Re: [RFC][Patch v9 2/6] KVM: Enables the kernel to isolate guest free pages

[Nitesh Narayan Lal <nitesh@redhat.com>]

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On 3/12/19 5:13 PM, Alexander Duyck wrote:
> On Tue, Mar 12, 2019 at 12:46 PM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>> On 3/8/19 4:39 PM, Alexander Duyck wrote:
>>> On Fri, Mar 8, 2019 at 11:39 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>>> On 3/8/19 2:25 PM, Alexander Duyck wrote:
>>>>> On Fri, Mar 8, 2019 at 11:10 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>>>>> On 3/8/19 1:06 PM, Alexander Duyck wrote:
>>>>>>> On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>>>>>> On Thu, Mar 07, 2019 at 02:35:53PM -0800, Alexander Duyck wrote:
>>>>>>>>> The only other thing I still want to try and see if I can do is to add
>>>>>>>>> a jiffies value to the page private data in the case of the buddy
>>>>>>>>> pages.
>>>>>>>> Actually there's one extra thing I think we should do, and that is make
>>>>>>>> sure we do not leave less than X% off the free memory at a time.
>>>>>>>> This way chances of triggering an OOM are lower.
>>>>>>> If nothing else we could probably look at doing a watermark of some
>>>>>>> sort so we have to have X amount of memory free but not hinted before
>>>>>>> we will start providing the hints. It would just be a matter of
>>>>>>> tracking how much memory we have hinted on versus the amount of memory
>>>>>>> that has been pulled from that pool.
>>>>>> This is to avoid false OOM in the guest?
>>>>> Partially, though it would still be possible. Basically it would just
>>>>> be a way of determining when we have hinted "enough". Basically it
>>>>> doesn't do us much good to be hinting on free memory if the guest is
>>>>> already constrained and just going to reallocate the memory shortly
>>>>> after we hinted on it. The idea is with a watermark we can avoid
>>>>> hinting until we start having pages that are actually going to stay
>>>>> free for a while.
>>>>>
>>>>>>>  It is another reason why we
>>>>>>> probably want a bit in the buddy pages somewhere to indicate if a page
>>>>>>> has been hinted or not as we can then use that to determine if we have
>>>>>>> to account for it in the statistics.
>>>>>> The one benefit which I can see of having an explicit bit is that it
>>>>>> will help us to have a single hook away from the hot path within buddy
>>>>>> merging code (just like your arch_merge_page) and still avoid duplicate
>>>>>> hints while releasing pages.
>>>>>>
>>>>>> I still have to check PG_idle and PG_young which you mentioned but I
>>>>>> don't think we can reuse any existing bits.
>>>>> Those are bits that are already there for 64b. I think those exist in
>>>>> the page extension for 32b systems. If I am not mistaken they are only
>>>>> used in VMA mapped memory. What I was getting at is that those are the
>>>>> bits we could think about reusing.
>>>>>
>>>>>> If we really want to have something like a watermark, then can't we use
>>>>>> zone->free_pages before isolating to see how many free pages are there
>>>>>> and put a threshold on it? (__isolate_free_page() does a similar thing
>>>>>> but it does that on per request basis).
>>>>> Right. That is only part of it though since that tells you how many
>>>>> free pages are there. But how many of those free pages are hinted?
>>>>> That is the part we would need to track separately and then then
>>>>> compare to free_pages to determine if we need to start hinting on more
>>>>> memory or not.
>>>> Only pages which are isolated will be hinted, and once a page is
>>>> isolated it will not be counted in the zone free pages.
>>>> Feel free to correct me if I am wrong.
>>> You are correct up to here. When we isolate the page it isn't counted
>>> against the free pages. However after we complete the hint we end up
>>> taking it out of isolation and returning it to the "free" state, so it
>>> will be counted against the free pages.
>>>
>>>> If I am understanding it correctly you only want to hint the idle pages,
>>>> is that right?
>>> Getting back to the ideas from our earlier discussion, we had 3 stages
>>> for things. Free but not hinted, isolated due to hinting, and free and
>>> hinted. So what we would need to do is identify the size of the first
>>> pool that is free and not hinted by knowing the total number of free
>>> pages, and then subtract the size of the pages that are hinted and
>>> still free.
>> To summarize, for now, I think it makes sense to stick with the current
>> approach as this way we can avoid any locking in the allocation path and
>> reduce the number of hypercalls for a bunch of MAX_ORDER - 1 page.
> I'm not sure what you are talking about by "avoid any locking in the
> allocation path". Are you talking about the spin on idle bit, if so
> then yes. 
Yeap!
> However I have been testing your patches and I was correct
> in the assumption that you forgot to handle the zone lock when you
> were freeing __free_one_page.
Yes, these are the steps other than the comments you provided in the
code. (One of them is to fix release_buddy_page())
>  I just did a quick copy/paste from your
> zone lock handling from the guest_free_page_hinting function into the
> release_buddy_pages function and then I was able to enable multiple
> CPUs without any issues.
>
>> For the next step other than the comments received in the code and what
>> I mentioned in the cover email, I would like to do the following:
>> 1. Explore the watermark idea suggested by Alex and bring down memhog
>> execution time if possible.
> So there are a few things that are hurting us on the memhog test:
> 1. The current QEMU patch is only madvising 4K pages at a time, this
> is disabling THP and hurts the test.
Makes sense, thanks for pointing this out.
>
> 2. The fact that we madvise the pages away makes it so that we have to
> fault the page back in in order to use it for the memhog test. In
> order to avoid that penalty we may want to see if we can introduce
> some sort of "timeout" on the pages so that we are only hinting away
> old pages that have not been used for some period of time.

