On Tue, Jun 25, 2019 at 02:14:46AM +0000, wenbinzeng(曾文斌) wrote: > Hi Ming, > > > -----Original Message----- > > From: Ming Lei <ming....@redhat.com> > > Sent: Tuesday, June 25, 2019 9:55 AM > > To: Wenbin Zeng <wenbin.z...@gmail.com> > > Cc: ax...@kernel.dk; keith.bu...@intel.com; h...@suse.com; osan...@fb.com; > > s...@grimberg.me; bvanass...@acm.org; linux-block@vger.kernel.org; > > linux-ker...@vger.kernel.org; wenbinzeng(曾文斌) <wenbinz...@tencent.com> > > Subject: Re: [PATCH] blk-mq: update hctx->cpumask at cpu-hotplug(Internet > > mail) > > > > On Mon, Jun 24, 2019 at 11:24:07PM +0800, Wenbin Zeng wrote: > > > Currently hctx->cpumask is not updated when hot-plugging new cpus, > > > as there are many chances kblockd_mod_delayed_work_on() getting > > > called with WORK_CPU_UNBOUND, workqueue blk_mq_run_work_fn may run > > > > There are only two cases in which WORK_CPU_UNBOUND is applied: > > > > 1) single hw queue > > > > 2) multiple hw queue, and all CPUs in this hctx become offline > > > > For 1), all CPUs can be found in hctx->cpumask. > > > > > on the newly-plugged cpus, consequently __blk_mq_run_hw_queue() > > > reporting excessive "run queue from wrong CPU" messages because > > > cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) returns false. > > > > The message means CPU hotplug race is triggered. > > > > Yeah, there is big problem in blk_mq_hctx_notify_dead() which is called > > after one CPU is dead, but still run this hw queue to dispatch request, > > and all CPUs in this hctx might become offline. > > > > We have some discussion before on this issue: > > > > https://lore.kernel.org/linux-block/CACVXFVN729SgFQGUgmu1iN7P6Mv5+puE78STz8hj > > 9j5bs82...@mail.gmail.com/ > > > > There is another scenario, you can reproduce it by hot-plugging cpus to kvm > guests via qemu monitor (I believe virsh setvcpus --live can do the same > thing), for example: > (qemu) cpu-add 1 > (qemu) cpu-add 2 > (qemu) cpu-add 3 > > In such scenario, cpu 1, 2 and 3 are not visible at boot, hctx->cpumask > doesn't get synced when these cpus are added.
It is CPU cold-plug, we suppose to support it. The new added CPUs should be visible to hctx, since we spread queues among all possible CPUs(), please see blk_mq_map_queues() and irq_build_affinity_masks(), which is like static allocation on CPU resources. Otherwise, you might use an old kernel or there is bug somewhere. > > > > > > > This patch added a cpu-hotplug handler into blk-mq, updating > > > hctx->cpumask at cpu-hotplug. > > > > This way isn't correct, hctx->cpumask should be kept as sync with > > queue mapping. > > Please advise what should I do to deal with the above situation? Thanks a lot. As I shared in last email, there is one approach discussed, which seems doable. Thanks, Ming
