From d73df887b6b8174dfbb7f5f878fbd1e0e2eb3f08 Mon Sep 17 00:00:00 2001 From: Huaixin Chang Date: Mon, 30 Aug 2021 11:22:15 +0800 Subject: sched/fair: Add document for burstable CFS bandwidth Basic description of usage and effect for CFS Bandwidth Control Burst. Co-developed-by: Shanpei Chen Signed-off-by: Shanpei Chen Co-developed-by: Tianchen Ding Signed-off-by: Tianchen Ding Signed-off-by: Huaixin Chang Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Daniel Jordan Acked-by: Tejun Heo Link: https://lore.kernel.org/r/20210830032215.16302-3-changhuaixin@linux.alibaba.com --- Documentation/scheduler/sched-bwc.rst | 84 +++++++++++++++++++++++++++++++---- 1 file changed, 75 insertions(+), 9 deletions(-) (limited to 'Documentation/scheduler') diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst index 1fc73555f5c4..173c14110c85 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/cgroup-v2.rst `. - 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. -- cgit v1.2.3