Re: [PATCH 4/4] x86, hotplug: Use hlt instead of mwait when resuming from hibernation

[Andy Lutomirski <luto@amacapital.net>]

On Sat, Oct 8, 2016 at 3:31 AM, Rafael J. Wysocki <rafael@kernel.org> wrote:
> On Fri, Oct 7, 2016 at 9:47 PM, Andy Lutomirski <luto@kernel.org> wrote:
>> On 06/25/2016 09:19 AM, Chen Yu wrote:
>>>
>>> Here's the story of what the problem is, why this
>>> happened, and why this patch looks like this:
>>>
>>> Stress test from Varun Koyyalagunta reports that, the
>>> nonboot CPU would hang occasionally, when resuming from
>>> hibernation. Further investigation shows that, the precise
>>> stage when nonboot CPU hangs, is the time when the nonboot
>>> CPU been woken up incorrectly, and tries to monitor the
>>> mwait_ptr for the second time, then an exception is
>>> triggered due to illegal vaddr access, say, something like,
>>> 'Unable to handler kernel address of 0xffff8800ba800010...'
>>>
>>> Further investigation shows that, the exception is caused
>>> by accessing a page without PRESENT flag, because the pte entry
>>> for this vaddr is zero. Here's the scenario how this problem
>>> happens: Page table for direct mapping is allocated dynamically
>>> by kernel_physical_mapping_init, it is possible that in the
>>> resume process, when the boot CPU is trying to write back pages
>>> to their original address, and just right to writes to the monitor
>>> mwait_ptr then wakes up one of the nonboot CPUs, since the page
>>> table currently used by the nonboot CPU might not the same as it
>>> is before the hibernation, an exception might occur due to
>>> inconsistent page table.
>>>
>>> First try is to get rid of this problem by changing the monitor
>>> address from task.flag to zero page, because one one would write
>>> to zero page. But this still have problem because of ping-pong
>>> wake up situation in mwait_play_dead:
>>>
>>> One possible implementation of a clflush is a read-invalidate snoop,
>>> which is what a store might look like, so cflush might break the mwait.
>>>
>>> 1. CPU1 wait at zero page
>>> 2. CPU2 cflush zero page, wake CPU1 up, then CPU2 waits at zero page
>>> 3. CPU1 is woken up, and invoke cflush zero page, thus wake up CPU2 again.
>>> then the nonboot CPUs never sleep for long.
>>>
>>> So it's better to monitor different address for each
>>> nonboot CPUs, however since there is only one zero page, at most:
>>> PAGE_SIZE/L1_CACHE_LINE CPUs are satisfied, which is usually 64
>>> on a x86_64, apparently it's not enough for servers, maybe more
>>> zero pages are required.
>>>
>>> So choose the solution as Brian suggested, to put the nonboot CPUs
>>> into hlt before resuming. But Rafael has mentioned that, if some of
>>> the CPUs have already been offline before hibernation, then the problem
>>> is still there. So this patch tries to kick the already offline CPUs woken
>>> up and fall into hlt, and then put the rest online CPUs into hlt.
>>> In this way, all the nonboot CPUs will wait at a safe state,
>>> without touching any memory during s/r. (It's not safe to modify
>>> mwait_play_dead, because once previous offline CPUs are woken up,
>>> it will either access text code, whose page table is not safe anymore
>>> across hibernation, due to:
>>> Commit ab76f7b4ab23 ("x86/mm: Set NX on gap between __ex_table and
>>> rodata").
>>>
>>
>> I realize I'm extremely late to the party, but I must admit that I don't get
>> it.  Sure, hibernation resume can spuriously wake the non-boot CPU, but at
>> some point it has to wake up for real.
>
> You mean during resume?  We reinit from scratch then.
>
>> What ensures that the text it was
>> running (native_play_dead or whatever) is still there when it wakes up?
>>
>> Or does the hibernation resume code actually send the remote CPU an
>> INIT-SIPI sequence a la wakeup_secondary_cpu_via_init()?
>
> That's what happens AFAICS.
>
>> If so, this seems
>> a bit odd to me.  Shouldn't we kick the CPU all the way to the wait-for-SIPI
>> state rather than getting it to play dead via hlt or mwait?
>
> We could do that.  It would be a bit cleaner than using the "hlt play
> dead" thing, but the practical difference would be very small (if
> observable at all).

Probably true.  It might be worth changing the "hlt" path to something like:

asm volatile ("hlt");
WARN(1, "CPU woke directly from halt-for-resume -- should have been
woken by SIPI\n");