Re: [PATCH v2] powerpc/smp: do not decrement idle task preempt count in CPU offline
From: Srikar Dronamraju <hidden>
Date: 2021-10-19 04:46:28
Also in:
lkml
* Nathan Lynch [off-list ref] [2021-10-15 12:39:02]:
With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we
get:
BUG: scheduling while atomic: swapper/1/0/0x00000000
no locks held by swapper/1/0.
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100
Call Trace:
dump_stack_lvl+0xac/0x108
__schedule_bug+0xac/0xe0
__schedule+0xcf8/0x10d0
schedule_idle+0x3c/0x70
do_idle+0x2d8/0x4a0
cpu_startup_entry+0x38/0x40
start_secondary+0x2ec/0x3a0
start_secondary_prolog+0x10/0x14
This is because powerpc's arch_cpu_idle_dead() decrements the idle task's
preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc:
Re-enable preemption before cpu_die()"), specifically "start_secondary()
expects a preempt_count() of 0."
However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle
task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core:
Initialize the idle task with preemption disabled"), that justification no
longer holds.
The idle task isn't supposed to re-enable preemption, so remove the
vestigial preempt_enable() from the CPU offline path.
Tested with pseries and powernv in qemu, and pseries on PowerVM.
Fixes: 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug")
Signed-off-by: Nathan Lynch <redacted>
Reviewed-by: Valentin Schneider <redacted>Looks good to me. Reviewed-by: Srikar Dronamraju <redacted>
quoted hunk ↗ jump to hunk
--- Notes: Changes since v1: - remove incorrect Fixes: tag, add Valentin's r-b. arch/powerpc/kernel/smp.c | 2 -- 1 file changed, 2 deletions(-)diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c index 9cc7d3dbf439..605bab448f84 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c@@ -1730,8 +1730,6 @@ void __cpu_die(unsigned int cpu) void arch_cpu_idle_dead(void) { - sched_preempt_enable_no_resched(); - /* * Disable on the down path. This will be re-enabled by * start_secondary() via start_secondary_resume() below-- 2.31.1
-- Thanks and Regards Srikar Dronamraju