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Re: ACPI/HT or Packet Scheduler BUG?

To: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Subject: Re: ACPI/HT or Packet Scheduler BUG?
From: Thomas Graf <tgraf@xxxxxxx>
Date: Sat, 16 Apr 2005 13:34:46 +0200
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>, hadi@xxxxxxxxxx, netdev <netdev@xxxxxxxxxxx>, Tarhon-Onu Victor <mituc@xxxxxxxxxxxxxx>, kuznet@xxxxxxxxxxxxx, devik@xxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Patrick McHardy <kaber@xxxxxxxxx>, "David S. Miller" <davem@xxxxxxxxxxxxx>
In-reply-to: <20050416112329.GA31847@xxxxxxxxxxxxxxxxxxx>
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* Herbert Xu <20050416112329.GA31847@xxxxxxxxxxxxxxxxxxx> 2005-04-16 21:23
> On Sat, Apr 16, 2005 at 01:06:39PM +0200, Thomas Graf wrote:
> > 
> > It's not completely useless, it speeds up the deletion classful
> > qdiscs having some depth. However, it's not worth the locking
> > troubles I guess.
> 
> RCU is meant to optimise the common reader path.  In this case
> that's the packet transmission code.  Unfortunately it fails
> miserably when judged by that criterion.

There is one case where it can do good for latency which is for
per flow qdiscs or any other scenarios implying hundreds or
thousands of leaf qdiscs where a destroyage of one such qdisc
tree will take up quite some cpu to traverse all the classes
under dev->queue_lock. I don't have any numbers on this, but
I don't completely dislike the method of hiding the qdiscs under
the lock and do the expensive traveling unlocked.

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