linux-kernel - Re: mm: deadlock between get_online_cpus/pcpu

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Date:   Wed, 8 Feb 2017 14:23:19 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mel Gorman <mgorman@...hsingularity.net>
cc:     Michal Hocko <mhocko@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc

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