On Wed, Feb 08, 2017 at 01:02:07PM +0100, Thomas Gleixner wrote:

On Wed, 8 Feb 2017, Michal Hocko wrote:

quoted

On Tue 07-02-17 23:25:17, Thomas Gleixner wrote:

quoted

On Tue, 7 Feb 2017, Christoph Lameter wrote:

quoted

On Tue, 7 Feb 2017, Michal Hocko wrote:

quoted

I am always nervous when seeing hotplug locks being used in low level
code. It has bitten us several times already and those deadlocks are
quite hard to spot when reviewing the code and very rare to hit so they
tend to live for a long time.

Yep. Hotplug events are pretty significant. Using stop_machine_XXXX() etc
would be advisable and that would avoid the taking of locks and get rid of all the
ocmplexity, reduce the code size and make the overall system much more
reliable.

Huch? stop_machine() is horrible and heavy weight. Don't go there, there
must be simpler solutions than that.

Absolutely agreed. We are in the page allocator path so using the
stop_machine* is just ridiculous. And, in fact, there is a much simpler
solution [1]

[1] http://lkml.kernel.org/r/20170207201950.20482-1-mhocko@kernel.org

Well, yes. It's simple, but from an RT point of view I really don't like
it as we have to fix it up again.

On RT we solved the problem of the page allocator differently which allows
us to do drain_all_pages() from the caller CPU as a side effect. That's
interesting not only for RT, it's also interesting for NOHZ FULL scenarios
because you don't inflict the work on the other CPUs.

https://git.kernel.org/cgit/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-4.9.y-rt-rebase&id=d577a017da694e29a06af057c517f2a7051eb305

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

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.

-- 
Mel Gorman
SUSE Labs

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