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

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Date:	Thu, 06 Aug 2015 10:55:37 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Jörn Engel <joern@...estorage.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	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 Thu, 2015-07-30 at 09:58 -0700, Jörn Engel wrote:
> On Thu, Jul 30, 2015 at 05:22:55PM +0200, Mike Galbraith wrote:
> > 
> > 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().
> 
> Block softirq is one of our problems as well.  It is a bit of a joke
> that __do_softirq() moves work to ksoftirqd after 2ms, but block softirq
> can take several 100ms in bad cases.

On my little desktop box, one blk_done_softirq() loop iteration can take
up to a few milliseconds, leaving me wondering if breaking that loop
will help a studly box much.  iow, I'd like to know how bad it gets, if
one iteration can be huge, loop breaking there is fairly pointless, and
I can stop fiddling.  Do you happen to know iteration time during a size
huge block softirq hit?  On my little box, loop break/re-raise and
whatnot improves the general case substantially, but doesn't do much at
all for worst case.. or rather the next worst case in a list of unknown
length ;-)

	-Mike

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