linux-kernel - Re: [PATCH] mm: add resched points to remap_pmd_range/ioremap_pmd

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Message-ID: <20150728133254.GI24972@dhcp22.suse.cz>
Date:	Tue, 28 Jul 2015 15:32:55 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Jörn Engel <joern@...estorage.com>
Cc:	Spencer Baugh <sbaugh@...ern.com>, Toshi Kani <toshi.kani@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Joern Engel <joern@...fs.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Shachar Raindel <raindel@...lanox.com>,
	Boaz Harrosh <boaz@...xistor.com>,
	Andy Lutomirski <luto@...capital.net>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Roman Pen <r.peniaev@...il.com>,
	Andrey Konovalov <adech.fo@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Rob Jones <rob.jones@...ethink.co.uk>,
	WANG Chao <chaowang@...hat.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	Spencer Baugh <Spencer.baugh@...estorage.com>
Subject: Re: [PATCH] mm: add resched points to
 remap_pmd_range/ioremap_pmd_range

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@...fs.org>
> > > > > 
> > > > > 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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