Re: [RFC PATCH v1 12/13] mm: split up release_pages into non-sentinel and sentinel passes

[Daniel Jordan <daniel.m.jordan@oracle.com>]

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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