Messages in this thread | | | Date | Mon, 23 Jul 2007 20:33:19 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC] Thread Migration Preemption |
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* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> Thread Migration Preemption > > This patch adds the ability to protect critical sections from > migration to another CPU without disabling preemption. > > This will be useful to minimize the amount of preemption disabling for > the -rt patch. [...]
unfortunately this abstraction has a number of fundamental disadvantages. (and hence the -rt patch does not make use of this facility)
firstly, its effects on load-balancing are quite brutal: it disables migration to another CPU, hence disturbing SMP load-balancing. It also adds an unplannable O(N) overhead into the load-balancer.
secondly, it has similar problems as preempt_disable(): it's opaque (not attached to a data structure) and thus it does not truly map the per-CPU data structure in any explicit way. It's little more than a per-task BKL for per-CPU data structures.
so in -rt we resisted the disable-migration approach from the early days on and are using two approaches to 'map' per-cpu data structures to PREEMPT_RT: firstly the use of PER_CPU_LOCKED data structures (which attach a per-cpu sleeping lock to per_cpu() data structures) - these are relatively easy first-approach conversions. As a second approach, if performance matters, the use of true atomic data structures.
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