and RUNNING are held. If some other
cpu enqueue'd during this release window (and they would have
returned after dropping the lock as they are pure enqueuer's),
qdisc_restart will find those skbs. Similarly if no skbs were
added, qdisc_restart will return 0.

Thanks,

- KK

> I hope what i described above helps. Off for about a day. CCing Herbert
> who last made changes to that area incase i missed something ..
>
> cheers,
> jamal
>
> PS:- Please dont use my temporary gmail account to respond; a reply-to
> will pick the right address (@cyberus.ca).
>

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