linux-kernel - Re: [PATCH 3/4] plist: add plist

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Date:	Tue, 6 May 2014 16:39:50 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Weijie Yang <weijieut@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/4] plist: add plist_rotate

On Tue, 6 May 2014 16:12:54 -0400
Dan Streetman <ddstreet@...e.org> wrote:

> On Mon, May 5, 2014 at 10:18 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Fri,  2 May 2014 15:02:29 -0400
> > Dan Streetman <ddstreet@...e.org> wrote:
> >
> >> Add plist_rotate(), which moves the specified plist_node after
> >> all other same-priority plist_nodes in the list.
> >
> > This is a little confusing? You mean it takes a plist_node from a plist
> > and simply moves it to the end of the list of all other nodes of the
> > same priority?
> 
> yes, exactly
> 
> > Kind of like what a sched_yield() would do with a
> > SCHED_FIFO task? I wonder if we should call this "plist_yield()" then?
> 
> I suppose it is similar, yes...I'll rename it in a v2 patch.

I'm open to other suggestions as well. What else can give you the idea
that it's putting a node at the end of its priority?

I added Peter to the Cc list because I know how much he loves
sched_yield() :-)

> 
> >
> >>
> >> This is needed by swap, which has a plist of swap_info_structs
> >> and needs to use each same-priority swap_info_struct equally.
> >
> > "needs to use each same-priority swap_info_struct equally"
> >
> > -ENOCOMPUTE
> 
> heh, yeah that needs a bit more explaining doesn't it :-)
> 
> by "equally", I mean as swap writes pages out to its swap devices, it
> must write to any same-priority devices on a round-robin basis.

OK, I think you are suffering from "being too involved to explain
clearly" syndrome. :)

I still don't see the connection between swap pages and plist, and even
more so, why something would already be in a plist and then needs to be
pushed to the end of its priority.

> 
> I'll update the comment in the v2 patch to try to explain clearer.
> 

Please do. But explain it to someone that has no idea how plists are
used by the swap subsystem, and why you need to move a node to the end
of its priority.

Thanks,

-- Steve
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