Current load balance scheme requires only one cpu in a
sched_group (balance_cpu) to look at other peer sched_groups for
imbalance and pull tasks towards itself from a busy cpu. Tasks
thus pulled by balance_cpu could later get picked up by cpus
that are in the same sched_group as that of balance_cpu.
This scheme however fails to pull tasks that are not allowed to
run on balance_cpu (but are allowed to run on other cpus in its
sched_group). That can affect fairness and in some worst case
scenarios cause starvation.
Consider a two core (2 threads/core) system running tasks as
below:
Core0 Core1
/ \ / \
C0 C1 C2 C3
| | | |
v v v v
F0 T1 F1 [idle]
T2
F0 = SCHED_FIFO task (pinned to C0)
F1 = SCHED_FIFO task (pinned to C2)
T1 = SCHED_OTHER task (pinned to C1)
T2 = SCHED_OTHER task (pinned to C1 and C2)
F1 could become a cpu hog, which will starve T2 unless C1 pulls
it. Between C0 and C1 however, C0 is required to look for
imbalance between cores, which will fail to pull T2 towards
Core0. T2 will starve eternally in this case. The same scenario
can arise in presence of non-rt tasks as well (say we replace F1
with high irq load).
We tackle this problem by having balance_cpu move pinned tasks
to one of its sibling cpus (where they can run). We first check
if load balance goal can be met by ignoring pinned tasks,
failing which we retry move_tasks() with a new env->dst_cpu.
This patch modifies load balance semantics on who can move load
towards a given cpu in a given sched_domain.
Before this patch, a given_cpu or a ilb_cpu acting on behalf of
an idle given_cpu is responsible for moving load to given_cpu.
With this patch applied, balance_cpu can in addition decide on
moving some load to a given_cpu.
There is a remote possibility that excess load could get moved
as a result of this (balance_cpu and given_cpu/ilb_cpu deciding
*independently* and at *same* time to move some load to a
given_cpu). However we should see less of such conflicting
decisions in practice and moreover subsequent load balance
cycles should correct the excess load moved to given_cpu.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Prashanth Nageshappa <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
[ minor edits ]
Signed-off-by: Ingo Molnar <[email protected]>
Move the code from Xen to debugfs to make the code common
for other users as well.
Accked-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Suzuki Poulose <[email protected]>
[v1: Fixed rebase issues]
[v2: Fixed PPC compile issues]
Signed-off-by: Raghavendra K T <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Handle cpu capacity being reported as 0 on cores with more number of
hardware threads. For example on a Power7 core with 4 hardware
threads, core power is 1177 and thus power of each hardware thread is
1177/4 = 294. This low power can lead to capacity for each hardware
thread being calculated as 0, which leads to tasks bouncing within the
core madly!
Fix this by reporting capacity for hardware threads as 1, provided
their power is not scaled down significantly because of frequency
scaling or real-time tasks usage of cpu.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Michael Neuling <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Arjan van de Ven <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
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: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Srivatsa Vaddagiri <[email protected]>
Cc: Mike Galbraith <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
The reason why we are getting better wakeup latencies for
!FAIR_USER_SCHED is because of this snippet of code in place_entity():
if (!initial) {
/* sleeps upto a single latency don't count. */
if (sched_feat(NEW_FAIR_SLEEPERS) && entity_is_task(se))
^^^^^^^^^^^^^^^^^^
vruntime -= sysctl_sched_latency;
/* ensure we never gain time by being placed backwards. */
vruntime = max_vruntime(se->vruntime, vruntime);
}
NEW_FAIR_SLEEPERS feature gives credit for sleeping only to tasks and
not group-level entities. With the patch attached, I could see that
wakeup latencies with FAIR_USER_SCHED are restored to the same level as
!FAIR_USER_SCHED.
Signed-off-by: Ingo Molnar <[email protected]>
The current load balancing scheme isn't good enough for precise
group fairness.
For example: on a 8-cpu system, I created 3 groups as under:
a = 8 tasks (cpu.shares = 1024)
b = 4 tasks (cpu.shares = 1024)
c = 3 tasks (cpu.shares = 1024)
a, b and c are task groups that have equal weight. We would expect each
of the groups to receive 33.33% of cpu bandwidth under a fair scheduler.
This is what I get with the latest scheduler git tree:
Signed-off-by: Ingo Molnar <[email protected]>
--------------------------------------------------------------------------------
Col1 | Col2 | Col3 | Col4
------|---------|-------|-------------------------------------------------------
a | 277.676 | 57.8% | 54.1% 54.1% 54.1% 54.2% 56.7% 62.2% 62.8% 64.5%
b | 116.108 | 24.2% | 47.4% 48.1% 48.7% 49.3%
c | 86.326 | 18.0% | 47.5% 47.9% 48.5%
--------------------------------------------------------------------------------
Explanation of o/p:
Col1 -> Group name
Col2 -> Cumulative execution time (in seconds) received by all tasks of that
group in a 60sec window across 8 cpus
Col3 -> CPU bandwidth received by the group in the 60sec window, expressed in
percentage. Col3 data is derived as:
Col3 = 100 * Col2 / (NR_CPUS * 60)
Col4 -> CPU bandwidth received by each individual task of the group.
