On Wed, Jun 30, 2021 at 11:43:41AM +0200, Hannes Reinecke wrote: > On 6/30/21 10:42 AM, Ming Lei wrote: > > On Wed, Jun 30, 2021 at 10:18:37AM +0200, Hannes Reinecke wrote: > > > On 6/29/21 9:49 AM, Ming Lei wrote: > > > > Hi, > > > > > > > > blk_mq_alloc_request_hctx() is used by NVMe fc/rdma/tcp/loop to connect > > > > io queue. Also the sw ctx is chosen as the 1st online cpu in hctx->cpumask. > > > > However, all cpus in hctx->cpumask may be offline. > > > > > > > > This usage model isn't well supported by blk-mq which supposes allocator is > > > > always done on one online CPU in hctx->cpumask. This assumption is > > > > related with managed irq, which also requires blk-mq to drain inflight > > > > request in this hctx when the last cpu in hctx->cpumask is going to > > > > offline. > > > > > > > > However, NVMe fc/rdma/tcp/loop don't use managed irq, so we should allow > > > > them to ask for request allocation when the specified hctx is inactive > > > > (all cpus in hctx->cpumask are offline). > > > > > > > > Fix blk_mq_alloc_request_hctx() by adding/passing flag of > > > > BLK_MQ_F_NOT_USE_MANAGED_IRQ. > > > > > > > > > > > > Ming Lei (2): > > > > blk-mq: not deactivate hctx if the device doesn't use managed irq > > > > nvme: pass BLK_MQ_F_NOT_USE_MANAGED_IRQ for fc/rdma/tcp/loop > > > > > > > > block/blk-mq.c | 6 +++++- > > > > drivers/nvme/host/fc.c | 3 ++- > > > > drivers/nvme/host/rdma.c | 3 ++- > > > > drivers/nvme/host/tcp.c | 3 ++- > > > > drivers/nvme/target/loop.c | 3 ++- > > > > include/linux/blk-mq.h | 1 + > > > > 6 files changed, 14 insertions(+), 5 deletions(-) > > > > > > > > Cc: Sagi Grimberg <sagi@xxxxxxxxxxx> > > > > Cc: Daniel Wagner <dwagner@suse. thede> > > > > Cc: Wen Xiong <wenxiong@xxxxxxxxxx> > > > > Cc: John Garry <john.garry@xxxxxxxxxx> > > > > > > > > > > > I have my misgivings about this patchset. > > > To my understanding, only CPUs present in the hctx cpumask are eligible to > > > submit I/O to that hctx. > > > > It is just true for managed irq, and should be CPUs online. > > > > However, no such constraint for non managed irq, since irq may migrate to > > other online CPUs if all CPUs in irq's current affinity become offline. > > > > But there shouldn't be any I/O pending during CPU offline (cf > blk_mq_hctx_notify_offline()), so no interrupts should be triggered, either, > no? > > > > Consequently if all cpus in that mask are offline, where is the point of > > > even transmitting a 'connect' request? > > > > nvmef requires to submit the connect request via one specified hctx > > which index has to be same with the io queue's index. > > > > Almost all nvmef drivers fail to setup controller in case of > > connect io queue error. > > > > And I would prefer to fix that, namely allowing blk-mq to run on a sparse > set of io queues. > The remaining io queues can be connected once the first cpu in the hctx > cpumask is onlined; we already have blk_mq_hctx_notify_online(), which could > easily be expanded to connect to relevant I/O queue... Then you need a big patches for doing that. > > > Also CPU can become offline & online, especially it is done in > > lots of sanity test. > > > > True, but then again all I/O on the hctx should be quiesced during cpu > offline. Again that is only necessary for managed irq. > > > So we should allow to allocate the connect request successful, and > > submit it to drivers given it is allowed in this way for non-managed > > irq. > > > > I'd rather not do this, as the 'connect' command runs on the 'normal' I/O > tagset, and hence runs into the risk of being issues against non-existing > CPUs. Can you explain what the risk is? > > > > Shouldn't we rather modify the tagset to only refer to the current online > > > CPUs _only_, thereby never submit a connect request for hctx with only > > > offline CPUs? > > > > Then you may setup very less io queues, and performance may suffer even > > though lots of CPUs become online later. > > ; > Only if we stay with the reduced number of I/O queues. Which is not what I'm > proposing; I'd rather prefer to connect and disconnect queues from the cpu > hotplug handler. For starters we could even trigger a reset once the first > cpu within a hctx is onlined. Yeah, that need one big/complicated patchset, but not see any advantages over this simple approach. Thanks, Ming
