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SubjectRe: [PATCH] arm64/kernel: Simplify __cpu_up() by bailing out early
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On 3/3/20 1:06 AM, Mark Rutland wrote:
> On Tue, Mar 03, 2020 at 12:38:48AM +1100, Gavin Shan wrote:
>> On 3/2/20 11:21 PM, Mark Rutland wrote:
>>> On Mon, Mar 02, 2020 at 01:03:40PM +1100, Gavin Shan wrote:
>>>> The function __cpu_up() is invoked to bring up the target CPU through
>>>> the backend, PSCI for example. The nested if statements won't be needed
>>>> if we bail out early on the following two conditions where the status
>>>> won't be checked. The code looks simplified in that case.
>>>>
>>>> * Error returned from the backend (e.g. PSCI)
>>>> * The target CPU has been marked as onlined
>>>>
>>>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>>>
>>> FWIW, this looks like a nice cleanup to me:
>>>
>>> Reviewed-by: Mark Rutland <mark.rutland@arm.com>
>>>
>>> While this patch leaves secondary_data.{task,stack} stale on a
>>> successful onlining, that was already the case for a timeout, and should
>>> be fine (since the next attempt at onlining will configure those before
>>> poking the CPU).
>>>
>>> Thanks,
>>> Mark.
>>>
>>
>> Thanks, Mark. Yeah, it should be fine as you said. There are something else,
>> which might be not relevant. @secondary_data could be accessed by multiple CPUs
>> in parallel. For example, the master CPU boots CPU#1 and timeouts to wait it
>> to be online in 5 seconds. CPU#1 isn't necessarily stuck in somewhere. After
>> that, CPU#2 is brought up and might be accessing @secondary_data. At this point,
>> CPU#1 can come back to access it either. However, @secondary_data isn't valid
>> for CPU#1 anymore.
>
> Sure; I'm aware of improvements that could be made here, but I don't
> think they need to block this patch.
>

Yep, I think this patch is ready to go in if nobody else objects.

>> I was thinking of something to improve the situation, but not sure if it makes
>> any sense to do so. There are several options: (1) Make @secondary_data per-cpu
>> variable, which looks a nature way to go. (2) To shutdown the CPU on timeout.
>> The shutdown request can be failed to be served in theory, but it seems still
>> an improvement.
>
> I think #2 is a bad idea, since if the CPU gets into the kernel at all,
> it may have done stuff (e.g. acquiring locks), and ripping it out is
> liable to cause more problems.
>
> I think doing #1 might be nice, but some caveats apply.
>
> I'd like to clean up the secondary stack/task hand-over to use an atomic
> cmpxchg pair, so that we can detect when the secondary has possibly
> tried to use the stack/task. That requires splitting that from the
> MMU-off bits from the MMU-on bits, and I'm not sure how well that
> interacts with #1. It might mean that the per-cpu part isn't that
> worthwhile.
>

Right, #2 isn't good if the acquired resource (e.g. lock) can't be released.

It's something like to introduce a lock to the shared @secondary_data with
atomic cmpxchg pair. With that, I don't think per-cpu part is needed, not
so useful at least.

