Re: [PATCH v11 0/6] mm / virtio: Provide support for unused page reporting

Nitesh Narayan Lal
2019-10-02T14:37:22Z

On 10/2/19 10:25 AM, Alexander Duyck wrote:
> On Wed, Oct 2, 2019 at 3:37 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>
>> On 10/1/19 8:55 PM, Alexander Duyck wrote:
>>> On Tue, Oct 1, 2019 at 12:16 PM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>>> On 10/1/19 12:21 PM, Alexander Duyck wrote:
>>>>> On Tue, 2019-10-01 at 17:35 +0200, David Hildenbrand wrote:
>>>>>> On 01.10.19 17:29, Alexander Duyck wrote:
> <snip>
>
>>>>> Do we have a working patch set for Nitesh's code? The last time I tried
>>>>> running his patch set I ran into issues with kernel panics. If we have a
>>>>> known working/stable patch set I can give it a try.
>>>> Did you try the v12 patch-set [1]?
>>>> I remember that you reported the CPU stall issue, which I fixed in the v12.
>>>>
>>>> [1] https://lkml.org/lkml/2019/8/12/593
>>> So I tried testing with the spin_lock calls replaced with spin_lock
>>> _irq to resolve the IRQ issue. I also had shuffle enabled in order to
>>> increase the number of pages being dirtied.
>>>
>>> With that setup the bitmap approach is running significantly worse
>>> then my approach, even with the boundary removed. Since I had to
>>> modify the code to even getting working I am not comfortable posting
>>> numbers.
>> I didn't face any issue in getting the code work or compile.
>> Before my v12 posting, I did try your previously suggested test
>> (will-it-scale/page_fault1 for 12 hours on a 60 GB) and didn't see any issues.
>> I think it would help more if you can share the setup which you are running.
> So one issue with the standard page_fault1 is that it is only
> operating at the 4K page level. You won't see much impact from you
> patches with that as the overhead of splitting a MAX_ORDER - 2 page
> down to a 4K page will end up being the biggest thing you are
> benchmarking.
>
> I think I have brought it up before but I am running with the
> page_fault1 modified to use THP. Making the change is pretty
> straightforward as all you have to do is add an madvise to the test
> code. All that is needed is to add "madvise(c, MEMSIZE,
> MADV_HUGEPAGE);" between the assert and the for loop in the
> page_fault1 code and then rebuild the test. I actually copied
> page_fault1.c into a file I named page_fault4.c and added the line. As
> a result it seems like the code will build it as an additional test.

Thanks for explaining.

>
> The only other alteration I can think of that might have much impact
> would be to enable the page shuffling. The idea is that it will cause
> us to use more pages because half of the pages freed are dumped to the
> tail of the list so we are constantly churning the memory.
>
>>> My suggestion would be to look at reworking the patch set and
>>> post numbers for my patch set versus the bitmap approach and we can
>>> look at them then.
>> Agreed. However, in order to fix an issue I have to reproduce it first.
> With the tweak I have suggested above it should make it much easier to
> reproduce. Basically all you need is to have the allocation competing
> against hinting. Currently the hinting isn't doing this because the
> allocations are mostly coming out of 4K pages instead of higher order
> ones.

Understood.

>
> Alternatively you could just make the suggestion I had proposed about
> using spin_lock/unlock_irq in your worker thread and that resolved it
> for me.

I will first reproduce as you suggested and then make the change.
That will help me to understand the issue in a better way.

>
>>> I would prefer not to spend my time fixing and
>>> tuning a patch set that I am still not convinced is viable.
>> You don't have to, I can fix the issues in my patch-set. :)
> Sounds good. Hopefully the stuff I pointed out above helps you to get
> a reproduction and resolve the issues.

Indeed, I will try these suggestions and fix this issue.
Did you run into any other issues while building or running?

>
> - Alex

--
Thanks
Nitesh