On Wed, Feb 08, 2017 at 01:02:07PM +0100, Thomas Gleixner wrote: > On Wed, 8 Feb 2017, Michal Hocko wrote: > > On Tue 07-02-17 23:25:17, Thomas Gleixner wrote: > > > On Tue, 7 Feb 2017, Christoph Lameter wrote: > > > > On Tue, 7 Feb 2017, Michal Hocko wrote: > > > > > > > > > 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@xxxxxxxxxx > > 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
