s_i = S * rw_i / \Sum_j rw_j

 -> \Sum_j rw_j = S * rw_i / s_i

 -> s'_i = S * (rw_i + w) / (\Sum_j rw_j + w)

delta s = s' - s = S * (rw + w) / ((S * rw / s) + w)
        = s * (S * (rw + w) / (S * rw + s * w) - 1)

 a = S*(rw+w), b = S*rw + s*w

delta s = s * (a-b) / b

IOW, trade one divide for two multiplies
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Currently task_h_load() computes the load of a task and uses that to either
subtract it from the total, or add to it.

However, removing or adding a task need not have any effect on the total load
at all. Imagine adding a task to a group that is local to one cpu - in that
case the total load of that cpu is unaffected.

So properly compute addition/removal:

 s_i = S * rw_i / \Sum_j rw_j
 s'_i = S * (rw_i + wl) / (\Sum_j rw_j + wg)

then s'_i - s_i gives the change in load.

Where s_i is the shares for cpu i, S the group weight, rw_i the runqueue weight
for that cpu, wl the weight we add (subtract) and wg the weight contribution to
the runqueue.
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

doing the load balance will change cfs_rq->load.weight (that's the whole point)
but since that's part of the scale factor, we'll scale back with a different
amount.

Weight getting smaller would result in an inflated moved_load which causes
it to stop balancing too soon.
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

With hierarchical grouping we can't just compare task weight to rq weight - we
need to scale the weight appropriately.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

While thinking about the previous patch - I realized that using per domain
aggregate load values in load_balance_fair() is wrong. We should use the
load value for that CPU.

By not needing per domain hierarchical load values we don't need to store
per domain aggregate shares, which greatly simplifies all the math.

It basically falls apart in two separate computations:
 - per domain update of the shares
 - per CPU update of the hierarchical load

Also get rid of the move_group_shares() stuff - just re-compute the shares
again after a successful load balance.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

We only need to know the task_weight of the busiest rq - nothing to do
if there are no tasks there.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The idea was to balance groups until we've reached the global goal, however
Vatsa rightly pointed out that we might never reach that goal this way -
hence take out this logic.

[ the initial rationale for this 'feature' was to promote max concurrency
  within a group - it does not however affect fairness ]
Reported-by: NSrivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Keeping the aggregate on the first cpu of the sched domain has two problems:
 - it could collide between different sched domains on different cpus
 - it could slow things down because of the remote accesses
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Uncouple buddy selection from wakeup granularity.

The initial idea was that buddies could run ahead as far as a normal task
can - do this by measuring a pair 'slice' just as we do for a normal task.

This means we can drop the wakeup_granularity back to 5ms.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Try again..

Initial commit: 18d95a28
Revert: 6363ca57Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Ok, so why are we in this mess, it was:

  1/w

but now we mixed that rw in the mix like:

 rw/w

rw being \Sum w suggests: fiddling w, we should also fiddle rw, humm?
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

calc_delta_asym() is supposed to do the same as calc_delta_fair() except
linearly shrink the result for negative nice processes - this causes them
to have a smaller preemption threshold so that they are more easily preempted.

The problem is that for task groups se->load.weight is the per cpu share of
the actual task group weight; take that into account.

Also provide a debug switch to disable the asymmetry (which I still don't
like - but it does greatly benefit some workloads)

This would explain the interactivity issues reported against group scheduling.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Try again..

initial commit: 8f1bc385
revert: f9305d4aSigned-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The currently logic inadvertently skips the last task on the run-queue,
resulting in missed balance opportunities.
Signed-off-by: NGregory Haskins <ghaskins@novell.com>
Signed-off-by: NDavid Bahi <dbahi@novell.com>
CC: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

Prevent short-running wakers of short-running threads from overloading a single
cpu via wakeup affinity, and wire up disconnected debug option.
Signed-off-by: NMike Galbraith <efault@gmx.de>
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Yanmin Zhang reported:

Comparing with 2.6.25, volanoMark has big regression with kernel 2.6.26-rc1.
It's about 50% on my 8-core stoakley, 16-core tigerton, and Itanium Montecito.

