Qais Yousef 13685c4a08 sched/uclamp: Add a new sysctl to control RT default boost value ... RT tasks by default run at the highest capacity/performance level. When uclamp is selected this default behavior is retained by enforcing the requested uclamp.min (p->uclamp_req[UCLAMP_MIN]) of the RT tasks to be uclamp_none(UCLAMP_MAX), which is SCHED_CAPACITY_SCALE; the maximum value. This is also referred to as 'the default boost value of RT tasks'. See commit 1a00d99997 ("sched/uclamp: Set default clamps for RT tasks"). On battery powered devices, it is desired to control this default (currently hardcoded) behavior at runtime to reduce energy consumed by RT tasks. For example, a mobile device manufacturer where big.LITTLE architecture is dominant, the performance of the little cores varies across SoCs, and on high end ones the big cores could be too power hungry. Given the diversity of SoCs, the new knob allows manufactures to tune the best performance/power for RT tasks for the particular hardware they run on. They could opt to further tune the value when the user selects a different power saving mode or when the device is actively charging. The runtime aspect of it further helps in creating a single kernel image that can be run on multiple devices that require different tuning. Keep in mind that a lot of RT tasks in the system are created by the kernel. On Android for instance I can see over 50 RT tasks, only a handful of which created by the Android framework. To control the default behavior globally by system admins and device integrator, introduce the new sysctl_sched_uclamp_util_min_rt_default to change the default boost value of the RT tasks. I anticipate this to be mostly in the form of modifying the init script of a particular device. To avoid polluting the fast path with unnecessary code, the approach taken is to synchronously do the update by traversing all the existing tasks in the system. This could race with a concurrent fork(), which is dealt with by introducing sched_post_fork() function which will ensure the racy fork will get the right update applied. Tested on Juno-r2 in combination with the RT capacity awareness [1]. By default an RT task will go to the highest capacity CPU and run at the maximum frequency, which is particularly energy inefficient on high end mobile devices because the biggest core[s] are 'huge' and power hungry. With this patch the RT task can be controlled to run anywhere by default, and doesn't cause the frequency to be maximum all the time. Yet any task that really needs to be boosted can easily escape this default behavior by modifying its requested uclamp.min value (p->uclamp_req[UCLAMP_MIN]) via sched_setattr() syscall. [1] 804d402fb6: ("sched/rt: Make RT capacity-aware") Signed-off-by: Qais Yousef <qais.yousef@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20200716110347.19553-2-qais.yousef@arm.com		2020-07-29 13:51:47 +02:00
..
acpi	Merge branch 'acpica'	2020-06-10 17:27:28 +02:00
asm-generic	Merge branch 'sched/urgent'	2020-07-22 10:22:02 +02:00
clocksource
crypto	crypto: af_alg - fix use-after-free in af_alg_accept() due to bh_lock_sock()	2020-06-18 17:09:54 +10:00
drm	drm/edid: Replace zero-length array with flexible-array	2020-06-15 23:08:31 -05:00
dt-bindings	- qcom :	2020-06-11 12:42:14 -07:00
keys	RxRPC: Replace zero-length array with flexible-array	2020-06-15 23:08:32 -05:00
kunit
kvm
linux	sched/uclamp: Add a new sysctl to control RT default boost value	2020-07-29 13:51:47 +02:00
math-emu
media	media updates for v5.8-rc1	2020-06-13 13:09:38 -07:00
misc
net	bpf: net: Avoid incorrect bpf_sk_reuseport_detach call	2020-07-09 22:03:31 +02:00
pcmcia
ras
rdma
scsi
soc
sound	ALSA: compress: fix partial_drain completion state	2020-07-07 11:52:18 +02:00
target	scsi: target: Rename target_setup_cmd_from_cdb() to target_cmd_parse_cdb()	2020-06-09 21:57:26 -04:00
trace	trace/events/sched.h: fix duplicated word	2020-07-22 10:22:05 +02:00
uapi	Char/Misc fixes for 5.8-rc6	2020-07-16 11:26:40 -07:00
vdso
video
xen	xen: Move xen_setup_callback_vector() definition to include/xen/hvm.h	2020-06-11 15:15:19 +02:00