linux-kernel - Re: [RFC][PATCH 0/6] Add group fairness to CFS

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Message-ID: <20070612055024.GA26957@in.ibm.com>
Date:	Tue, 12 Jun 2007 11:20:24 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, efault@....de,
	kernel@...ivas.org, containers@...ts.osdl.org,
	ckrm-tech@...ts.sourceforge.net, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, pwil3058@...pond.net.au,
	tingy@...umass.edu, tong.n.li@...el.com, wli@...omorphy.com,
	linux-kernel@...r.kernel.org, dmitry.adamushko@...il.com,
	balbir@...ibm.com, Kirill Korotaev <dev@...ru>
Subject: Re: [RFC][PATCH 0/6] Add group fairness to CFS - v1

On Mon, Jun 11, 2007 at 09:37:35PM +0200, Ingo Molnar wrote:
> > Patches 1-3 introduce the essential changes in CFS core to support 
> > this concept. They rework existing code w/o any (intended!) change in 
> > functionality.
> 
> i currently have these 3 patches applied to the CFS queue and it's 
> looking pretty good so far! If it continues to be problem-free i'll 
> release them as part of -v17, just to check that they truly have no bad 
> side-effects (they shouldnt). Then #4 can go into -v18.

ok. I am also most concerned about not upsetting current performance of
CFS when CONFIG_FAIR_GROUP_SCHED is turned off. Staging these patches in
incremental versions of CFS is a good idea to test that.

> i've attached my current -v17 tree - it should apply mostly cleanly 
> ontop of the -mm queue (with a minor number of fixups). Could you 
> refactor the remaining 3 patches ontop of this base? There's some 
> rejects in the last 3 patches due to the update_load_fair() change.

sure, i will rework them on this -v17 snapshot.

> > Patch 4 fixes some bad interaction between SCHED_RT and SCHED_NORMAL
> > tasks in current CFS.
> 
> btw., the plan here is to turn off 'bit 0' in sched_features: i.e. to 
> use the precise statistics to calculate lrq->cpu_load[], not the 
> timer-irq-sampled imprecise statistics. Dmitry has fixed a couple of 
> bugs in it that made it not work too well in previous CFS versions, but 
> now we are ready to turn it on for -v17. (indeed in my tree it's already 
> turned on - i.e. sched_features defaults to '14')

On Mon, Jun 11, 2007 at 09:39:31PM +0200, Ingo Molnar wrote:
> i mean bit 6, value 64. I flipped around its meaning in -v17-rc4, so the
> new precise stats code there is now default-enabled - making SMP
> load-balancing more accurate.

I must be missing something here. AFAICS, cpu_load calculation still is
timer-interrupt driven in the -v17 snapshot you sent me. Besides, there
is no change in default value of bit 6 b/n v16 and v17:

-unsigned int sysctl_sched_features __read_mostly = 1 | 2 | 4 | 8 | 0 | 0;
+unsigned int sysctl_sched_features __read_mostly = 0 | 2 | 4 | 8 | 0 | 0;

So where's this precise stats based calculation of cpu_load?

Anyway, do you agree that splitting the cpu_load/nr_running fields so that:

rq->nr_running 	   	  = total count of -all- tasks in runqueue
rq->raw_weighted_load	  = total weight of -all- tasks in runqueue
rq->lrq.nr_running 	  = total count of SCHED_NORMAL/BATCH tasks in runqueue
rq->lrq.raw_weighted_load = total weight of SCHED_NORMAL/BATCH tasks in runqueue

is a good thing to avoid SCHED_RT<->SCHED_NORMAL/BATCH mixup (as accomplished 
in Patch #4)?

If you don't agree, then I will make this split dependent on
CONFIG_FAIR_GROUP_SCHED 

> > Patch 5 introduces basic changes in CFS core to support group 
> > fairness.
> >
> > Patch 6 hooks up scheduler with container patches in mm (as an 
> > interface for task-grouping functionality).

Just to be clear, by container patches, I am referring to "process" container
patches from Paul Menage [1]. They aren't necessarily tied to
"virtualization-related" container support in -mm tree, although I
believe that "virtualization-related" container patches will make use of the 
same "process-related" container patches for their task-grouping requirements. 
Phew ..we need better names!

> ok. Kirill, how do you like Srivatsa's current approach? Would be nice 
> to kill two birds with the same stone, if possible :-)

One thing the current patches don't support is the notion of virtual
cpus (which Kirill and other "virtualization-related" container folks would 
perhaps want). IMHO, the current patches can still be usefull for
containers to load balance between those virtual cpus (as and when it is 
introduced).

> you'll get the best hackbench results by using SCHED_BATCH:
> 
>    chrt -b 0 ./hackbench 10

thanks for this tip. Will try out and let you know how it fares for me.

> or indeed increasing the runtime_limit would work too.


References:

1.  https://lists.linux-foundation.org/pipermail/containers/2007-May/005261.html

-- 
Regards,
vatsa
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