Date: Wed, 29 Jul 2015 11:54:40 +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
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On Tue 28-07-15 10:08:44, J?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.

-- 
Michal Hocko
SUSE Labs
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