Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Thomas Gleixner]

On Wed, 8 Feb 2017, Mel Gorman wrote:
> It may be worth noting that patches in Andrew's tree no longer disable
> interrupts in the per-cpu allocator and now per-cpu draining will
> be from workqueue context. The reasoning was due to the overhead of
> the page allocator with figures included. Interrupts will bypass the
> per-cpu allocator and use the irq-safe zone->lock to allocate from
> the core.  It'll collide with the RT patch. Primary patch of interest is
> http://www.ozlabs.org/~akpm/mmots/broken-out/mm-page_alloc-only-use-per-cpu-allocator-for-irq-safe-requests.patch

Yeah, we'll sort that out once it hits Linus tree and we move RT forward.
Though I have once complaint right away:

+	preempt_enable_no_resched();

This is a nono, even in mainline. You effectively disable a preemption
point.

> The draining from workqueue context may be a problem for RT but one
> option would be to move the drain to only drain for high-order pages
> after direct reclaim combined with only draining for order-0 if
> __alloc_pages_may_oom is about to be called.

Why would the draining from workqueue context be an issue on RT?

Thanks,

	tglx