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

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On Wed, 2015-07-29 at 11:54 +0200, Michal Hocko wrote:
> On Tue 28-07-15 10:08:44, Jörn 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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[PATCH] mm: add resched points to remap_pmd_range/ioremap_pmd_range Spencer Baugh <sbaugh@catern.com> - 2015-07-24 00:00 +0200
  Re: [PATCH] mm: add resched points to  remap_pmd_range/ioremap_pmd_range Toshi Kani <toshi.kani@hp.com> - 2015-07-24 01:40 +0200
  Re: [PATCH] mm: add resched points to  remap_pmd_range/ioremap_pmd_range Michal Hocko <mhocko@kernel.org> - 2015-07-29 12:00 +0200
    Re: [PATCH] mm: add resched points to  remap_pmd_range/ioremap_pmd_range Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-07-30 17:30 +0200
      Re: [PATCH] mm: add resched points to  remap_pmd_range/ioremap_pmd_range Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-07-30 21:00 +0200

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