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    SubjectRe: [RFC] Thread Migration Preemption

    * Mathieu Desnoyers <> 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

    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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      Last update: 2007-07-23 20:37    [W:0.024 / U:0.812 seconds]
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