On Thu, May 30, 2019 at 10:31:34AM +0100, John Garry wrote: > 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? > > > > *As I understand*, splitting the tagset has issues with dual-mode HBAs - as > in supporting NVMe and SCSI, as some HBAs do. The patchset I mentioned doesn't split tagset. The patch just converts SCSI's reply queue into blk_mq hw queue, and all hw queues share the host-wide tags. You can get unique tag too. This way isn't very different with the current single hw queue(tags), that is why I think the performance shouldn't be bad compared with the current single hw queue. Meantime, drivers can get simplified. > > > 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. > > > > We are testing further, and I will tentatively say that we're getting better > results (than previously) after fixing something in the LLDD. TBC. > > > > > > > > > > > > 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, > > Yeah, the comment reads that it's going away, but it's actually gone. > > > 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? > > > > > > > > > > > > > Last time, it was reported that the patchset causes performance regression, > > > > which is actually caused by duplicated io accounting in > > > > blk_mq_queue_tag_busy_iter(), > > > > which should be fixed easily. > > > > > > > > What do you think of this approach? > > > > > > It would still be good to have a forward port of this patchset for testing, > > > if we're serious about it. Or at least this bug you mention fixed. > > > > I plan to make this patchset workable on 5.2-rc for your test first. > > > > ok, thanks. I assume that we're still open to not adding support for global > tags in blk-mq, but rather the LLDD generating the unique tag with sbitmap. Actually it is global tags. Thanks, Ming
