RE: [PATCH] blk-mq: update hctx->cpumask at cpu-hotplug(Internet mail)

["wenbinzeng(曾文斌)" <wenbinzeng@tencent.com>]

Hi Ming,

> -----Original Message-----
> From: Ming Lei <ming.lei@redhat.com>
> Sent: Tuesday, June 25, 2019 9:55 AM
> To: Wenbin Zeng <wenbin.zeng@gmail.com>
> Cc: axboe@kernel.dk; keith.busch@intel.com; hare@suse.com; osandov@fb.com;
> sagi@grimberg.me; bvanassche@acm.org; linux-block@vger.kernel.org;
> linux-kernel@vger.kernel.org; wenbinzeng(曾文斌) <wenbinzeng@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
> 9J5bS828Ng@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.

> >
> > 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.

> 
> Thanks,
> Ming