This patchset restarts the work about adding a latency priority to describe the latency tolerance of cfs tasks. Patch [1] is a new one that has been added with v6. It fixes an unfairness for low prio tasks because of wakeup_gran() being bigger than the maximum vruntime credit that a waking task can keep after sleeping. The patches [2-4] have been done by Parth: https://lore.kernel.org/lkml/20200228090755.22829-1-parth@xxxxxxxxxxxxx/ I have just rebased and moved the set of latency priority outside the priority update. I have removed the reviewed tag because the patches are 2 years old. This aims to be a generic interface and the following patches is one use of it to improve the scheduling latency of cfs tasks. Patch [5] uses latency nice priority to define a latency offset and then decide if a cfs task can or should preempt the current running task. The patch gives some tests results with cyclictests and hackbench to highlight the benefit of latency priority for short interactive task or long intensive tasks. Patch [6] adds the support of latency nice priority to task group by adding a cpu.latency.nice field. The range is [-20:19] as for setting task latency priority. Patch [7] makes sched_core taking into account the latency offset. Patch [8] adds a rb tree to cover some corner cases where the latency sensitive task (priority < 0) is preempted by high priority task (RT/DL) or fails to preempt them. This patch ensures that tasks will have at least a slice of sched_min_granularity in priority at wakeup. I have also backported the patchset on a dragonboard RB3 with an android mainline kernel based on v5.18 for a quick test. I have used the TouchLatency app which is part of AOSP and described to be a very good test to highlight jitter and jank frame sources of a system [1]. In addition to the app, I have added some short running tasks waking-up regularly (to use the 8 cpus for 4 ms every 37777us) to stress the system without overloading it (and disabling EAS). The 1st results shows that the patchset helps to reduce the missed deadline frames from 5% to less than 0.1% when the cpu.latency.nice of task group are set. I haven't rerun the test with latest version. I have also tested the patchset with the modified version of the alsa latency test that has been shared by Tim. The test quickly xruns with default latency nice priority 0 but is able to run without underuns with a latency -20 and hackbench running simultaneously. While preparing the version 8, I have evaluated the benefit of using an augmented rbtree instead of adding a rbtree for latency sensitive entities, which was a relevant suggestion done by PeterZ. Although the augmented rbtree enables to sort additional information in the tree with a limited overhead, it has more impact on legacy use cases (latency_nice >= 0) because the augmented callbacks are always called to maintain this additional information even when there is no sensitive tasks. In such cases, the dedicated rbtree remains empty and the overhead is reduced to loading a cached null node pointer. Nevertheless, we might want to reconsider the augmented rbtree once the use of negative latency_nice will be more widlely deployed. At now, the different tests that I have done, have not shown improvements with augmented rbtree. Below are some hackbench results (from v10): 2 rbtrees augmented rbtree augmented rbtree sorted by vruntime sorted by wakeup_vruntime sched pipe avg 26311,000 25976,667 25839,556 stdev 0,15 % 0,28 % 0,24 % vs tip 0,50 % -0,78 % -1,31 % hackbench 1 group avg 1,315 1,344 1,359 stdev 0,88 % 1,55 % 1,82 % vs tip -0,47 % -2,68 % -3,87 % hackbench 4 groups avg 1,339 1,365 1,367 stdev 2,39 % 2,26 % 3,58 % vs tip -0,08 % -2,01 % -2,22 % hackbench 8 groups avg 1,233 1,286 1,301 stdev 0,74 % 1,09 % 1,52 % vs tip 0,29 % -4,05 % -5,27 % hackbench 16 groups avg 1,268 1,313 1,319 stdev 0,85 % 1,60 % 0,68 % vs tip -0,02 % -3,56 % -4,01 % [1] https://source.android.com/docs/core/debug/eval_perf#touchlatency Change since v11: - init latency_node of task group entity Change since v10: - remove sched_latency_to_weight array and use a calc_latency_offset() instead - save latency_prio instead for task group instead of latency offset - enqueue back an entity when changing the latency nice prio fo a task group Change since v9: - Rebase - add tags Change since v8: - Rename get_sched_latency by get_sleep_latency - move latency nice defines in sched/prio.h and fix latency_prio init value - Fix typo and comments Change since v7: - Replaced se->on_latency by using RB_CLEAR_NODE() and RB_EMPTY_NODE() - Clarify the limit behavior fo the cgroup cpu.latenyc_nice Change since v6: - Fix compilation error for !CONFIG_SCHED_DEBUG Change since v5: - Add patch 1 to fix unfairness for low prio task. This has been discovered while studying Youssef's tests results with latency nice which were hitting the same problem. - Fixed latency_offset computation to take into account GENTLE_FAIR_SLEEPERS. This has diseappeared with v2and has been raised by Youssef's tests. - Reworked and optimized how latency_offset in used to check for preempting current task at wakeup and tick. This cover more cases too. - Add patch 9 to remove check_preempt_from_others() which is not needed anymore with the rb tree. Change since v4: - Removed permission checks to set latency priority. This enables user without elevated privilege like audio application to set their latency priority as requested by Tim. - Removed cpu.latency and replaced it by cpu.latency.nice so we keep a generic interface not tied to latency_offset which can be used to implement other latency features. - Added an entry in Documentation/admin-guide/cgroup-v2.rst to describe cpu.latency.nice. - Fix some typos. Change since v3: - Fix 2 compilation warnings raised by kernel test robot <lkp@xxxxxxxxx> Change since v2: - Set a latency_offset field instead of saving a weight and computing it on the fly. - Make latency_offset available for task group: cpu.latency - Fix some corner cases to make latency sensitive tasks schedule first and add a rb tree for latency sensitive task. Change since v1: - fix typo - move some codes in the right patch to make bisect happy - simplify and fixed how the weight is computed - added support of sched core patch 7 Parth Shah (3): sched: Introduce latency-nice as a per-task attribute sched/core: Propagate parent task's latency requirements to the child task sched: Allow sched_{get,set}attr to change latency_nice of the task Vincent Guittot (5): sched/fair: fix unfairness at wakeup sched/fair: Take into account latency priority at wakeup sched/fair: Add sched group latency support sched/core: Support latency priority with sched core sched/fair: Add latency list Documentation/admin-guide/cgroup-v2.rst | 10 ++ include/linux/sched.h | 4 + include/linux/sched/prio.h | 27 +++ include/uapi/linux/sched.h | 4 +- include/uapi/linux/sched/types.h | 19 ++ init/init_task.c | 1 + kernel/sched/core.c | 65 +++++++ kernel/sched/debug.c | 1 + kernel/sched/fair.c | 222 ++++++++++++++++++++---- kernel/sched/sched.h | 50 +++++- tools/include/uapi/linux/sched.h | 4 +- 11 files changed, 371 insertions(+), 36 deletions(-) -- 2.34.1
