On Thu, May 30, 2019 at 10:28 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > On Wed, May 29, 2019 at 05:10:38PM +0100, John Garry wrote: > > > > > > > > > > And we should be careful to handle the multiple reply queue case, given the queue > > > > shouldn't be stopped or quieseced because other reply queues are still active. > > > > > > > > The new CPUHP state for blk-mq should be invoked after the to-be-offline > > > > CPU is quiesced and before it becomes offline. > > > > > > Hi John, > > > > > > > Hi Ming, > > > > > Thinking of this issue further, so far, one doable solution is to > > > expose reply queues > > > as blk-mq hw queues, as done by the following patchset: > > > > > > https://lore.kernel.org/linux-block/20180205152035.15016-1-ming.lei@xxxxxxxxxx/ > > > > I thought that this patchset had fundamental issues, in terms of working for > > all types of hosts. FYI, I did the backport of latest hisi_sas_v3 to v4.15 > > Could you explain it a bit about the fundamental issues for all types of > host? > > It is just for hosts with multiple reply queues, such as hisi_sas v3, > megaraid_sas, mpt3sas and hpsa. > > > with this patchset (as you may have noticed in my git send mistake), but we > > have not got to test it yet. > > > > On a related topic, we did test exposing reply queues as blk-mq hw queues > > and generating the host-wide tag internally in the LLDD with sbitmap, and > > unfortunately we were experiencing a significant performance hit, like 2300K > > -> 1800K IOPs for 4K read. > > > > We need to test this further. I don't understand why we get such a big hit. > > The performance regression shouldn't have been introduced in theory, and it is > because blk_mq_queue_tag_busy_iter() iterates over the same duplicated tags multiple > times, which can be fixed easily. > > > > > > > > > In which global host-wide tags are shared for all blk-mq hw queues. > > > > > > Also we can remove all the reply_map stuff in drivers, then solve the problem of > > > draining in-flight requests during unplugging CPU in a generic approach. > > > > So you're saying that removing this reply queue stuff can make the solution > > to the problem more generic, but do you have an idea of the overall > > solution? > > 1) convert reply queue into blk-mq hw queue first > > 2) then all drivers are in same position wrt. handling requests vs. > unplugging CPU (shutdown managed IRQ) > > The current handling in blk_mq_hctx_notify_dead() is actually wrong, > at that time, all CPUs on the hctx are dead, blk_mq_run_hw_queue() > still dispatches requests on driver's hw queue, and driver is invisible > to DEAD CPUs mapped to this hctx, and finally interrupt for these > requests on the hctx are lost. > > Frankly speaking, the above 2nd problem is still hard to solve. > > 1) take_cpu_down() shutdown managed IRQ first, then run teardown callback > for states in [CPUHP_AP_ONLINE, CPUHP_AP_OFFLINE) on the to-be-offline > CPU > > 2) However, all runnable tasks are removed from the CPU in the teardown > callback for CPUHP_AP_SCHED_STARTING, which is run after managed IRQs > are shutdown. That said it is hard to avoid new request queued to > the hctx with all DEAD CPUs. > > 3) we don't support to freeze queue for specific hctx yet, or that way > may not be accepted because of extra cost in fast path > > 4) once request is allocated, it should be submitted to driver no matter > if CPU hotplug happens or not. Or free it and re-allocate new request > on proper sw/hw queue? That looks doable, we may steal bios from the old in-queue request, then re-submit them via generic_make_request(), and finally free the old request, but RQF_DONTPREP has to be addressed via one new callback. So follows the overall solution for waiting request vs. CPU hotplug, which is done in two stages: 1) in the teardown callback of new CPUHP state of CPUHP_BLK_MQ_PREP, which is run before CPUHP_AP_ONLINE_IDLE, at that time the CPU & managed IRQ is still alive: - stopped the hctx - wait in-flight requests from this hctx until all are completed 2) in the teardown callback of CPUHP_BLK_MQ_DEAD, which is run after the CPU is dead - dequeue request queued in sw queue or scheduler queue from this hctx - steal bios from the dequeued request, and re-submit them via generic_make_request() - free the dequeued request, and need to free driver resource via new callback for RQF_DONTPREP, looks only SCSI needs it. - restart this hctx Thanks, Ming Lei
