Asymmetric systems may not offer the same level of userspace ISA support
across all CPUs, meaning that some applications cannot be executed by
some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
not feature support for 32-bit applications on both clusters.
Although we take care to prevent explicit hot-unplug of all 32-bit
capable CPUs on such a system, this is required when suspending on some
SoCs where the firmware mandates that the suspend/resume operation is
handled by CPU 0, which may not be capable of running 32-bit tasks.
Consequently, there is a window on the resume path where no 32-bit
capable CPUs are available for scheduling and waking up a 32-bit task
will result in a scheduler BUG() due to failure of select_fallback_rq():
| kernel BUG at kernel/sched/core.c:2858!
| Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
| ...
| Call trace:
| select_fallback_rq+0x4b0/0x4e4
| try_to_wake_up.llvm.4388853297126348405+0x460/0x5b0
| default_wake_function+0x1c/0x30
| autoremove_wake_function+0x1c/0x60
| __wake_up_common.llvm.11763074518265335900+0x100/0x1b8
| __wake_up+0x78/0xc4
| ep_poll_callback+0x20c/0x3fc
Prevent wakeups of unschedulable frozen tasks in ttwu() and instead
defer the wakeup to __thaw_tasks(), which runs only once all the
secondary CPUs are back online.
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/linux-arch/20210525151432.16875-17-will@kernel.org/
Bug: 186372082
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I5a0531b48d537a79e1926289b5a87edcd7dd78ad
(cherry picked from commit 94155f60a54df82652117b58c124a529556db6be)
6ebb3c5057