Col4 = 100 * cpu_time_recd_by_task / 60
[I can share the test case that produces a similar o/p if reqd]
The deviation from desired group fairness is as below:
a = +24.47%
b = -9.13%
c = -15.33%
which is quite high.
After the patch below is applied, here are the results:
--------------------------------------------------------------------------------
Col1 | Col2 | Col3 | Col4
------|---------|-------|-------------------------------------------------------
a | 163.112 | 34.0% | 33.2% 33.4% 33.5% 33.5% 33.7% 34.4% 34.8% 35.3%
b | 156.220 | 32.5% | 63.3% 64.5% 66.1% 66.5%
c | 160.653 | 33.5% | 85.8% 90.6% 91.4%
--------------------------------------------------------------------------------
Deviation from desired group fairness is as below:
a = +0.67%
b = -0.83%
c = +0.17%
which is far better IMO. Most of other runs have yielded a deviation within
+-2% at the most, which is good.
Why do we see bad (group) fairness with current scheuler?
=========================================================
Currently cpu's weight is just the summation of individual task weights.
This can yield incorrect results. For ex: consider three groups as below
on a 2-cpu system:
CPU0 CPU1
---------------------------
A (10) B(5)
C(5)
---------------------------
Group A has 10 tasks, all on CPU0, Group B and C have 5 tasks each all
of which are on CPU1. Each task has the same weight (NICE_0_LOAD =
1024).
The current scheme would yield a cpu weight of 10240 (10*1024) for each cpu and
the load balancer will think both CPUs are perfectly balanced and won't
move around any tasks. This, however, would yield this bandwidth:
A = 50%
B = 25%
C = 25%
which is not the desired result.
What's changing in the patch?
=============================
- How cpu weights are calculated when CONFIF_FAIR_GROUP_SCHED is
defined (see below)
- API Change
- Two tunables introduced in sysfs (under SCHED_DEBUG) to
control the frequency at which the load balance monitor
thread runs.
The basic change made in this patch is how cpu weight (rq->load.weight) is
calculated. Its now calculated as the summation of group weights on a cpu,
rather than summation of task weights. Weight exerted by a group on a
cpu is dependent on the shares allocated to it and also the number of
tasks the group has on that cpu compared to the total number of
(runnable) tasks the group has in the system.
Let,
W(K,i) = Weight of group K on cpu i
T(K,i) = Task load present in group K's cfs_rq on cpu i
T(K) = Total task load of group K across various cpus
S(K) = Shares allocated to group K
NRCPUS = Number of online cpus in the scheduler domain to
which group K is assigned.
Then,
W(K,i) = S(K) * NRCPUS * T(K,i) / T(K)
A load balance monitor thread is created at bootup, which periodically
runs and adjusts group's weight on each cpu. To avoid its overhead, two
min/max tunables are introduced (under SCHED_DEBUG) to control the rate
at which it runs.
Fixes from: Peter Zijlstra <[email protected]>
- don't start the load_balance_monitor when there is only a single cpu.
- rename the kthread because its currently longer than TASK_COMM_LEN
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
doms_cur[] array represents various scheduling domains which are
mutually exclusive. Currently cpusets code can modify this array (by
calling partition_sched_domains()) as a result of user modifying
sched_load_balance flag for various cpusets.
This patch introduces a mutex and corresponding API (only when
CONFIG_FAIR_GROUP_SCHED is defined) which allows a reader to safely read
the doms_cur[] array w/o worrying abt concurrent modifications to the
array.
The fair group scheduler code (introduced in next patch of this series)
makes use of this mutex to walk thr' doms_cur[] array while rebalancing
shares of task groups across cpus.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
This patch changes how the cpu load exerted by fair_sched_class tasks
is calculated. Load exerted by fair_sched_class tasks on a cpu is now
a summation of the group weights, rather than summation of task weights.
Weight exerted by a group on a cpu is dependent on the shares allocated
to it.
This version of patch has a minor impact on code size, but should have
no runtime/functional impact for !CONFIG_FAIR_GROUP_SCHED.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Minor bug fixes for the group scheduler:
- Use a mutex to serialize add/remove of task groups and also when
changing shares of a task group. Use the same mutex when printing
cfs_rq debugging stats for various task groups.
- Use list_for_each_entry_rcu in for_each_leaf_cfs_rq macro (when
walking task group list)
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Commit cfb5285660 removed a useful feature for
us, which provided a cpu accounting resource controller. This feature would be
useful if someone wants to group tasks only for accounting purpose and doesnt
really want to exercise any control over their cpu consumption.
