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

[Michal Hocko]

On Mon 27-07-15 08:18:14, Jörn Engel wrote:
> On Mon, Jul 27, 2015 at 09:08:42AM +0200, Michal Hocko wrote:
> > On Fri 24-07-15 09:56:27, Jörn Engel wrote:
> > > On Fri, Jul 24, 2015 at 09:04:21AM +0200, Michal Hocko wrote:
> > > > On Thu 23-07-15 14:54:33, Spencer Baugh wrote:
> > > > > From: Joern Engel <joern@xxxxxxxxx>
> > > > > 
> > > > > Mapping large memory spaces can be slow and prevent high-priority
> > > > > realtime threads from preempting lower-priority threads for a long time.
> > > > 
> > > > How can a lower priority task block the high priority one? Do you have
> > > > preemption disabled?
> > > 
> > > Yes.
> 
> 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.
-- 
Michal Hocko
SUSE Labs
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