[PATCH 0/2] Add statistics and document for cfs bandwidth burst

[PATCH 0/2] Add statistics and document for cfs bandwidth burst

[Huaixin Chang]

The comparison in __refill_cfs_bandwidth_runtime is further simplified.

Huaixin Chang (2):
  sched/fair: Add cfs bandwidth burst statistics
  sched/fair: Add document for burstable CFS bandwidth

 Documentation/admin-guide/cgroup-v2.rst |  8 ++++
 Documentation/scheduler/sched-bwc.rst   | 85 +++++++++++++++++++++++++++++----
 kernel/sched/core.c                     | 13 +++--
 kernel/sched/fair.c                     |  9 ++++
 kernel/sched/sched.h                    |  3 ++
 5 files changed, 105 insertions(+), 13 deletions(-)

-- 
2.14.4.44.g2045bb6

[PATCH 1/2] sched/fair: Add cfs bandwidth burst statistics

[Huaixin Chang]

Two new statistics are introduced to show the internal of burst feature
and explain why burst helps or not.

nr_bursts:  number of periods bandwidth burst occurs
burst_usec: cumulative wall-time that any cpus has
	    used above quota in respective periods

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>
Acked-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/core.c  | 13 ++++++++++---
 kernel/sched/fair.c  |  9 +++++++++
 kernel/sched/sched.h |  3 +++
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d9ff40f4661..9a286c8a1354 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10088,6 +10088,9 @@ static int cpu_cfs_stat_show(struct seq_file *sf, void *v)
 		seq_printf(sf, "wait_sum %llu\n", ws);
 	}
 
+	seq_printf(sf, "nr_bursts %d\n", cfs_b->nr_burst);
+	seq_printf(sf, "burst_usec %llu\n", cfs_b->burst_time);
+
 	return 0;
 }
 #endif /* CONFIG_CFS_BANDWIDTH */
@@ -10184,16 +10187,20 @@ static int cpu_extra_stat_show(struct seq_file *sf,
 	{
 		struct task_group *tg = css_tg(css);
 		struct cfs_bandwidth *cfs_b = &tg->cfs_bandwidth;
-		u64 throttled_usec;
+		u64 throttled_usec, burst_usec;
 
 		throttled_usec = cfs_b->throttled_time;
 		do_div(throttled_usec, NSEC_PER_USEC);
+		burst_usec = cfs_b->burst_time;
+		do_div(burst_usec, NSEC_PER_USEC);
 
 		seq_printf(sf, "nr_periods %d\n"
 			   "nr_throttled %d\n"
-			   "throttled_usec %llu\n",
+			   "throttled_usec %llu\n"
+			   "nr_bursts %d\n"
+			   "burst_usec %llu\n",
 			   cfs_b->nr_periods, cfs_b->nr_throttled,
-			   throttled_usec);
+			   throttled_usec, cfs_b->nr_burst, burst_usec);
 	}
 #endif
 	return 0;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 44c452072a1b..464371f364f1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4655,11 +4655,20 @@ static inline u64 sched_cfs_bandwidth_slice(void)
  */
 void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
 {
+	s64 runtime;
+
 	if (unlikely(cfs_b->quota == RUNTIME_INF))
 		return;
 
 	cfs_b->runtime += cfs_b->quota;
+	runtime = cfs_b->runtime_snap - cfs_b->runtime;
+	if (runtime > 0) {
+		cfs_b->burst_time += runtime;
+		cfs_b->nr_burst++;
+	}
+
 	cfs_b->runtime = min(cfs_b->runtime, cfs_b->quota + cfs_b->burst);
+	cfs_b->runtime_snap = cfs_b->runtime;
 }
 
 static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 14a41a243f7b..80e4322727b4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -367,6 +367,7 @@ struct cfs_bandwidth {
 	u64			quota;
 	u64			runtime;
 	u64			burst;
+	u64			runtime_snap;
 	s64			hierarchical_quota;
 
 	u8			idle;
@@ -379,7 +380,9 @@ struct cfs_bandwidth {
 	/* Statistics: */
 	int			nr_periods;
 	int			nr_throttled;
+	int			nr_burst;
 	u64			throttled_time;
+	u64			burst_time;
 #endif
 };
 
-- 
2.14.4.44.g2045bb6

[PATCH 2/2] sched/fair: Add document for burstable CFS bandwidth

[Huaixin Chang]

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>
Acked-by: Tejun Heo <tj@kernel.org>
---
 Documentation/admin-guide/cgroup-v2.rst |  8 ++++
 Documentation/scheduler/sched-bwc.rst   | 85 +++++++++++++++++++++++++++++----
 2 files changed, 83 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 5c7377b5bd3e..c79477089c53 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1016,6 +1016,8 @@ 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
@@ -1047,6 +1049,12 @@ 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, $QUOTA].
+
   cpu.pressure
 	A read-write nested-keyed file.
 
diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst
index 1fc73555f5c4..0b2a3b2e3369 100644
--- a/Documentation/scheduler/sched-bwc.rst
+++ b/Documentation/scheduler/sched-bwc.rst
@@ -22,39 +22,89 @@ 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
    to cgroup v1. For cgroup v2, see
    :ref:`Documentation/admin-guide/cgroup-v2.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 +124,7 @@ 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 +132,9 @@ 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_usec: Cumulative wall-time that any CPUs has used above quota in
+  respective periods
 
 This interface is read-only.
 
