BFQ: the purpose of idle rb-tree in bfq_service_tree

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Hi Paolo,

I am reading the code of BFQ scheduler and having a question about the purpose
of idle rb-tree in bfq_service_tree.

>From the comment in code, the idle rb-tree is used to keep the bfq_queue which
doesn't have any request and has a finish time greater than the vtime of the
service tree.

The only function that I can find and reveals the purpose of the idle rb-tree is
__bfq_activate_entity(). In this function, when the activated queue had been on
the idle tree, the start time of the queue will be reset to the greater value
between min_vstart and finish time. It seems to me that the idle rb-tree is used
to delay the schedule of the queue which issues requests periodically, but I don't
known why the delay is needed. Could you please explain the purpose of the idle
rb-tree in bf_service_tree and its main scenario ?

Thanks,
Tao




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