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Date: Tue, 6 May 2014 16:39:50 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Dan Streetman <ddstreet@ieee.org>
Cc: Hugh Dickins <hughd@google.com>, Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@suse.cz>,
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        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 3/4] plist: add plist_rotate
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On Tue, 6 May 2014 16:12:54 -0400
Dan Streetman <ddstreet@ieee.org> wrote:

> On Mon, May 5, 2014 at 10:18 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Fri,  2 May 2014 15:02:29 -0400
> > Dan Streetman <ddstreet@ieee.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