@@ -179,3 +232,15 @@ 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.
-- 
2.14.4.44.g2045bb6

Re: [PATCH 1/2] sched/fair: Add cfs bandwidth burst statistics

[Daniel Jordan]

On Mon, Aug 16, 2021 at 03:08:48PM +0800, Huaixin Chang wrote:
> Two new statistics are introduced to show the internal of burst feature
> and explain why burst helps or not.
> 
> nr_bursts:  number of periods bandwidth burst occurs
> burst_usec: cumulative wall-time that any cpus has
> 	    used above quota in respective periods
> 
> 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>
> Acked-by: Tejun Heo <tj@kernel.org>
> ---
>  kernel/sched/core.c  | 13 ++++++++++---
>  kernel/sched/fair.c  |  9 +++++++++
>  kernel/sched/sched.h |  3 +++
>  3 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2d9ff40f4661..9a286c8a1354 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10088,6 +10088,9 @@ static int cpu_cfs_stat_show(struct seq_file *sf, void *v)
>  		seq_printf(sf, "wait_sum %llu\n", ws);
>  	}
>  
> +	seq_printf(sf, "nr_bursts %d\n", cfs_b->nr_burst);
> +	seq_printf(sf, "burst_usec %llu\n", cfs_b->burst_time);

burst_time is nsec, not usec.

Just "burst_time" seems the way to go for consistency with
throttled_time in the same file, though "burst_usec" nicely has units
and would have the same name between cgroup1 and 2.

Re: [PATCH 1/2] sched/fair: Add cfs bandwidth burst statistics

[Tejun Heo]

On Mon, Aug 23, 2021 at 02:32:10PM -0400, Daniel Jordan wrote:
> burst_time is nsec, not usec.
> 
> Just "burst_time" seems the way to go for consistency with
> throttled_time in the same file, though "burst_usec" nicely has units
> and would have the same name between cgroup1 and 2.

The cgroup2 interface rule is that time units are in usecs. The right thing
to do is keeping the _usec postfix (to avoid confusion due to the unit
change from cgroup1) and converting it to usecs for printing.

Thanks.

-- 
tejun

Re: [PATCH 2/2] sched/fair: Add document for burstable CFS bandwidth

[Daniel Jordan]

On Mon, Aug 16, 2021 at 03:08:49PM +0800, Huaixin Chang wrote:
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 5c7377b5bd3e..c79477089c53 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1047,6 +1049,12 @@ 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, $QUOTA].

FWIW, max/$MAX are used in this file instead of quota, so it seems
clearer to stick to that.

[PATCH 2/2] sched/fair: Add document for burstable CFS bandwidth

[Huaixin Chang]

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>
---
 Documentation/admin-guide/cgroup-v2.rst |  8 ++++
 Documentation/scheduler/sched-bwc.rst   | 85 +++++++++++++++++++++++++++++----
 2 files changed, 83 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 5c7377b5bd3e..f9d749d2daf2 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1016,6 +1016,8 @@ 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
@@ -1047,6 +1049,12 @@ 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 1fc73555f5c4..05cfefdeebb2 100644
--- a/Documentation/scheduler/sched-bwc.rst
+++ b/Documentation/scheduler/sched-bwc.rst
@@ -22,39 +22,89 @@ 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
    to cgroup v1. For cgroup v2, see
    :ref:`Documentation/admin-guide/cgroup-v2.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 +124,7 @@ 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 +132,9 @@ 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 +232,15 @@ 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.
-- 
2.14.4.44.g2045bb6

[PATCH 2/2] sched/fair: Add document for burstable CFS bandwidth

[Huaixin Chang]

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>
---
 Documentation/admin-guide/cgroup-v2.rst |  8 ++++
 Documentation/scheduler/sched-bwc.rst   | 85 +++++++++++++++++++++++++++++----
 2 files changed, 83 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 5c7377b5bd3e..c79477089c53 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1016,6 +1016,8 @@ 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
@@ -1047,6 +1049,12 @@ 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, $QUOTA].
+
   cpu.pressure
 	A read-write nested-keyed file.
 
diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst
index 1fc73555f5c4..0b2a3b2e3369 100644
--- a/Documentation/scheduler/sched-bwc.rst
+++ b/Documentation/scheduler/sched-bwc.rst
@@ -22,39 +22,89 @@ 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
    to cgroup v1. For cgroup v2, see
    :ref:`Documentation/admin-guide/cgroup-v2.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 +124,7 @@ 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 +132,9 @@ 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_usec: Cumulative wall-time that any CPUs has used above quota in
+  respective periods
 
 This interface is read-only.
 
@@ -179,3 +232,15 @@ 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.
-- 
2.14.4.44.g2045bb6