netdev - Re: [RFC] NET_SCHED: restore HZ value to /proc/net/psched

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Date:	Mon, 23 Jun 2008 21:36:08 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] NET_SCHED: restore HZ value to /proc/net/psched

Stephen Hemminger wrote:
> Some changes back in  2.6.22 broke a number of
> other places where the iproute2 utilities depended on kernel HZ
> value. The use of HZ was a bad original ABI design choice; the
> question is how to fix the impacts.
> 
> During 2.6.22 development, the psched clock was converted over
> to use high resolution timers, and one of the changes was to the
> return value of /proc/net/psched. This file is used by utilities
> to determine conversion between time and jiffies.
> It would be better to go back to the original format
> output where the fourth field was the kernel HZ.
> 
> The motivation for the change was to make the HTB burst size
> default smaller because the kernel clock resolution was better.  
> But the change broke the use of jiffies by dst_metrics.
> 
> With this revert all the metric RTT values will work again.
> The only use of hz in htb is to set the buffer and ceiling buffer default,
> and having the old value would cause a slightly larger value. Note: the comment
> in q_htb is incorrect, the value calculated is the default not the
> minimum.  As an example if HTB rate is 1mbit, then the default buffer
> would be come 11500 (with 100 HZ) vs 1500 (with 2.6.25).
> 
> This effectively reverts commit 4361cb17f0df5491fe6e2c3ae1defc98e9a64a79
> I am not 100% convinced this is the best or only solution.

Me neither. As you've mentioned yourself, the fact that
the interfaces use kernel HZ as unit is a bug, and the use
of /proc/net/psched is another bug (one that isn't present
for *that* long I should add, the iproute git history
unfortunately only contains one huge patch which includes
this change).

IMO qdiscs shouldn't be punished for bugs in other code
and the main problem would still remain. Two alternative
approaches:

- just fix the interface (something we've done multiple
   times, even after 2.6 was released).

- add a new proc file to get the value of kernel HZ (the
   fix needs a kernel change anyway, this would additionally
   require a new iproute version).

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