Possibly using MADVISE_FREE should also help in this, I will try this as
well.

If we could come up with something bit which we could reuse then we may
be able to  tackle this issue easily. I will look into this.

>
> 3. Currently we are still doing a large amount of processing in the
> page free path. Ideally we should look at getting away from trying to
> do so much per-cpu work and instead just have some small tasks that
> put the data needed in the page, and then have a separate thread
> walking the free_list checking that data, isolating the pages, hinting
> them, and then returning them back to the free_list.
I will probably defer this analysis for now, once we have other things
fixed. I can possibly evaluate/compare the performance impact with both
the approach and chose from them.
>
>> 2. Benchmark hinting v/s non-hinting more extensively.
>> Let me know if you have any specific suggestions in terms of the tools I
>> can run to do the same. (I am planning to run atleast netperf, hackbench
>> and stress for this).
> So I have been running the memhog 32g test and the will-it-scale
> page_fault1 test as my primary two tests for this so far.
>
> What I have seen so far has been pretty promising. I had to do some
> build fixes, fixes to QEMU to hint on the full size page instead of 4K
> page, and fixes for locking so this isn't exactly your original patch
> set, but with all that I am seeing data comparable to the original
> patch set I had.
>
> For memhog 32g I am seeing performance similar to a VM that was fresh
> booted. I make that the comparison because you will have to take page
> faults on a fresh boot as you access additional memory. However after
> the first run of the runtime drops  from 22s to 20s without the
> hinting enabled.
>
> The big one that probably still needs some work will be the multi-cpu
> scaling. With the per-cpu locking for the zone lock to pull pages out,
> and put them back in the free list I am seeing what looks like about a
> 10% drop in the page_fault1 test. Here are the results as I have seen
> so far on a 16 cpu 32G VM:
>
> -- baseline --
> ./runtest.py page_fault1
> tasks,processes,processes_idle,threads,threads_idle,linear
> 0,0,100,0,100,0
> 1,522242,93.73,514965,93.74,522242
> 2,929433,87.48,857280,87.50,1044484
> 3,1360651,81.25,1214224,81.48,1566726
> 4,1693709,75.01,1437156,76.33,2088968
> 5,2062392,68.77,1743294,70.78,2611210
> 6,2271363,62.54,1787238,66.75,3133452
> 7,2564479,56.33,1924684,61.77,3655694
> 8,2699897,50.09,2205783,54.28,4177936
> 9,2931697,43.85,2135788,50.20,4700178
> 10,2939384,37.63,2258725,45.04,5222420
> 11,3039010,31.41,2209401,41.04,5744662
> 12,3022976,25.19,2177655,35.68,6266904
> 13,3015683,18.98,2123546,31.73,6789146
> 14,2921798,12.77,2160489,27.30,7311388
> 15,2846758,6.51,1815036,17.40,7833630
> 16,2703146,0.36,2121018,18.21,8355872
>
> -- modified rh patchset --
> ./runtest.py page_fault1
> tasks,processes,processes_idle,threads,threads_idle,linear
> 0,0,100,0,100,0
> 1,527216,93.72,517459,93.70,527216
> 2,911239,87.48,843278,87.51,1054432
> 3,1295059,81.22,1193523,81.61,1581648
> 4,1649332,75.02,1439403,76.17,2108864
> 5,1985780,68.81,1745556,70.44,2636080
> 6,2174751,62.56,1769433,66.84,3163296
> 7,2433273,56.33,2121777,58.46,3690512
> 8,2537356,50.17,1901743,57.23,4217728
> 9,2737689,43.87,1859179,54.17,4744944
> 10,2718474,37.65,2188891,43.69,5272160
> 11,2743381,31.47,2205112,38.00,5799376
> 12,2738717,25.26,2117281,38.09,6326592
> 13,2643648,19.06,1887956,35.31,6853808
> 14,2598001,12.92,1916544,27.87,7381024
> 15,2498325,6.70,1992580,26.10,7908240
> 16,2424587,0.45,2137742,21.37,8435456
>
> As we discussed earlier, it would probably be good to focus on only
> pulling something like 4 to 8 (MAX_ORDER - 1) pages per round of
> hinting. 
I agree that I should bring down the page-set on which I am working.
> You might also look at only working one zone at a time. Then
> what you could do is look at placing the pages you have already hinted
> on at the tail end of the free_list and pull a new set of pages out to
> hint on.
I think for this we still need a way to check if a particular page is
hinted or not.
>  You could do this all in one shot while holding the zone
> lock.
-- 
Regards
Nitesh


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