With bisect, I located the following patch:

| 18d95a28 is first bad commit
| commit 18d95a28
| Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
| Date:   Sat Apr 19 19:45:00 2008 +0200
|
|     sched: fair-group: SMP-nice for group scheduling

Revert it so that we get v2.6.25 behavior.
Bisected-by: NYanmin Zhang <yanmin_zhang@linux.intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Yanmin Zhang reported:

Comparing with kernel 2.6.25, sysbench+mysql(oltp, readonly) has many
regressions with 2.6.26-rc1:

 1) 8-core stoakley: 28%;
 2) 16-core tigerton: 20%;
 3) Itanium Montvale: 50%.

Bisect located this patch:

| 8f1bc385 is first bad commit
| commit 8f1bc385
| Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
| Date:   Sat Apr 19 19:45:00 2008 +0200
|
|     sched: fair: weight calculations

Revert it to the 2.6.25 state.
Bisected-by: NYanmin Zhang <yanmin_zhang@linux.intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The conversion between virtual and real time is as follows:

  dvt = rw/w * dt <=> dt = w/rw * dvt

Since we want the fair sleeper granularity to be in real time, we actually
need to do:

  dvt = - rw/w * l

This bug could be related to the regression reported by Yanmin Zhang:

| Comparing with kernel 2.6.25, sysbench+mysql(oltp, readonly) has lots
| of regressions with 2.6.26-rc1:
|
| 1) 8-core stoakley: 28%;
| 2) 16-core tigerton: 20%;
| 3) Itanium Montvale: 50%.
Reported-by: N"Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Signed-off-by: NMike Galbraith <efault@gmx.de>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

this replaces the rq->clock stuff (and possibly cpu_clock()).

 - architectures that have an 'imperfect' hardware clock can set
   CONFIG_HAVE_UNSTABLE_SCHED_CLOCK

 - the 'jiffie' window might be superfulous when we update tick_gtod
   before the __update_sched_clock() call in sched_clock_tick()

 - cpu_clock() might be implemented as:

     sched_clock_cpu(smp_processor_id())

   if the accuracy proves good enough - how far can TSC drift in a
   single jiffie when considering the filtering and idle hooks?

[ mingo@elte.hu: various fixes and cleanups ]
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Revert debugging commit 7ba2e74a.
print_cfs_rq_tasks() can induce live-lock if a task is dequeued
during list traversal.
Signed-off-by: NMike Galbraith <efault@gmx.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

We currently use an optimization to skip the overhead of wake-idle
processing if more than one task is assigned to a run-queue.  The
assumption is that the system must already be load-balanced or we
wouldnt be overloaded to begin with.

The problem is that we are looking at rq->nr_running, which may include
RT tasks in addition to CFS tasks.  Since the presence of RT tasks
really has no bearing on the balance status of CFS tasks, this throws
the calculation off.

This patch changes the logic to only consider the number of CFS tasks
when making the decision to optimze the wake-idle.
Signed-off-by: NGregory Haskins <ghaskins@novell.com>
CC: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Noticed by sparse:
kernel/sched.c:760:20: warning: symbol 'sched_feat_names' was not declared. Should it be static?
kernel/sched.c:767:5: warning: symbol 'sched_feat_open' was not declared. Should it be static?
kernel/sched_fair.c:845:3: warning: returning void-valued expression
kernel/sched.c:4386:3: warning: returning void-valued expression
Signed-off-by: NHarvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Normalized sleeper uses calc_delta*() which requires that the rq load is
already updated, so move account_entity_enqueue() before place_entity()
Tested-by: NFrans Pop <elendil@planet.nl>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Print a tree of weights.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

In order to level the hierarchy, we need to calculate load based on the
root view. That is, each task's load is in the same unit.

             A
            / \
           B   1
          / \
         2   3

To compute 1's load we do:

	   weight(1)
	--------------
	 rq_weight(A)

To compute 2's load we do:

	  weight(2)      weight(B)
	------------ * -----------
	rq_weight(B)   rw_weight(A)

This yields load fractions in comparable units.

The consequence is that it changes virtual time. We used to have:

                time_{i}
  vtime_{i} = ------------
               weight_{i}

  vtime = \Sum vtime_{i} = time / rq_weight.

But with the new way of load calculation we get that vtime equals time.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

De-couple load-balancing from the rb-trees, so that I can change their
organization.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Currently FAIR_GROUP sched grows the scheduler latency outside of
sysctl_sched_latency, invert this so it stays within.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Implement SMP nice support for the full group hierarchy.

