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SubjectRe: [RFC PATCH v1 12/13] mm: split up release_pages into non-sentinel and sentinel passes
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On 02/02/2018 12:00 PM, Laurent Dufour wrote:
> On 02/02/2018 15:40, Laurent Dufour wrote:
>>
>>
>> On 01/02/2018 00:04, daniel.m.jordan@oracle.com wrote:
>>> A common case in release_pages is for the 'pages' list to be in roughly
>>> the same order as they are in their LRU. With LRU batch locking, when a
>>> sentinel page is removed, an adjacent non-sentinel page must be promoted
>>> to a sentinel page to follow the locking scheme. So we can get behavior
>>> where nearly every page in the 'pages' array is treated as a sentinel
>>> page, hurting the scalability of this approach.
>>>
>>> To address this, split up release_pages into non-sentinel and sentinel
>>> passes so that the non-sentinel pages can be locked with an LRU batch
>>> lock before the sentinel pages are removed.
>>>
>>> For the prototype, just use a bitmap and a temporary outer loop to
>>> implement this.
>>>
>>> Performance numbers from a single microbenchmark at this point in the
>>> series are included in the next patch.
>>>
>>> Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
>>> ---
>>> mm/swap.c | 20 +++++++++++++++++++-
>>> 1 file changed, 19 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/swap.c b/mm/swap.c
>>> index fae766e035a4..a302224293ad 100644
>>> --- a/mm/swap.c
>>> +++ b/mm/swap.c
>>> @@ -731,6 +731,7 @@ void lru_add_drain_all(void)
>>> put_online_cpus();
>>> }
>>>
>>> +#define LRU_BITMAP_SIZE 512
>>> /**
>>> * release_pages - batched put_page()
>>> * @pages: array of pages to release
>>> @@ -742,16 +743,32 @@ void lru_add_drain_all(void)
>>> */
>>> void release_pages(struct page **pages, int nr)
>>> {
>>> - int i;
>>> + int h, i;
>>> LIST_HEAD(pages_to_free);
>>> struct pglist_data *locked_pgdat = NULL;
>>> spinlock_t *locked_lru_batch = NULL;
>>> struct lruvec *lruvec;
>>> unsigned long uninitialized_var(flags);
>>> + DECLARE_BITMAP(lru_bitmap, LRU_BITMAP_SIZE);
>>> +
>>> + VM_BUG_ON(nr > LRU_BITMAP_SIZE);
>>
>> While running your series rebased on v4.15-mmotm-2018-01-31-16-51, I'm
>> hitting this VM_BUG sometimes on a ppc64 system where page size is set to 64K.
>
> I can't see any link between nr and LRU_BITMAP_SIZE, caller may pass a
> larger list of pages which is not relative to the LRU list.

You're correct, I used the hard-coded size to quickly prototype, just to
see how this approach performs. That's unfortunate that it bit you.
> To move forward seeing the benefit of this series with the SPF one, I
> declared the bit map based on nr. This is still not a valid option but this
> at least allows to process all the passed pages.

Yes, the bitmap's not for the final version.

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