Thanks,
Gavin

>>
>>>> ---
>>>> arch/arm64/kernel/smp.c | 79 +++++++++++++++++++----------------------
>>>> 1 file changed, 37 insertions(+), 42 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>>>> index d4ed9a19d8fe..2a9d8f39dc58 100644
>>>> --- a/arch/arm64/kernel/smp.c
>>>> +++ b/arch/arm64/kernel/smp.c
>>>> @@ -115,60 +115,55 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
>>>> update_cpu_boot_status(CPU_MMU_OFF);
>>>> __flush_dcache_area(&secondary_data, sizeof(secondary_data));
>>>> - /*
>>>> - * Now bring the CPU into our world.
>>>> - */
>>>> + /* Now bring the CPU into our world */
>>>> ret = boot_secondary(cpu, idle);
>>>> - if (ret == 0) {
>>>> - /*
>>>> - * CPU was successfully started, wait for it to come online or
>>>> - * time out.
>>>> - */
>>>> - wait_for_completion_timeout(&cpu_running,
>>>> - msecs_to_jiffies(5000));
>>>> -
>>>> - if (!cpu_online(cpu)) {
>>>> - pr_crit("CPU%u: failed to come online\n", cpu);
>>>> - ret = -EIO;
>>>> - }
>>>> - } else {
>>>> + if (ret) {
>>>> pr_err("CPU%u: failed to boot: %d\n", cpu, ret);
>>>> return ret;
>>>> }
>>>> + /*
>>>> + * CPU was successfully started, wait for it to come online or
>>>> + * time out.
>>>> + */
>>>> + wait_for_completion_timeout(&cpu_running,
>>>> + msecs_to_jiffies(5000));
>>>> + if (cpu_online(cpu))
>>>> + return 0;
>>>> +
>>>> + pr_crit("CPU%u: failed to come online\n", cpu);
>>>> secondary_data.task = NULL;
>>>> secondary_data.stack = NULL;
>>>> __flush_dcache_area(&secondary_data, sizeof(secondary_data));
>>>> status = READ_ONCE(secondary_data.status);
>>>> - if (ret && status) {
>>>> -
>>>> - if (status == CPU_MMU_OFF)
>>>> - status = READ_ONCE(__early_cpu_boot_status);
>>>> + if (status == CPU_MMU_OFF)
>>>> + status = READ_ONCE(__early_cpu_boot_status);
>>>> - switch (status & CPU_BOOT_STATUS_MASK) {
>>>> - default:
>>>> - pr_err("CPU%u: failed in unknown state : 0x%lx\n",
>>>> - cpu, status);
>>>> - cpus_stuck_in_kernel++;
>>>> - break;
>>>> - case CPU_KILL_ME:
>>>> - if (!op_cpu_kill(cpu)) {
>>>> - pr_crit("CPU%u: died during early boot\n", cpu);
>>>> - break;
>>>> - }
>>>> - pr_crit("CPU%u: may not have shut down cleanly\n", cpu);
>>>> - /* Fall through */
>>>> - case CPU_STUCK_IN_KERNEL:
>>>> - pr_crit("CPU%u: is stuck in kernel\n", cpu);
>>>> - if (status & CPU_STUCK_REASON_52_BIT_VA)
>>>> - pr_crit("CPU%u: does not support 52-bit VAs\n", cpu);
>>>> - if (status & CPU_STUCK_REASON_NO_GRAN)
>>>> - pr_crit("CPU%u: does not support %luK granule \n", cpu, PAGE_SIZE / SZ_1K);
>>>> - cpus_stuck_in_kernel++;
>>>> + switch (status & CPU_BOOT_STATUS_MASK) {
>>>> + default:
>>>> + pr_err("CPU%u: failed in unknown state : 0x%lx\n",
>>>> + cpu, status);
>>>> + cpus_stuck_in_kernel++;
>>>> + break;
>>>> + case CPU_KILL_ME:
>>>> + if (!op_cpu_kill(cpu)) {
>>>> + pr_crit("CPU%u: died during early boot\n", cpu);
>>>> break;
>>>> - case CPU_PANIC_KERNEL:
>>>> - panic("CPU%u detected unsupported configuration\n", cpu);
>>>> }
>>>> + pr_crit("CPU%u: may not have shut down cleanly\n", cpu);
>>>> + /* Fall through */
>>>> + case CPU_STUCK_IN_KERNEL:
>>>> + pr_crit("CPU%u: is stuck in kernel\n", cpu);
>>>> + if (status & CPU_STUCK_REASON_52_BIT_VA)
>>>> + pr_crit("CPU%u: does not support 52-bit VAs\n", cpu);
>>>> + if (status & CPU_STUCK_REASON_NO_GRAN) {
>>>> + pr_crit("CPU%u: does not support %luK granule\n",
>>>> + cpu, PAGE_SIZE / SZ_1K);
>>>> + }
>>>> + cpus_stuck_in_kernel++;
>>>> + break;
>>>> + case CPU_PANIC_KERNEL:
>>>> + panic("CPU%u detected unsupported configuration\n", cpu);
>>>> }
>>>> return ret;
>>>> --
>>>> 2.23.0
>>>>
>>>
>>
>

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