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SubjectRe: High contention on the sk_buff_head.lock
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On Wed, 2009-03-18 at 18:03 -0700, David Miller wrote:
> From: Gregory Haskins <ghaskins@novell.com>
> Date: Wed, 18 Mar 2009 17:54:04 -0400
>
> > Note that -rt doesnt typically context-switch under contention anymore
> > since we introduced adaptive-locks. Also note that the contention
> > against the lock is still contention, regardless of whether you have -rt
> > or not. Its just that the slow-path to handle the contended case for
> > -rt is more expensive than mainline. However, once you have the
> > contention as stated, you have already lost.
>
> First, contention is not implicitly a bad thing.
>
> Second, if the -rt kernel is doing adaptive spinning I see no
> reason why that adaptive spinning is not kicking in here to
> make this problem just go away.

The basic 'problem' with comparing RT adaptive spinning to non-rt
spinlocks is that if the lock owner is !oncpu, all spinners must break
and go to sleep, otherwise we (potentially) deadlock. This does not
exist for non-rt spinners.

Best,
-PWM


>
> This lock is held for mere cycles, just to unlink an SKB from
> the networking qdisc, and then it is immediately released.
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