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Subject[tip: sched/core] sched/fair: Add document for burstable CFS bandwidth
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 9de47777ee77c2b9bedf43ba92900b199cf1457e
Gitweb: https://git.kernel.org/tip/9de47777ee77c2b9bedf43ba92900b199cf1457e
Author: Huaixin Chang <changhuaixin@linux.alibaba.com>
AuthorDate: Mon, 30 Aug 2021 11:22:15 +08:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 09 Sep 2021 11:27:32 +02:00

sched/fair: Add document for burstable CFS bandwidth

Basic description of usage and effect for CFS Bandwidth Control Burst.

Co-developed-by: Shanpei Chen <shanpeic@linux.alibaba.com>
Signed-off-by: Shanpei Chen <shanpeic@linux.alibaba.com>
Co-developed-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20210830032215.16302-3-changhuaixin@linux.alibaba.com
---
Documentation/admin-guide/cgroup-v2.rst | 8 ++-
Documentation/scheduler/sched-bwc.rst | 84 +++++++++++++++++++++---
2 files changed, 83 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index b1e81aa..4cee0f3 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1000,6 +1000,8 @@ All time durations are in microseconds.
- nr_periods
- nr_throttled
- throttled_usec
+ - nr_bursts
+ - burst_usec

cpu.weight
A read-write single value file which exists on non-root
@@ -1031,6 +1033,12 @@ All time durations are in microseconds.
$PERIOD duration. "max" for $MAX indicates no limit. If only
one number is written, $MAX is updated.

+ cpu.max.burst
+ A read-write single value file which exists on non-root
+ cgroups. The default is "0".
+
+ The burst in the range [0, $MAX].
+
cpu.pressure
A read-write nested-keyed file.

diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst
index 845eee6..023b366 100644
--- a/Documentation/scheduler/sched-bwc.rst
+++ b/Documentation/scheduler/sched-bwc.rst
@@ -22,9 +22,52 @@ cfs_quota units at each period boundary. As threads consume this bandwidth it
is transferred to cpu-local "silos" on a demand basis. The amount transferred
within each of these updates is tunable and described as the "slice".

+Burst feature
+-------------
+This feature borrows time now against our future underrun, at the cost of
+increased interference against the other system users. All nicely bounded.
+
+Traditional (UP-EDF) bandwidth control is something like:
+
+ (U = \Sum u_i) <= 1
+
+This guaranteeds both that every deadline is met and that the system is
+stable. After all, if U were > 1, then for every second of walltime,
+we'd have to run more than a second of program time, and obviously miss
+our deadline, but the next deadline will be further out still, there is
+never time to catch up, unbounded fail.
+
+The burst feature observes that a workload doesn't always executes the full
+quota; this enables one to describe u_i as a statistical distribution.
+
+For example, have u_i = {x,e}_i, where x is the p(95) and x+e p(100)
+(the traditional WCET). This effectively allows u to be smaller,
+increasing the efficiency (we can pack more tasks in the system), but at
+the cost of missing deadlines when all the odds line up. However, it
+does maintain stability, since every overrun must be paired with an
+underrun as long as our x is above the average.
+
+That is, suppose we have 2 tasks, both specify a p(95) value, then we
+have a p(95)*p(95) = 90.25% chance both tasks are within their quota and
+everything is good. At the same time we have a p(5)p(5) = 0.25% chance
+both tasks will exceed their quota at the same time (guaranteed deadline
+fail). Somewhere in between there's a threshold where one exceeds and
+the other doesn't underrun enough to compensate; this depends on the
+specific CDFs.
+
+At the same time, we can say that the worst case deadline miss, will be
+\Sum e_i; that is, there is a bounded tardiness (under the assumption
+that x+e is indeed WCET).
+
+The interferenece when using burst is valued by the possibilities for
+missing the deadline and the average WCET. Test results showed that when
+there many cgroups or CPU is under utilized, the interference is
+limited. More details are shown in:
+https://lore.kernel.org/lkml/5371BD36-55AE-4F71-B9D7-B86DC32E3D2B@linux.alibaba.com/
+
Management
----------
-Quota and period are managed within the cpu subsystem via cgroupfs.
+Quota, period and burst are managed within the cpu subsystem via cgroupfs.

