On 21/09/2020 18:35, Peter Zijlstra wrote:
> Hi,
>
> Here's my take on migrate_disable(). It avoids growing a second means of
> changing the affinity, documents how the things violates locking rules but
> still mostly works.
>
> It also avoids blocking completely, so no more futex band-aids required.
>
> Also, no more atomics/locks on the fast path.
>
> I also put in a comment that explains how the whole concept is fundamentally
> flawed but a necessary evil to get PREEMPT_RT going -- for now.
>
> Somewhat tested with PREEMPT_RT.
>
> ---
> include/linux/cpuhotplug.h | 1
> include/linux/preempt.h | 60 +++
> include/linux/sched.h | 4
> include/linux/sched/hotplug.h | 2
> include/linux/stop_machine.h | 5
> kernel/cpu.c | 9
> kernel/sched/core.c | 673
> +++++++++++++++++++++++++++++++-----------
> kernel/sched/deadline.c | 5
> kernel/sched/sched.h | 24 +
> kernel/stop_machine.c | 23 +
> lib/dump_stack.c | 2
> lib/smp_processor_id.c | 5
> 12 files changed, 635 insertions(+), 178 deletions(-)
>
I get this when running 6 (periodic) RT50 tasks with CPU hp stress on my
6 CPU JUNO board (!CONFIG_PREEMPT_RT).
[ 55.490263] ------------[ cut here ]------------
[ 55.505261] Modules linked in:
[ 55.508322] CPU: 3 PID: 24 Comm: migration/3 Not tainted
5.9.0-rc1-00132-gc096e6406c50-dirty #90
[ 55.517119] Hardware name: ARM Juno development board (r0) (DT)
[ 55.523058] Stopper: multi_cpu_stop+0x0/0x170 <- 0x0
[ 55.528029] pstate: 20000085 (nzCv daIf -PAN -UAO BTYPE=--)
[ 55.533612] pc : sched_cpu_dying+0x124/0x130
[ 55.537887] lr : sched_cpu_dying+0xd8/0x130
[ 55.542071] sp : ffff800011f0bca0
[ 55.545385] x29: ffff800011f0bca0 x28: 0000000000000002
[ 55.550703] x27: 0000000000000000 x26: 0000000000000060
[ 55.556022] x25: 0000000000000000 x24: 0000000000000001
[ 55.561340] x23: 0000000000000000 x22: 0000000000000003
[ 55.566659] x21: 0000000000000080 x20: 0000000000000003
[ 55.571977] x19: ffff00097ef9e1c0 x18: 0000000000000010
[ 55.577295] x17: 0000000000000000 x16: 0000000000000000
[ 55.582613] x15: 0000000000000000 x14: 000000000000015c
[ 55.587932] x13: 0000000000000000 x12: 00000000000006f1
[ 55.593250] x11: 0000000000000080 x10: 0000000000000000
[ 55.598567] x9 : 0000000000000003 x8 : ffff0009743f5900
[ 55.603886] x7 : 0000000000000003 x6 : 0000000000000000
[ 55.609204] x5 : 0000000000000001 x4 : 0000000000000002
[ 55.614521] x3 : 0000000000000000 x2 : 0000000000000013
[ 55.619839] x1 : 0000000000000008 x0 : 0000000000000003
[ 55.625158] Call trace:
[ 55.627607] sched_cpu_dying+0x124/0x130
[ 55.631535] cpuhp_invoke_callback+0x88/0x210
[ 55.635897] take_cpu_down+0x7c/0xd8
[ 55.639475] multi_cpu_stop+0xac/0x170
[ 55.643227] cpu_stopper_thread+0x98/0x130
[ 55.647327] smpboot_thread_fn+0x1c4/0x280
[ 55.651427] kthread+0x140/0x160
[ 55.654658] ret_from_fork+0x10/0x34
[ 55.658239] Code: f000e1c1 913fc021 1400034a 17ffffde (d4210000)
[ 55.664342] ---[ end trace c5b8988b7b701e56 ]---
[ 55.668963] note: migration/3[24] exited with preempt_count 3
7309 int sched_cpu_dying(unsigned int cpu)
...
BUG_ON(rq->nr_running != 1 || rq_has_pinned_tasks(rq));
...
rq->nr_running is always 2 here in this cases.
balance_hotplug_wait and sched_cpu_wait_empty run in cpuhp/X (CFS)
whereas sched_cpu_dying in migration/X ?