On each load-balance action, compile a sched_domain wide view of the full
task_group tree. We compute the domain wide view when walking down the
hierarchy, and readjust the weights when walking back up.

After collecting and readjusting the domain wide view, we try to balance the
tasks within the task_groups. The current approach is a naively balance each
task group until we've moved the targeted amount of load.

Inspired by Srivatsa Vaddsgiri's previous code and Abhishek Chandra's H-SMP
paper.

XXX: there will be some numerical issues due to the limited nature of
     SCHED_LOAD_SCALE wrt to representing a task_groups influence on the
     total weight. When the tree is deep enough, or the task weight small
     enough, we'll run out of bits.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
CC: Abhishek Chandra <chandra@cs.umn.edu>
CC: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

[rebased for sched-devel/latest]

 - Add a new cpuset file, having levels:
     sched_relax_domain_level

 - Modify partition_sched_domains() and build_sched_domains()
   to take attributes parameter passed from cpuset.

 - Fill newidle_idx for node domains which currently unused but
   might be required if sched_relax_domain_level become higher.

 - We can change the default level by boot option 'relax_domain_level='.
Signed-off-by: NHidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

This patch allows tasks and groups to exist in the same cfs_rq. With this
change the CFS group scheduling follows a 1/(M+N) model from a 1/(1+N)
fairness model where M tasks and N groups exist at the cfs_rq level.

[a.p.zijlstra@chello.nl: rt bits and assorted fixes]
Signed-off-by: NDhaval Giani <dhaval@linux.vnet.ibm.com>
Signed-off-by: NSrivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Balbir Singh reported:

> 1:mon> t
> [c0000000e7677da0] c000000000067de0 .sys_sched_yield+0x6c/0xbc
> [c0000000e7677e30] c000000000008748 syscall_exit+0x0/0x40
> --- Exception: c01 (System Call) at 00000400001d09e4
> SP (4000664cb10) is in userspace
> 1:mon> r
> cpu 0x1: Vector: 300 (Data Access) at [c0000000e7677aa0]
>     pc: c000000000068e50: .yield_task_fair+0x94/0xc4
>     lr: c000000000067de0: .sys_sched_yield+0x6c/0xbc

the check that should have avoided that is:

        /*
         * Are we the only task in the tree?
         */
        if (unlikely(rq->load.weight == curr->se.load.weight))
                return;

But I guess that overlooks rt tasks, they also increase the load.
So I guess something like this ought to fix it..
Signed-off-by: NIngo Molnar <mingo@elte.hu>

it's unused.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The wakeup buddy logic didn't use the same wakeup granularity logic as the
wakeup preemption did, this might cause the ->next buddy to be selected past
the point where we would have preempted had the task been a single running
instance.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

re-apply:

| commit e22ecef1
| Author: Ingo Molnar <mingo@elte.hu>
| Date:   Fri Mar 14 22:16:08 2008 +0100
|
|     sched: fix fair sleepers
|
|     Fair sleepers need to scale their latency target down by runqueue
|     weight. Otherwise busy systems will gain ever larger sleep bonus.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

revert "sched: fix fair sleepers" (e22ecef1),
because it is causing audio skipping, see:

   http://bugzilla.kernel.org/show_bug.cgi?id=10428

the patch is correct and the real cause of the skipping is not
understood (tracing makes it go away), but time has run out so we'll
revert it and re-try in 2.6.26.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

TREE_AVG and APPROX_AVG are initial task placement policies that have been
disabled for a long while.. time to remove them.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
CC: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

reduce wake-up granularity for better interactivity.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

improve affine wakeups. Maintain the 'overlap' metric based on CFS's
sum_exec_runtime - which means the amount of time a task executes
after it wakes up some other task.

Use the 'overlap' for the wakeup decisions: if the 'overlap' is short,
it means there's strong workload coupling between this task and the
woken up task. If the 'overlap' is large then the workload is decoupled
and the scheduler will move them to separate CPUs more easily.

( Also slightly move the preempt_check within try_to_wake_up() - this has
  no effect on functionality but allows 'early wakeups' (for still-on-rq
  tasks) to be correctly accounted as well.)
Signed-off-by: NIngo Molnar <mingo@elte.hu>