[PATCH v4 4/4] sched/fair: Add document for burstable CFS bandwidth control

Huaixin Chang
2021-03-16T04:51:03Z

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>
Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com>
---
Documentation/admin-guide/cgroup-v2.rst | 16 +++++----
Documentation/scheduler/sched-bwc.rst | 64 ++++++++++++++++++++++++++++++---
2 files changed, 69 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 64c62b979f2f..17ec571ab4a8 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -997,6 +997,8 @@ All time durations are in microseconds.
- nr_periods
- nr_throttled
- throttled_usec
+ - nr_burst
+ - burst_usec

cpu.weight
A read-write single value file which exists on non-root
@@ -1017,16 +1019,18 @@ All time durations are in microseconds.
the closest approximation of the current weight.

cpu.max
- A read-write two value file which exists on non-root cgroups.
- The default is "max 100000".
+ A read-write three value file which exists on non-root cgroups.
+ The default is "max 100000 0".

The maximum bandwidth limit. It's in the following format::

- $MAX $PERIOD
+ $MAX $PERIOD $BURST

- which indicates that the group may consume upto $MAX in each
- $PERIOD duration. "max" for $MAX indicates no limit. If only
- one number is written, $MAX is updated.
+ which indicates that the group may consume upto $MAX from this
+ period plus $BURST carried over from previous periods in each
+ $PERIOD duration. "max" for $MAX indicates no limit. "0" for
+ $BURST indicates no bandwidth can be carried over. On partial
+ writing, values are updated accordingly.

cpu.pressure
A read-write nested-keyed file.
diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst
index 845eee659199..42e0773c0eed 100644
--- a/Documentation/scheduler/sched-bwc.rst
+++ b/Documentation/scheduler/sched-bwc.rst
@@ -22,24 +22,51 @@ 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".

+By default, CPU bandwidth consumption is strictly limited to quota within each
+given period. For the sequence of CPU usage u_i served under CFS bandwidth
+control, if for any j <= k N(j,k) is the number of periods from u_j to u_k:
+
+ u_j+...+u_k <= quota * N(j,k)
+
+For a bursty sequence among which interval u_j...u_k are at the peak, CPU
+requests might have to wait for more periods to replenish enough quota.
+Otherwise, larger quota is required.
+
+With "burst" buffer, CPU requests might be served as long as:
+
+ u_j+...+u_k <= B_j + quota * N(j,k)
+
+if for any j <= k N(j,k) is the number of periods from u_j to u_k and B_j is
+the accumulated quota from previous periods in burst buffer serving u_j.
+Burst buffer helps in that serving whole bursty CPU requests without throttling
+them can be done with moderate quota setting and accumulated quota in burst
+buffer, if:
+
+ u_0+...+u_n <= B_0 + quota * N(0,n)
+
+where B_0 is the initial state of burst buffer. The maximum accumulated quota in
+the burst buffer is capped by burst. With proper burst setting, the available
+bandwidth is still determined by quota and period on the long run.
+
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
to cgroup v1. For cgroup v2, see
: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
@@ -55,6 +82,11 @@ 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) 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.

@@ -72,9 +104,15 @@ This is tunable via procfs::
Larger slice values will reduce transfer overheads, while smaller values allow
for more fine-grained consumption.

+There is also a global switch to turn off burst for all groups::
+ /proc/sys/kernel/sched_cfs_bw_burst_enabled (default=1)
+
+By default it is enabled. Writing a 0 value means no accumulated CPU time can be
+used for any group, even if cpu.cfs_burst_us is configured.
+
Statistics
----------
-A group's bandwidth statistics are exported via 3 fields in cpu.stat.
+A group's bandwidth statistics are exported via 6 fields in cpu.stat.

cpu.stat:

@@ -82,6 +120,10 @@ 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_burst: Number of periods burst occurs.
+- burst_time: Cumulative wall-time that any CPUs has used above quota in
+ respective periods
+

This interface is read-only.

@@ -179,3 +221,15 @@ 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 20% of 1 CPU, and allow accumulate up to 60% of 1 CPU
+ additionally, in case accumulation has been done.
+
+ With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU.
+ And 30ms burst will be equivalent to 60% of 1 CPU.
+
+ # echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
+ # echo 50000 > cpu.cfs_period_us /* period = 50ms */
+ # echo 30000 > cpu.cfs_burst_us /* burst = 30ms */
+
+ Larger buffer setting allows greater burst capacity.
--
2.14.4.44.g2045bb6