linux-kernel - Re: [PATCH v3 2/2] sched: Handle on_list ancestor in list_add_leaf_cfs

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Date:	Tue, 30 Aug 2011 15:35:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"\"Jan H." Schönherr" 
	<schnhrr@...tu-berlin.de>
Cc:	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] sched: Handle on_list ancestor in
 list_add_leaf_cfs_rq()

On Thu, 2011-08-25 at 00:40 +0200, "Jan H. Schönherr" wrote:
> Am 24.08.2011 23:32, schrieb Paul Turner:
> >>> Now I don't really like the above because its hard to make the code go
> >>> away in the !FAIR_GROUP case, but maybe we can come up with something
> >>> for that.
> >>
> >> Hmmm... you might want to reconsider my original approach to solve this:
> >> http://lkml.org/lkml/2011/7/18/86
> >>
> >> That might have been the cleanest one in this respect.
> >>
> >> Paul Turner did not like the introduced in-order removal, but the
> >> out-of-order removal is causing most problems.
> >>
> > 
> > Sorry for the delayed reply -- I owe you some feedback on the updated
> > versions but have been buried with other work.
> 
> No problem.
> 
> > What I didn't like about the original approach was specifically the
> > positional dependence on enqueue/dequeue. 
> 
> Maybe I misunderstood you, then.
> 
> If we can guarantee in-order removal of leaf_cfs_rqs, then there is
> no positional dependency. Any SE can be enqueued and dequeued anytime.
> 
> OTOH, the RCU splice variant has a positional dependence: calling
> enqueue_entity() outside of enqueue_task_fair() can go wrong easily as it
> depends on being called bottom-up and requires its caller to maintain state.
> 
> This is also partly true for the leaf_insertion_point variant: if a caller
> maintains state, then the pair enqueue_entity/enqueue_leaf_cfs_rq() also
> depends on being called bottom up.
> 
> 
> > If we can't do the splicing
> > properly then I think we want something like:
> > https://lkml.org/lkml/2011/7/18/348 to avoid shooting ourselves in the
> > future later.
> > 
> > See: https://lkml.org/lkml/2011/7/19/178 for why this should be cheap.
> 
> As far as I can tell, all three variants proposed so far work.
> 
> It is probably a matter of taste in the end. I'll happily help with
> whatever version tastes best. :)


pjt, you said you were rewriting the cgroup stuff yet-again, any
preference for which solution would least get in your way?
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