The patch below reintroduces the feature. It is based on Paul Menage's
original patch (Commit 62d0df6406), with
these differences:
- Removed load average information. I felt it needs more thought (esp
to deal with SMP and virtualized platforms) and can be added for
2.6.25 after more discussions.
- Convert group cpu usage to be nanosecond accurate (as rest of the cfs
stats are) and invoke cpuacct_charge() from the respective scheduler
classes
- Make accounting scalable on SMP systems by splitting the usage
counter to be per-cpu
- Move the code from kernel/cpu_acct.c to kernel/sched.c (since the
code is not big enough to warrant a new file and also this rightly
needs to live inside the scheduler. Also things like accessing
rq->lock while reading cpu usage becomes easier if the code lived in
kernel/sched.c)
The patch also modifies the cpu controller not to provide the same accounting
information.
Tested-by: Balbir Singh <[email protected]>
Tested the patches on top of 2.6.24-rc3. The patches work fine. Ran
some simple tests like cpuspin (spin on the cpu), ran several tasks in
the same group and timed them. Compared their time stamps with
cpuacct.usage.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Balbir Singh <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Sukadev Bhattiprolu reported a kernel crash with control groups.
There are couple of problems discovered by Suka's test:
- The test requires the cgroup filesystem to be mounted with
atleast the cpu and ns options (i.e both namespace and cpu
controllers are active in the same hierarchy).
# mkdir /dev/cpuctl
# mount -t cgroup -ocpu,ns none cpuctl
(or simply)
# mount -t cgroup none cpuctl -> Will activate all controllers
in same hierarchy.
- The test invokes clone() with CLONE_NEWNS set. This causes a a new child
to be created, also a new group (do_fork->copy_namespaces->ns_cgroup_clone->
cgroup_clone) and the child is attached to the new group (cgroup_clone->
attach_task->sched_move_task). At this point in time, the child's scheduler
related fields are uninitialized (including its on_rq field, which it has
inherited from parent). As a result sched_move_task thinks its on
runqueue, when it isn't.
As a solution to this problem, I moved sched_fork() call, which
initializes scheduler related fields on a new task, before
copy_namespaces(). I am not sure though whether moving up will
cause other side-effects. Do you see any issue?
- The second problem exposed by this test is that task_new_fair()
assumes that parent and child will be part of the same group (which
needn't be as this test shows). As a result, cfs_rq->curr can be NULL
for the child.
The solution is to test for curr pointer being NULL in
task_new_fair().
With the patch below, I could run ns_exec() fine w/o a crash.
Reported-by: Sukadev Bhattiprolu <[email protected]>
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Peter Zijlstra noticed that the rcu_head object need not be present
in every cfs_rq of a group. Move it to the task_group structure
instead.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Child task may be added on a different cpu that the one on which parent
is running. In which case, task_new_fair() should check whether the new
born task's parent entity should be added as well on the cfs_rq.
Patch below fixes the problem in task_new_fair.
This could fix the put_prev_task_fair() crashes reported.
Reported-by: Kamalesh Babulal <[email protected]>
Reported-by: Andy Whitcroft <[email protected]>
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Generate uevents when a user is being created/destroyed. These events
can be used to configure cpu share of a new user.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Dhaval Giani <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
There is a possibility that because of task of a group moving from one
cpu to another, it may gain more cpu time that desired. See
http://marc.info/?l=linux-kernel&m=119073197730334 for details.
This is an attempt to fix that problem. Basically it simulates dequeue
of higher level entities as if they are going to sleep. Similarly it
simulate wakeup of higher level entities as if they are waking up from
sleep.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
group scheduler SMP migration fix: use task_cfs_rq(p) to get
to the relevant fair-scheduling runqueue of a task, rq->cfs
is not the right one.
Signed-off-by: Ingo Molnar <[email protected]>
group scheduler wakeup latency fix: when checking for preemption
we must check cross-group too, not just intra-group.
Signed-off-by: Ingo Molnar <[email protected]>
Enable user-id based fair group scheduling. This is useful for anyone
who wants to test the group scheduler w/o having to enable
CONFIG_CGROUPS.
A separate scheduling group (i.e struct task_grp) is automatically created for
every new user added to the system. Upon uid change for a task, it is made to
move to the corresponding scheduling group.
A /proc tunable (/proc/root_user_share) is also provided to tune root
user's quota of cpu bandwidth.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Dhaval Giani <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
With the view of supporting user-id based fair scheduling (and not just
container-based fair scheduling), this patch renames several functions
and makes them independent of whether they are being used for container
or user-id based fair scheduling.
Also fix a problem reported by KAMEZAWA Hiroyuki (wrt allocating
less-sized array for tg->cfs_rq[] and tf->se[]).
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Dhaval Giani <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
- fix a minor bug in yield (seen for CONFIG_FAIR_GROUP_SCHED),
group scheduling would skew when yield was called.
Signed-off-by: Srivatsa Vaddagiri <[email protected]>
Signed-off-by: Dhaval Giani <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Srivatsa Vaddagiri