Messages in this thread | | | Date | Thu, 8 Nov 2007 09:43:53 -0600 | From | Matt Mackall <> | Subject | Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23? |
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On Thu, Nov 08, 2007 at 10:11:21AM +0100, Peter Zijlstra wrote: > On Wed, 2007-11-07 at 18:20 -0600, Matt Mackall wrote: > > > This and other cases > > (lots of per_cpu users, IIRC) actually want a migrate_disable() which > > is a proper subset. > > The disadvantage of migrate_disable() is that it complicates the > load-balancer
Hmm, I'm surprised it's more than a one-liner. Something like
if (cannot_migrate(task)) continue;
But I'm certainly not the expert here. Perhaps this one-liner introduces the "unplannable O(N) overhead" Ingo mentions in the email you referenced.
> but more importantly, that it does bring a form of latencies with it > that are hard to measure. Using preempt_disable() for these current > per-cpu users basically forces them to keep it short.
Ok, so maybe we've got implementation issues to tackle. But still: a) preempt_disable is inherently misdocumentation (preemption is not the real problem) and b) preemption is a bigger hammer than needed. At the very least, we should introduce migrate_disable as an alias for preempt_disable so we can document intent.
At any rate, in the current context, we're talking about actually disabling preempt in a *delay loop*. Let's find a fix for that.
> Also see: > http://lkml.org/lkml/2007/7/23/338
Heh, I'd completely forgotten about this thread and thought I was having an original idea! Must have never seen Ingo's followup.
Ingo's complaint about it being "a per-task BKL" is interesting. I've occassionally wondered if it makes sense to make some of these primitives take a "documentation parameter" - a pointer to an object that does nothing more than indicate the object of interest.
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