linux-kernel - Re: [PATCH 0/4] Convert khugepaged to a task

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Message-ID: <20141030002333.GA28052@node.dhcp.inet.fi>
Date:	Thu, 30 Oct 2014 02:23:33 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Alex Thorlton <athorlton@....com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bob Liu <lliubbo@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Hugh Dickins <hughd@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] Convert khugepaged to a task_work function

On Wed, Oct 29, 2014 at 04:58:39PM -0500, Alex Thorlton wrote:
> On Tue, Oct 28, 2014 at 05:12:26AM -0700, Andi Kleen wrote:
> > Alex Thorlton <athorlton@....com> writes:
> > 
> > > Last week, while discussing possible fixes for some unexpected/unwanted behavior
> > > from khugepaged (see: https://lkml.org/lkml/2014/10/8/515) several people
> > > mentioned possibly changing changing khugepaged to work as a task_work function
> > > instead of a kernel thread.  This will give us finer grained control over the
> > > page collapse scans, eliminate some unnecessary scans since tasks that are
> > > relatively inactive will not be scanned often, and eliminate the unwanted
> > > behavior described in the email thread I mentioned.
> > 
> > With your change, what would happen in a single threaded case?
> > 
> > Previously one core would scan and another would run the workload.
> > With your change both scanning and running would be on the same
> > core.
> > 
> > Would seem like a step backwards to me.
> 
> I suppose from the single-threaded point of view, it could be.  Maybe we
> could look at this a bit differently.  What if we allow processes to
> choose their collapse mechanism on fork?

Yet another knob nobody uses? Let's just do it right.

-- 
 Kirill A. Shutemov
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