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

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Wed, 2015-07-29 at 11:54 +0200, Michal Hocko wrote:
> On Tue 28-07-15 10:08:44, JA?rn Engel wrote:
> > On Tue, Jul 28, 2015 at 03:32:55PM +0200, Michal Hocko wrote:
> > > > 
> > > > We have kernel preemption disabled.  A lower-priority task in a system
> > > > call will block higher-priority tasks.
> > > 
> > > This is an inherent problem of !PREEMPT, though. There are many
> > > loops which can take quite some time but we do not want to sprinkle
> > > cond_resched all over the kernel. On the other hand these io/remap resp.
> > > vunmap page table walks do not have any cond_resched points AFAICS so we
> > > can at least mimic zap_pmd_range which does cond_resched.
> > 
> > Even for !PREEMPT we don't want infinite scheduler latencies.  Real
> > question is how much we are willing to accept and at what point we
> > should start sprinkling cond_resched.  I would pick 100ms, but that is
> > just a personal choice.  If we decide on 200ms or 500ms, I can live with
> > that too.
> 
> I do not thing this is about a magic value. It is more about natural
> places for scheduling point. As I've written above cond_resched at pmd
> level of the page table walk sounds reasonable to me as we do that
> already for zap_pmd_range and consistency would make sense to me.

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().

	-Mike

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