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The Return of Simple
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Classifieds Local Job Listings. Website powered by Foundation. It is unlikely to happen without a strong and convincing argument in its favor. In this case, the drive for simple wait queues is as is the code itself coming from the realtime project. The realtime developers seek determinism at all times, and, as it turns out, current mainline wait queues get in the way. The most problematic aspect of ordinary wait queues appears to be the ability to add custom wakeup callbacks.
But there is a mechanism provided to allow specialized users to change the wake-up behavior of wait queues: The userfaultfd code added for the 4. The exit code similarly uses a custom wake function to only wake processes that have an interest in the exiting process. It is a feature that cannot be removed.
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The problem with this feature, from the realtime developers' point of view, is that they have no control over how long the custom wake function will take to run. This feature thus makes it harder for them to provide response-time guarantees. Beyond that, these callbacks require that the wait-queue structure be protected by an ordinary spinlock, which is a sleeping lock in the realtime tree.
Simple wait queues dispense with custom callbacks and many other wait-queue features, allowing the entire data structure to be reduced to: Regular wait queues, instead, may have to search the list for specific processes to wake.
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The lack of custom wakeup callbacks means that the time required to wake any individual process on the list is known and short , so a raw spinlock can be used to protect the whole thing. This patch set has been posted by Daniel Wagner, who has taken on the challenge of getting it into the mainline, but the core wait-queue work was done by Peter Zijlstra. It has seen a few revisions in the last few months, but comments appear to be slowing down.
One never knows with such things the patches looked mostly ready in as well , but it seems like there is not much keeping this work from going into the 4. Log in to post comments The return of simple wait queues.