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SubjectRe: [PATCH v2 2/3] mm/page_reporting: Allow driver to specify threshold
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On 6/23/21 3:39 AM, Alexander Duyck wrote:
> On Mon, Jun 21, 2021 at 10:48 PM Gavin Shan <gshan@redhat.com> wrote:
>>
>> The page reporting threshold is currently sticky to @pageblock_order.
>> The page reporting can never be triggered because the freeing page
>> can't come up with a free area like that huge. The situation becomes
>> worse when the system memory becomes heavily fragmented.
>>
>> For example, the following configurations are used on ARM64 when 64KB
>> base page size is enabled. In this specific case, the page reporting
>> won't be triggered until the freeing page comes up with a 512MB free
>> area. That's hard to be met, especially when the system memory becomes
>> heavily fragmented.
>>
>> PAGE_SIZE: 64KB
>> HPAGE_SIZE: 512MB
>> pageblock_order: 13 (512MB)
>> MAX_ORDER: 14
>>
>> This allows the drivers to specify the threshold when the page
>> reporting device is registered. The threshold falls back to
>> @pageblock_order if it's not specified by the driver. The existing
>> users (hv_balloon and virtio_balloon) don't specify the threshold
>> and @pageblock_order is still taken as their page reporting order.
>> So this shouldn't introduce functional changes.
>>
>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>> ---
>> include/linux/page_reporting.h | 3 +++
>> mm/page_reporting.c | 14 ++++++++++----
>> mm/page_reporting.h | 10 ++--------
>> 3 files changed, 15 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
>> index 3b99e0ec24f2..fe648dfa3a7c 100644
>> --- a/include/linux/page_reporting.h
>> +++ b/include/linux/page_reporting.h
>> @@ -18,6 +18,9 @@ struct page_reporting_dev_info {
>>
>> /* Current state of page reporting */
>> atomic_t state;
>> +
>> + /* Minimal order of page reporting */
>> + unsigned int order;
>> };
>>
>> /* Tear-down and bring-up for page reporting devices */
>> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
>> index df9c5054e1b4..27670360bae6 100644
>> --- a/mm/page_reporting.c
>> +++ b/mm/page_reporting.c
>
> <snip>
>
>> @@ -324,6 +324,12 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>> goto err_out;
>> }
>>
>> + /*
>> + * We need to choose the minimal order of page reporting if it's
>> + * not specified by the driver.
>> + */
>> + prdev->order = prdev->order ? prdev->order : pageblock_order;
>> +
>> /* initialize state and work structures */
>> atomic_set(&prdev->state, PAGE_REPORTING_IDLE);
>> INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);
>
> Rather than using prdev->order directly it might be better to have a
> reporting_order value you could export for use by
> page_reporting_notify_free. That way you avoid the overhead of having
> to make a function call per page freed.
>

Yes, I obviously missed the point to reduce the overhead because of
function call. In next revision, I will introduce @page_reporting_order
for this. Besides, it will be exported as a module parameter so that
it can be changed dynamically, as David suggested before.

>> diff --git a/mm/page_reporting.h b/mm/page_reporting.h
>> index 2c385dd4ddbd..d9f972e72649 100644
>> --- a/mm/page_reporting.h
>> +++ b/mm/page_reporting.h
>> @@ -10,11 +10,9 @@
>> #include <linux/pgtable.h>
>> #include <linux/scatterlist.h>
>>
>> -#define PAGE_REPORTING_MIN_ORDER pageblock_order
>> -
>> #ifdef CONFIG_PAGE_REPORTING
>> DECLARE_STATIC_KEY_FALSE(page_reporting_enabled);
>> -void __page_reporting_notify(void);
>> +void __page_reporting_notify(unsigned int order);
>>
>> static inline bool page_reported(struct page *page)
>> {
>> @@ -37,12 +35,8 @@ static inline void page_reporting_notify_free(unsigned int order)
>> if (!static_branch_unlikely(&page_reporting_enabled))
>> return;
>>
>> - /* Determine if we have crossed reporting threshold */
>> - if (order < PAGE_REPORTING_MIN_ORDER)
>> - return;
>> -
>> /* This will add a few cycles, but should be called infrequently */
>> - __page_reporting_notify();
>> + __page_reporting_notify(order);
>> }
>> #else /* CONFIG_PAGE_REPORTING */
>> #define page_reported(_page) false
>
> With us making the function call per page freed we are likely to have
> a much more significant impact on performance with page reporting
> enabled. Ideally we want to limit this impact so that we only take the
> cost for the conditional check on the lower order pages.
>

Yep, thanks for the explanation, Alex.

Thanks,
Gavin

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