sched: group scheduler, fix fairness of cpu bandwidth allocation for task groups
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: NIngo Molnar <mingo@elte.hu> -------------------------------------------------------------------------------- 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 <a.p.zijlstra@chello.nl> - 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: NSrivatsa Vaddagiri <vatsa@linux.vnet.ibm.com> Signed-off-by: NIngo Molnar <mingo@elte.hu>
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