On 6/25/19 10:27 AM, Ming Lei wrote: > On Tue, Jun 25, 2019 at 02:14:46AM +0000, wenbinzeng(曾文斌) wrote: >> Hi Ming, >> >>> -----Original Message----- >>> From: Ming Lei <ming.lei@xxxxxxxxxx> >>> Sent: Tuesday, June 25, 2019 9:55 AM >>> To: Wenbin Zeng <wenbin.zeng@xxxxxxxxx> >>> Cc: axboe@xxxxxxxxx; keith.busch@xxxxxxxxx; hare@xxxxxxxx; osandov@xxxxxx; >>> sagi@xxxxxxxxxxx; bvanassche@xxxxxxx; linux-block@xxxxxxxxxxxxxxx; >>> linux-kernel@xxxxxxxxxxxxxxx; wenbinzeng(曾文斌) <wenbinzeng@xxxxxxxxxxx> >>> 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 >>> 9J5bS828Ng@xxxxxxxxxxxxxx/ >>> >> >> 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. Here is how I play with it with the most recent qemu and linux. Boot VM with 1 out of 4 vcpu online. # qemu-system-x86_64 -hda disk.img \ -smp 1,maxcpus=4 \ -m 4096M -enable-kvm \ -device nvme,drive=lightnvme,serial=deadbeaf1 \ -drive file=nvme.img,if=none,id=lightnvme \ -vnc :0 \ -kernel /.../mainline-linux/arch/x86_64/boot/bzImage \ -append "root=/dev/sda1 init=/sbin/init text" \ -monitor stdio -net nic -net user,hostfwd=tcp::5022-:22 As Ming mentioned, after boot: # cat /proc/cpuinfo | grep processor processor : 0 # cat /sys/block/nvme0n1/mq/0/cpu_list 0 # cat /sys/block/nvme0n1/mq/1/cpu_list 1 # cat /sys/block/nvme0n1/mq/2/cpu_list 2 # cat /sys/block/nvme0n1/mq/3/cpu_list 3 # cat /proc/interrupts | grep nvme 24: 11 PCI-MSI 65536-edge nvme0q0 25: 78 PCI-MSI 65537-edge nvme0q1 26: 0 PCI-MSI 65538-edge nvme0q2 27: 0 PCI-MSI 65539-edge nvme0q3 28: 0 PCI-MSI 65540-edge nvme0q4 I hotplug with "device_add qemu64-x86_64-cpu,id=core1,socket-id=1,core-id=0,thread-id=0" in qemu monitor. Dongli Zhang > > 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 >
