linux-kernel - Re: [PATCH -mm v8 4/7] proc: add kpagecgroup file

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Message-ID: <20150717092744.GF2001@esperanza>
Date:	Fri, 17 Jul 2015 12:27:44 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Andres Lagar-Cavilla <andreslc@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jonathan Corbet <corbet@....net>, <linux-api@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v8 4/7] proc: add kpagecgroup file

On Thu, Jul 16, 2015 at 12:04:59PM -0700, Andres Lagar-Cavilla wrote:
> On Thu, Jul 16, 2015 at 2:28 AM, Vladimir Davydov <vdavydov@...allels.com>
> wrote:
> 
> > On Wed, Jul 15, 2015 at 12:03:18PM -0700, Andres Lagar-Cavilla wrote:
> > > For both /proc/kpage* interfaces you add (and more critically for the
> > > rmap-causing one, kpageidle):
> > >
> > > It's a good idea to do cond_sched(). Whether after each pfn, each Nth
> > > pfn, each put_user, I leave to you, but a reasonable cadence is
> > > needed, because user-space can call this on the entire physical
> > > address space, and that's a lot of work to do without re-scheduling.
> >
> > I really don't think it's necessary. These files can only be
> > read/written by the root, who has plenty ways to kill the system anyway.
> > The program that is allowed to read/write these files must be conscious
> > and do it in batches of reasonable size. AFAICS the same reasoning
> > already lays behind /proc/kpagecount and /proc/kpageflag, which also do
> > not thrust the "right" batch size on their readers.
> >
> 
> Beg to disagree. You're conflating intended use with system health. A
> cond_sched() is a one-liner.

I would still prefer not to clutter the code with cond_resched's, but I
don't think it's a matter worth arguing upon, so I'll prepare a patch
that makes all /proc/kapge* files issue cond_resched periodically and
leave it up to Andrew to decide if it should be applied or not.

Thanks,
Vladimir
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