Date: Tue, 28 Jul 2015 15:32:55 +0200
From: Michal Hocko <mhocko@kernel.org>
To: =?iso-8859-1?Q?J=F6rn?= Engel <joern@purestorage.com>
Cc: Spencer Baugh <sbaugh@catern.com>, Toshi Kani <toshi.kani@hp.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Fengguang Wu <fengguang.wu@intel.com>, Joern Engel <joern@logfs.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mel Gorman <mgorman@suse.de>, Johannes Weiner <hannes@cmpxchg.org>,
        Shachar Raindel <raindel@mellanox.com>,
        Boaz Harrosh <boaz@plexistor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrey Ryabinin <a.ryabinin@samsung.com>,
        Roman Pen <r.peniaev@gmail.com>, Andrey Konovalov <adech.fo@gmail.com>,
        Eric Dumazet <edumazet@google.com>, Dmitry Vyukov <dvyukov@google.com>,
        Rob Jones <rob.jones@codethink.co.uk>, WANG Chao <chaowang@redhat.com>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        Spencer Baugh <Spencer.baugh@purestorage.com>
Subject: Re: [PATCH] mm: add resched points to
 remap_pmd_range/ioremap_pmd_range
Message-ID: <20150728133254.GI24972@dhcp22.suse.cz>
References: <1437688476-3399-3-git-send-email-sbaugh@catern.com>
 <20150724070420.GF4103@dhcp22.suse.cz>
 <20150724165627.GA3458@Sligo.logfs.org>
 <20150727070840.GB11317@dhcp22.suse.cz>
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On Mon 27-07-15 08:18:14, J?rn Engel wrote:
> On Mon, Jul 27, 2015 at 09:08:42AM +0200, Michal Hocko wrote:
> > On Fri 24-07-15 09:56:27, J?rn 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@logfs.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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