Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)

[Khalid Aziz]

On 09/12/2018 09:37 AM, Julian Stecklina wrote:
> Julian Stecklina <jsteckli@xxxxxxxxx> writes:
> 
>> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
>>
>>> On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <jsteckli@xxxxxxxxx> wrote:
>>>>
>>>> I've been spending some cycles on the XPFO patch set this week. For the
>>>> patch set as it was posted for v4.13, the performance overhead of
>>>> compiling a Linux kernel is ~40% on x86_64[1]. The overhead comes almost
>>>> completely from TLB flushing. If we can live with stale TLB entries
>>>> allowing temporary access (which I think is reasonable), we can remove
>>>> all TLB flushing (on x86). This reduces the overhead to 2-3% for
>>>> kernel compile.
>>>
>>> I have to say, even 2-3% for a kernel compile sounds absolutely horrendous.
>>
>> Well, it's at least in a range where it doesn't look hopeless.
>>
>>> Kernel bullds are 90% user space at least for me, so a 2-3% slowdown
>>> from a kernel is not some small unnoticeable thing.
>>
>> The overhead seems to come from the hooks that XPFO adds to
>> alloc/free_pages. These hooks add a couple of atomic operations per
>> allocated (4K) page for book keeping. Some of these atomic ops are only
>> for debugging and could be removed. There is also some opportunity to
>> streamline the per-page space overhead of XPFO.
> 
> I've updated my XPFO branch[1] to make some of the debugging optional
> and also integrated the XPFO bookkeeping with struct page, instead of
> requiring CONFIG_PAGE_EXTENSION, which removes some checks in the hot
> path. These changes push the overhead down to somewhere between 1.5 and
> 2% for my quad core box in kernel compile. This is close to the
> measurement noise, so I take suggestions for a better benchmark here.
> 
> Of course, if you hit contention on the xpfo spinlock then performance
> will suffer. I guess this is what happened on Khalid's large box.
> 
> I'll try to remove the spinlocks and add fixup code to the pagefault
> handler to see whether this improves the situation on large boxes. This
> might turn out to be ugly, though.
> 

Hi Julian,

I ran tests with your updated code and gathered lock statistics. Change in system time for "make -j60" was in the noise margin (It actually went up by about 2%). There is some contention on xpfo_lock. Average wait time does not look high compared to other locks. Max hold time looks a little long. From /proc/lock_stat:

              &(&page->xpfo_lock)->rlock:         29698          29897           0.06         134.39       15345.58           0.51      422474670      960222532           0.05       30362.05   195807002.62           0.20

Nevertheless even a smaller average wait time can add up.

--
Khalid