Re: [PATCH v3 15/15] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Ming Lei <ming.lei@redhat.com>]

On Wed, Aug 07, 2024 at 02:40:11PM +0200, Daniel Wagner wrote:
> On Tue, Aug 06, 2024 at 10:55:09PM GMT, Ming Lei wrote:
> > On Tue, Aug 06, 2024 at 02:06:47PM +0200, Daniel Wagner wrote:
> > > When isolcpus=io_queue is enabled all hardware queues should run on the
> > > housekeeping CPUs only. Thus ignore the affinity mask provided by the
> > > driver. Also we can't use blk_mq_map_queues because it will map all CPUs
> > > to first hctx unless, the CPU is the same as the hctx has the affinity
> > > set to, e.g. 8 CPUs with isolcpus=io_queue,2-3,6-7 config
> > 
> > What is the expected behavior if someone still tries to submit IO on isolated
> > CPUs?
> 
> If a user thread is issuing an IO the IO is handled by the housekeeping
> CPU, which will cause some noise on the submitting CPU. As far I was
> told this is acceptable. Our customers really don't want to have any
> IO not from their application ever hitting the isolcpus. When their
> application is issuing an IO.
> 
> > BTW, I don't see any change in blk_mq_get_ctx()/blk_mq_map_queue() in this
> > patchset,
> 
> I was trying to figure out what you tried to explain last time with
> hangs, but didn't really understand what the conditions are for this
> problem to occur.

Isolated CPUs are removed from queue mapping in this patchset, when someone
submit IOs from the isolated CPU, what is the correct hctx used for handling
these IOs?

From current implementation, it depends on implied zero filled
tag_set->map[type].mq_map[isolated_cpu], so hctx 0 is used.

During CPU offline, in blk_mq_hctx_notify_offline(),
blk_mq_hctx_has_online_cpu() returns true even though the last cpu in
hctx 0 is offline because isolated cpus join hctx 0 unexpectedly, so IOs in
hctx 0 won't be drained.

However managed irq core code still shutdowns the hw queue's irq because all
CPUs in this hctx are offline now. Then IO hang is triggered, isn't it?

The current blk-mq takes static & global queue/CPUs mapping, in which all CPUs
are covered. This patchset removes isolated CPUs from the mapping, and the
change is big from viewpoint of blk-mq queue mapping.

> 
> > that means one random hctx(or even NULL) may be used for submitting
> > IO from isolated CPUs,
> > then there can be io hang risk during cpu hotplug, or
> > kernel panic when submitting bio.
> 
> Can you elaborate a bit more? I must miss something important here.
> 
> Anyway, my understanding is that when the last CPU of a hctx goes
> offline the affinity is broken and assigned to an online HK CPU. And we
> ensure all flight IO have finished and also ensure we don't submit any
> new IO to a CPU which goes offline.
> 
> FWIW, I tried really hard to get an IO hang with cpu hotplug.
 
Please see above.


thanks,
Ming