kernel_optimize_test/arch/sparc
Valentin Schneider 3c51d82d0b sched/core: Initialize the idle task with preemption disabled
[ Upstream commit f1a0a376ca0c4ef1fc3d24e3e502acbb5b795674 ]

As pointed out by commit

  de9b8f5dcb ("sched: Fix crash trying to dequeue/enqueue the idle thread")

init_idle() can and will be invoked more than once on the same idle
task. At boot time, it is invoked for the boot CPU thread by
sched_init(). Then smp_init() creates the threads for all the secondary
CPUs and invokes init_idle() on them.

As the hotplug machinery brings the secondaries to life, it will issue
calls to idle_thread_get(), which itself invokes init_idle() yet again.
In this case it's invoked twice more per secondary: at _cpu_up(), and at
bringup_cpu().

Given smp_init() already initializes the idle tasks for all *possible*
CPUs, no further initialization should be required. Now, removing
init_idle() from idle_thread_get() exposes some interesting expectations
with regards to the idle task's preempt_count: the secondary startup always
issues a preempt_disable(), requiring some reset of the preempt count to 0
between hot-unplug and hotplug, which is currently served by
idle_thread_get() -> idle_init().

Given the idle task is supposed to have preemption disabled once and never
see it re-enabled, it seems that what we actually want is to initialize its
preempt_count to PREEMPT_DISABLED and leave it there. Do that, and remove
init_idle() from idle_thread_get().

Secondary startups were patched via coccinelle:

  @begone@
  @@

  -preempt_disable();
  ...
  cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210512094636.2958515-1-valentin.schneider@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
2021-07-14 16:55:50 +02:00
..
boot .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
configs scsi: sr: remove references to BLK_DEV_SR_VENDOR, leave it enabled 2020-02-24 14:59:01 -05:00
crypto crypto: sparc - rename sha256 to sha256_alg 2020-07-16 21:49:04 +10:00
include sparc64: Use arch_validate_flags() to validate ADI flag 2021-03-17 17:06:24 +01:00
kernel sched/core: Initialize the idle task with preemption disabled 2021-07-14 16:55:50 +02:00
lib sparc32: fix a user-triggerable oops in clear_user() 2021-03-04 11:38:43 +01:00
math-emu treewide: Use fallthrough pseudo-keyword 2020-08-23 17:36:59 -05:00
mm sparc32: Limit memblock allocation to low memory 2021-03-17 17:06:24 +01:00
net treewide: Use fallthrough pseudo-keyword 2020-08-23 17:36:59 -05:00
oprofile
power
prom
vdso treewide: remove DISABLE_LTO 2020-10-21 00:28:53 +09:00
Kbuild
Kconfig sparc64: only select COMPAT_BINFMT_ELF if BINFMT_ELF is set 2021-03-04 11:38:10 +01:00
Kconfig.debug
Makefile