On Tue, Feb 19, 2019 at 10:24 PM Hannes Reinecke <hare@xxxxxxx> wrote: > > On 2/19/19 3:19 AM, Ming Lei wrote: > > On Tue, Feb 5, 2019 at 11:30 PM Hannes Reinecke <hare@xxxxxxx> wrote: > >> > >> Hi all, > >> > >> this came up during discussion on the mailing list (cf thread "Question > >> on handling managed IRQs when hotplugging CPUs"). > >> The problem is that with managed IRQs and block-mq I/O will be routed to > >> individual CPUs, and the response will be send to the IRQ assigned to > >> that CPU. > >> > >> If now a CPU hotplug event occurs when I/O is still in-flight the IRQ > >> will _still_ be assigned to the CPU, causing any pending interrupt to be > >> lost. > >> Hence the driver will never notice that an interrupt has happened, and > >> an I/O timeout occurs. > > > > Lots of driver's timeout handler only returns BLK_EH_RESET_TIMER, > > and this situation can't be covered by IO timeout for these devices. > > > > For example, we have see IO hang issue on HPSA, megaraid_sas > > before when wrong msi vector is set on IO command. Even one such > > issue on aacraid isn't fixed yet. > > > >> > >> One proposal was to quiesce the device when a CPU hotplug event occurs, > >> and only allow for CPU hotplugging once it's fully quiesced. > > > > That is the original solution, but big problem is that queue dependency > > exists, such as loop/DM's queue depends on underlying's queue, NVMe > > IO queue depends on its admin queue. > > > >> > >> While this would be working, it will be introducing quite some system > >> stall, and it actually a rather big impact in the system. > >> Another possiblity would be to have the driver abort the requests > >> itself, but this requires specific callbacks into the driver, and, of > >> course, the driver having the ability to actually do so. > >> > >> I would like to discuss at LSF/MM how these issues can be addressed best. > > > > One related topic is that the current static queue mapping without CPU hotplug > > handler involved may waste lots of IRQ vectors[1], and how to deal > > with this problem? > > > > [1] http://lists.infradead.org/pipermail/linux-nvme/2019-January/021961.html > > > Yes, ideally I would like to touch upon that, too. > Additionally we have the issue raised by the mpt3sas folks [2], where > they ran into a CPU lockup when having more CPU cores than interrupts. In theory, if number of submission queues is bigger than number of completion queues, the softlock issue might be triggered. But in reality, that often means the irq handler takes too much CPU. I saw such NVMe devices in which queue mapping is 2:1(2 submission: 1 completion) and IOPS may reach millions without any soft lockup. Threaded irq handler may help this case too, at least for avoiding soft lockup. Thanks, Ming Lei
