Re: [PATCH v4 8/9] blk-mq: use hk cpus only when isolcpus=managed_irq is enabled

Daniel Wagner
2025-01-10T09:23:26Z

Hi Ming,

On Fri, Dec 20, 2024 at 04:54:21PM +0800, Ming Lei wrote:
> On Thu, Dec 19, 2024 at 04:38:43PM +0100, Daniel Wagner wrote:
>
> > When isolcpus=managed_irq is enabled all hardware queues should run on
> > the housekeeping CPUs only. Thus ignore the affinity mask provided by
> > the driver.
>
> Compared with in-tree code, the above words are misleading.
>
> - irq core code respects isolated CPUs by trying to exclude isolated
> CPUs from effective masks
>
> - blk-mq won't schedule blockd on isolated CPUs

I see your point, the commit should highlight the fact when an
application is issuing an I/O, this can lead to stalls.

What about a commit message like:

When isolcpus=managed_irq is enabled, and the last housekeeping CPU for
a given hardware context goes offline, there is no CPU left which
handles the IOs anymore. If isolated CPUs mapped to this hardware
context are online and an application running on these isolated CPUs
issue an IO this will lead to stalls.

The kernel will not schedule IO to isolated CPUS thus this avoids IO
stalls.

Thus issue a warning when housekeeping CPUs are offlined for a hardware
context while there are still isolated CPUs online.

> If application aren't run on isolated CPUs, IO interrupt usually won't
> be triggered on isolated CPUs, so isolated CPUs are _not_ ignored.

FWIW, the 'usually' part is what made our customers nervous. They saw
some IRQ noise on the isolated CPUs with their workload and reported
with these changes all was good. Unfortunately, we never got the hands
on the workload, hard to say what was causing it.

> > On Thu, Dec 19, 2024 at 05:20:44PM +0800, Ming Lei wrote:
> > > > + cpumask_andnot(isol_mask,
> > > > + cpu_possible_mask,
> > > > + housekeeping_cpumask(HK_TYPE_MANAGED_IRQ));
> > > > +
> > > > + for_each_cpu(cpu, isol_mask) {
> > > > + qmap->mq_map[cpu] = qmap->queue_offset + queue;
> > > > + queue = (queue + 1) % qmap->nr_queues;
> > > > + }
> > >
> > > Looks the IO hang issue in V3 isn't addressed yet, is it?
> > >
> > > https://lore.kernel.org/linux-block/ZrtX4pzqwVUEgIPS@fedora/
> >
> > I've added an explanation in the cover letter why this is not
> > addressed. From the cover letter:
> >
> > I've experimented for a while and all solutions I came up were horrible
> > hacks (the hotpath needs to be touched) and I don't want to slow down all
> > other users (which are almost everyone). IMO, it's just not worth trying
>
> IMO, this patchset is one improvement on existed best-effort approach, which
> works fine most of times, so why you do think it slows down everyone?

I was talking about implementing the feature which would remap the
isolated CPUs to online hardware context when the current hardware
context goes offline. I didn't find a solution which I think would be
worth presenting. All involved some sort of locking/refcounting in the
hotpath, which I think we should just avoid.

> > to fix this corner case. If the user is using isolcpus and does CPU
> > hotplug, we can expect that the user can also first offline the isolated
> > CPUs. I've discussed this topic during ALPSS and the room came to the
> > same conclusion. Thus I just added a patch which issues a warning that
> > IOs are likely to hang.
>
> If the change need userspace cooperation for using 'managed_irq', the exact
> behavior need to be documented in both this commit and Documentation/admin-guide/kernel-parameters.txt,
> instead of cover-letter only.
>
> But this patch does cause regression for old applications which can't
> follow the new introduced rule:
>
> ```
> If the user is using isolcpus and does CPU hotplug, we can expect that the
> user can also first offline the isolated CPUs.
> ```

Indeed, I forgot to update the documentation. I'll update it accordingly.

Thanks,
Daniel