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

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Date:	Thu, 30 Jul 2015 17:22:55 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Jörn Engel <joern@...estorage.com>,
	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 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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