.. note::
The cgroupfs files described in this section are only applicable
@@ -32,29 +75,37 @@ Quota and period are managed within the cpu subsystem via cgroupfs.
:ref:`Documentation/admin-guide/cgroupv2.rst <cgroup-v2-cpu>`.

- cpu.cfs_quota_us: the total available run-time within a period (in
- microseconds)
+- cpu.cfs_quota_us: run-time replenished within a period (in microseconds)
- cpu.cfs_period_us: the length of a period (in microseconds)
- cpu.stat: exports throttling statistics [explained further below]
+- cpu.cfs_burst_us: the maximum accumulated run-time (in microseconds)

The default values are::

cpu.cfs_period_us=100ms
- cpu.cfs_quota=-1
+ cpu.cfs_quota_us=-1
+ cpu.cfs_burst_us=0

A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
bandwidth restriction in place, such a group is described as an unconstrained
bandwidth group. This represents the traditional work-conserving behavior for
CFS.

-Writing any (valid) positive value(s) will enact the specified bandwidth limit.
-The minimum quota allowed for the quota or period is 1ms. There is also an
-upper bound on the period length of 1s. Additional restrictions exist when
-bandwidth limits are used in a hierarchical fashion, these are explained in
-more detail below.
+Writing any (valid) positive value(s) no smaller than cpu.cfs_burst_us will
+enact the specified bandwidth limit. The minimum quota allowed for the quota or
+period is 1ms. There is also an upper bound on the period length of 1s.
+Additional restrictions exist when bandwidth limits are used in a hierarchical
+fashion, these are explained in more detail below.

Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
and return the group to an unconstrained state once more.

+A value of 0 for cpu.cfs_burst_us indicates that the group can not accumulate
+any unused bandwidth. It makes the traditional bandwidth control behavior for
+CFS unchanged. Writing any (valid) positive value(s) no larger than
+cpu.cfs_quota_us into cpu.cfs_burst_us will enact the cap on unused bandwidth
+accumulation.
+
Any updates to a group's bandwidth specification will result in it becoming
unthrottled if it is in a constrained state.

@@ -74,7 +125,7 @@ for more fine-grained consumption.

Statistics
----------
-A group's bandwidth statistics are exported via 3 fields in cpu.stat.
+A group's bandwidth statistics are exported via 5 fields in cpu.stat.

cpu.stat:

@@ -82,6 +133,9 @@ cpu.stat:
- nr_throttled: Number of times the group has been throttled/limited.
- throttled_time: The total time duration (in nanoseconds) for which entities
of the group have been throttled.
+- nr_bursts: Number of periods burst occurs.
+- burst_time: Cumulative wall-time (in nanoseconds) that any CPUs has used
+ above quota in respective periods

This interface is read-only.

@@ -179,3 +233,15 @@ Examples

By using a small period here we are ensuring a consistent latency
response at the expense of burst capacity.
+
+4. Limit a group to 40% of 1 CPU, and allow accumulate up to 20% of 1 CPU
+ additionally, in case accumulation has been done.
+
+ With 50ms period, 20ms quota will be equivalent to 40% of 1 CPU.
+ And 10ms burst will be equivalent to 20% of 1 CPU.
+
+ # echo 20000 > cpu.cfs_quota_us /* quota = 20ms */
+ # echo 50000 > cpu.cfs_period_us /* period = 50ms */
+ # echo 10000 > cpu.cfs_burst_us /* burst = 10ms */
+
+ Larger buffer setting (no larger than quota) allows greater burst capacity.
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