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SubjectRe: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf
On Fri, Sep 8, 2017 at 6:39 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>> On Sep 8, 2017, at 6:05 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>
>>> On Fri, Sep 8, 2017 at 5:00 PM, Andy Lutomirski <luto@kernel.org> wrote:
>>>
>>> I'm not convinced. The SDM says (Vol 3, 11.3, under WC):
>>>
>>> If the WC buffer is partially filled, the writes may be delayed until
>>> the next occurrence of a serializing event; such as, an SFENCE or
>>> MFENCE instruction, CPUID execution, a read or write to uncached
>>> memory, an interrupt occurrence, or a LOCK instruction execution.
>>>
>>> Thanks, Intel, for definiing "serializing event" differently here than
>>> anywhere else in the whole manual.
>>
>> Yeah, it's really badly defined. Ok, maybe a locked instruction does
>> actually wait for it.. It should be invisible to anything, regardless.
>>
>>> 1. The kernel wants to reclaim a page of normal memory, so it unmaps
>>> it and flushes. Another CPU has an entry for that page in its WC
>>> buffer. I don't think we care whether the flush causes the WC write
>>> to really hit RAM because it's unobservable -- we just need to make
>>> sure it is ordered, as seen by software, before the flush operation
>>> completes. From the quote above, I think we're okay here.
>>
>> Agreed.
>>
>>> 2. The kernel is unmapping some IO memory (e.g. a GPU command buffer).
>>> It wants a guarantee that, when flush_tlb_mm_range returns, all CPUs
>>> are really done writing to it. Here I'm less convinced. The SDM
>>> quote certainly suggests to me that we have a promise that the WC
>>> write has *started* before flush_tlb_mm_range returns, but I'm not
>>> sure I believe that it's guaranteed to have retired.
>>
>> If others have writable TLB entries, what keeps them from just
>> continuing to write for a long time afterwards?
>
> Whoever unmaps the resource by kicking out their drm fd? I admit I'm just trying to think of the worst case.
>
>>
>>> I'd prefer to leave it as is except on the buggy AMD CPUs, though,
>>> since the current code is nice and fast.
>>
>> So is there a patch to detect the 383 erratum and serialize for those?
>> I may have missed that part.
>>
>
> The patch is in my head. It's imaginarily attached to this email.

After contemplating the info from Boris and Markus, I think I need to
add a #3 to the list of reasons my patch could be problematic:

3. If a CPU frees a page table (or PUD or PMD or whatever), that CPU
will flush before the memory goes back to the system. If that flush
is deferred on a different CPU that has the pointer to the freed table
cached in its TLB, then that CPU can speculatively load complete
garbage into its TLB.

I don't think this should be observable, but I can easily imagine it
triggering errata or weird ill-advised machine checks.

Anyway, if I need change the behavior back, I can do it in one of two
ways. I can just switch to init_mm instead of going lazy, which is
expensive, but not *that* expensive on CPUs with PCID. Or I can do it
the way we used to do it and send the flush IPI to lazy CPUs. The
latter will only have a performance impact when a flush happens, but
the performance hit is much higher when there's a flush.

--Andy

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