Re: [PATCH] mm: add resched points to remap_pmd_range/ioremap_pmd_range

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Thu, 2015-07-30 at 09:58 -0700, JA?rn Engel wrote:
> On Thu, Jul 30, 2015 at 05:22:55PM +0200, Mike Galbraith wrote:
> > 
> > I piddled about with the thought that it might be nice to be able to
> > sprinkle cond_resched() about to cut rt latencies without wrecking
> > normal load throughput, cobbled together a cond_resched_rt().
> > 
> > On my little box that was a waste of time, as the biggest hits are block
> > softirq and free_hot_cold_page_list().
> 
> Block softirq is one of our problems as well.  It is a bit of a joke
> that __do_softirq() moves work to ksoftirqd after 2ms, but block softirq
> can take several 100ms in bad cases.
> 
> We could give individual softirqs a time budget.  If they exceed the
> budget they should complete, but reassert themselves.  Not sure about
> the rest, but that would be pretty simple to implement for block
> softirq.

Yeah, it wants something, not sure what though.  Fix up every spot that
hinders rt performance, you'll end up with PREEMPT_RT, and generic
performance falls straight through the floor.  Darn.

	-Mike

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