Messages in this thread | | | Date | Thu, 25 Jun 2015 16:07:19 +0800 | Subject | Re: [PATCH 3/4] blk-mq: establish new mapping before cpu starts handling requests | From | Ming Lei <> |
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On Thu, Jun 25, 2015 at 10:56 AM, Akinobu Mita <akinobu.mita@gmail.com> wrote: > 2015-06-25 1:24 GMT+09:00 Ming Lei <tom.leiming@gmail.com>: >> On Wed, Jun 24, 2015 at 10:34 PM, Akinobu Mita <akinobu.mita@gmail.com> wrote: >>> Hi Ming, >>> >>> 2015-06-24 18:46 GMT+09:00 Ming Lei <tom.leiming@gmail.com>: >>>> On Sun, Jun 21, 2015 at 9:52 PM, Akinobu Mita <akinobu.mita@gmail.com> wrote: >>>>> ctx->index_hw is zero for the CPUs which have never been onlined since >>>>> the block queue was initialized. If one of those CPUs is hotadded and >>>>> starts handling request before new mappings are established, pending >>>> >>>> Could you explain a bit what the handling request is? The fact is that >>>> blk_mq_queue_reinit() is run after all queues are put into freezing. >>> >>> Notifier callbacks for CPU_ONLINE action can be run on the other CPU >>> than the CPU which was just onlined. So it is possible for the >>> process running on the just onlined CPU to insert request and run >>> hw queue before blk_mq_queue_reinit_notify() is actually called with >>> action=CPU_ONLINE. >> >> You are right because blk_mq_queue_reinit_notify() is alwasy run after >> the CPU becomes UP, so there is a tiny window in which the CPU is up >> but the mapping is updated. Per current design, the CPU just onlined >> is still mapped to hw queue 0 until the mapping is updated by >> blk_mq_queue_reinit_notify(). >> >> But I am wondering why it is a problem and why you think flush_busy_ctxs >> can't find the requests on the software queue in this situation? > > The problem happens when the CPU has just been onlined first time > since the request queue was initialized. At this time ctx->index_hw > for the CPU is still zero before blk_mq_queue_reinit_notify is called. > > The request can be inserted to ctx->rq_list, but blk_mq_hctx_mark_pending() > marks busy for wrong bit position as ctx->index_hw is zero.
It isn't wrong bit since the CPU onlined just is still mapped to hctx 0 at that time .
> > flush_busy_ctxs() only retrieves the requests from software queues > which are marked busy. So the request just inserted is ignored as > the corresponding bit position is not busy.
Before making the remap in blk_mq_queue_reinit() for the CPU topo change, the request queue will be put into freezing first and all requests inserted to hctx 0 should be retrieved and scheduled out. So can the request be igonred by flush_busy_ctxs()?
-- Ming Lei
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