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

On 10/9/19 12:50 PM, Alexander Duyck wrote:
> On Wed, 2019-10-09 at 12:25 -0400, Nitesh Narayan Lal wrote:
>> On 10/7/19 1:20 PM, Alexander Duyck wrote:
>>> On Mon, Oct 7, 2019 at 10:07 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>>> On 10/7/19 12:27 PM, Alexander Duyck wrote:
>>>>> On Mon, 2019-10-07 at 12:19 -0400, Nitesh Narayan Lal wrote:
>>>>>> On 10/7/19 11:33 AM, Alexander Duyck wrote:
>>>>>>> On Mon, 2019-10-07 at 08:29 -0400, Nitesh Narayan Lal wrote:
>>>>>>>> On 10/2/19 10:25 AM, Alexander Duyck wrote:
>>> <snip>
>>>>>>>> page_reporting.c change:
>>>>>>>> @@ -101,8 +101,12 @@ static void scan_zone_bitmap(struct page_reporting_config
>>>>>>>> *phconf,
>>>>>>>>                 /* Process only if the page is still online */
>>>>>>>>                 page = pfn_to_online_page((setbit << PAGE_REPORTING_MIN_ORDER) +
>>>>>>>>                                           zone->base_pfn);
>>>>>>>> -               if (!page)
>>>>>>>> +               if (!page || !PageBuddy(page)) {
>>>>>>>> +                       clear_bit(setbit, zone->bitmap);
>>>>>>>> +                       atomic_dec(&zone->free_pages);
>>>>>>>>                         continue;
>>>>>>>> +               }
>>>>>>> I suspect the zone->free_pages is going to be expensive for you to deal
>>>>>>> with. It is a global atomic value and is going to have the cacheline
>>>>>>> bouncing that it is contained in. As a result thinks like setting the
>>>>>>> bitmap with be more expensive as every tome a CPU increments free_pages it
>>>>>>> will likely have to take the cache line containing the bitmap pointer as
>>>>>>> well.
>>>>>> I see I will have to explore this more. I am wondering if there is a way to
>>>>>> measure this If its effect is not visible in will-it-scale/page_fault1. If
>>>>>> there is a noticeable amount of degradation, I will have to address this.
>>>>> If nothing else you might look at seeing if you can split up the
>>>>> structures so that the bitmap and nr_bits is in a different region
>>>>> somewhere since those are read-mostly values.
>>>> ok, I will try to understand the issue and your suggestion.
>>>> Thank you for bringing this up.
>>>>> Also you are now updating the bitmap and free_pages both inside and
>>>>> outside of the zone lock so that will likely have some impact.
>>>> So as per your previous suggestion, I have made the bitmap structure
>>>> object as a rcu protected pointer. So we are safe from that side.
>>>> The other downside which I can think of is a race where one page
>>>> trying to increment free_pages and other trying to decrements it.
>>>> However, being an atomic variable that should not be a problem.
>>>> Did I miss anything?
>>> I'm not so much worried about a race as the cache line bouncing
>>> effect. Basically your notifier combined within this hinting thread
>>> will likely result in more time spent by the thread that holds the
>>> lock since it will be trying to access the bitmap to set the bit and
>>> the free_pages to report the bit, but at the same time you will have
>>> this thread clearing bits and decrementing the free_pages values.
>>> One thing you could consider in your worker thread would be to do
>>> reallocate and replace the bitmap every time you plan to walk it. By
>>> doing that you would avoid the cacheline bouncing on the bitmap since
>>> you would only have to read it, and you would no longer have another
>>> thread dirtying it. You could essentially reset the free_pages at the
>>> same time you replace the bitmap. It would need to all happen with the
>>> zone lock held though when you swap it out.
>> If I am not mistaken then from what you are suggesting, I will have to hold
>> the zone lock for the entire duration of swap & scan which would be costly if
>> the bitmap is large, isn't? Also, we might end up missing free pages that are
>> getting
>> freed while we are scanning.
> You would only need to hold the zone lock when you swap the bitmap. Once
> it is swapped you wouldn't need to worry about the locking again for
> bitmap access since your worker thread would be the only one holding the
> current bitmap. Think of it as a batch clearing of the bits.

I see.

> You already end up missing pages freed while scanning since you are doing
> it linearly.

I was referring to free pages for whom bits will not be set while we
are doing the batch clearing of the bits.

>> As far as free_pages count is concerned, I am thinking if I should
>> replace it with zone->free_area[REPORTING_ORDER].nr_free which is already there
>> (I still need to explore this in a bit more depth).
>>> - Alex
> So there ends up being two ways you could use nr_free. One is to track it
> the way I did with the number of reported pages being tracked, however
> that requires reducing the count when reported pages are pulled from the
> free_area and identifying reported pages vs unreported ones.
> The other option would be to look at converting nr_free into a pair of
> free running counters, one tracking frees, and another tracking
> allocations. Then you just need to record a snapshot of the nr_free values
> when you do something like the bitmap swap, and then you would be able to
> track churn, but it wouldn't give you an exact count of unreported pages
> since it is possible to just alloc/free a single page multiple times to
> make it look like you have freed a number of pages even though you really
> haven't.

Yeah possibly. I will think about it a little bit more to see what
is the best way to do it.


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