Re: [PATCHv3 1/3] z3fold: make counters atomic

[Dan Streetman <ddstreet@ieee.org>]

On Tue, Nov 1, 2016 at 5:03 PM, Vitaly Wool <vitalywool@gmail.com> wrote:
> On Tue, Nov 1, 2016 at 9:03 PM, Dan Streetman <ddstreet@ieee.org> wrote:
>> On Thu, Oct 27, 2016 at 7:08 AM, Vitaly Wool <vitalywool@gmail.com> wrote:
>>> This patch converts pages_nr per-pool counter to atomic64_t.
>>> It also introduces a new counter, unbuddied_nr, which is
>>> atomic64_t, too, to track the number of unbuddied (compactable)
>>> z3fold pages.
>>
>> so, with the use of workqueues, do we still need the unbuddied_nr
>> counter?  It doesn't seem to be used in later patches?
>
> I'm going to add sysfs/debugfs accounting a bit later so it won't rest unused.
>
> Also, with a per-page lock (if I come up with something reasonable) it could
> be still worth going back to shrinker.

let's wait to add it once the code that uses it is ready, yeah?

>
>> changing the pages_nr to atomic is a good idea though, so we can
>> safely read it without needing the pool lock.
>>
>>>
>>> Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
>>> ---
>>>  mm/z3fold.c | 33 +++++++++++++++++++++++++--------
>>>  1 file changed, 25 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>>> index 8f9e89c..5ac325a 100644
>>> --- a/mm/z3fold.c
>>> +++ b/mm/z3fold.c
>>> @@ -69,6 +69,7 @@ struct z3fold_ops {
>>>   * @lru:       list tracking the z3fold pages in LRU order by most recently
>>>   *             added buddy.
>>>   * @pages_nr:  number of z3fold pages in the pool.
>>> + * @unbuddied_nr:      number of unbuddied z3fold pages in the pool.
>>>   * @ops:       pointer to a structure of user defined operations specified at
>>>   *             pool creation time.
>>>   *
>>> @@ -80,7 +81,8 @@ struct z3fold_pool {
>>>         struct list_head unbuddied[NCHUNKS];
>>>         struct list_head buddied;
>>>         struct list_head lru;
>>> -       u64 pages_nr;
>>> +       atomic64_t pages_nr;
>>> +       atomic64_t unbuddied_nr;
>>>         const struct z3fold_ops *ops;
>>>         struct zpool *zpool;
>>>         const struct zpool_ops *zpool_ops;
>>> @@ -234,7 +236,8 @@ static struct z3fold_pool *z3fold_create_pool(gfp_t gfp,
>>>                 INIT_LIST_HEAD(&pool->unbuddied[i]);
>>>         INIT_LIST_HEAD(&pool->buddied);
>>>         INIT_LIST_HEAD(&pool->lru);
>>> -       pool->pages_nr = 0;
>>> +       atomic64_set(&pool->pages_nr, 0);
>>> +       atomic64_set(&pool->unbuddied_nr, 0);
>>>         pool->ops = ops;
>>>         return pool;
>>>  }
>>> @@ -334,6 +337,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>>>                                         continue;
>>>                                 }
>>>                                 list_del(&zhdr->buddy);
>>> +                               atomic64_dec(&pool->unbuddied_nr);
>>>                                 goto found;
>>>                         }
>>>                 }
>>> @@ -346,7 +350,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>>>         if (!page)
>>>                 return -ENOMEM;
>>>         spin_lock(&pool->lock);
>>> -       pool->pages_nr++;
>>> +       atomic64_inc(&pool->pages_nr);
>>>         zhdr = init_z3fold_page(page);
>>>
>>>         if (bud == HEADLESS) {
>>> @@ -369,6 +373,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>>>                 /* Add to unbuddied list */
>>>                 freechunks = num_free_chunks(zhdr);
>>>                 list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
>>> +               atomic64_inc(&pool->unbuddied_nr);
>>>         } else {
>>>                 /* Add to buddied list */
>>>                 list_add(&zhdr->buddy, &pool->buddied);
>>> @@ -412,6 +417,10 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
>>>                 /* HEADLESS page stored */
>>>                 bud = HEADLESS;
>>>         } else {
>>> +               bool is_unbuddied = zhdr->first_chunks == 0 ||
>>> +                               zhdr->middle_chunks == 0 ||
>>> +                               zhdr->last_chunks == 0;
>>> +
>>>                 bud = handle_to_buddy(handle);
>>>
>>>                 switch (bud) {
>>> @@ -431,6 +440,8 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
>>>                         spin_unlock(&pool->lock);
>>>                         return;
>>>                 }
>>> +               if (is_unbuddied)
>>> +                       atomic64_dec(&pool->unbuddied_nr);
>>>         }
>>>
>>>         if (test_bit(UNDER_RECLAIM, &page->private)) {
>>> @@ -451,12 +462,13 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
>>>                 list_del(&page->lru);
>>>                 clear_bit(PAGE_HEADLESS, &page->private);
>>>                 free_z3fold_page(zhdr);
>>> -               pool->pages_nr--;
>>> +               atomic64_dec(&pool->pages_nr);
>>>         } else {
>>>                 z3fold_compact_page(zhdr);
>>>                 /* Add to the unbuddied list */
>>>                 freechunks = num_free_chunks(zhdr);
>>>                 list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
>>> +               atomic64_inc(&pool->unbuddied_nr);
>>>         }
>>>
>>>         spin_unlock(&pool->lock);
>>> @@ -520,6 +532,11 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>>>                 zhdr = page_address(page);
>>>                 if (!test_bit(PAGE_HEADLESS, &page->private)) {
>>>                         list_del(&zhdr->buddy);
>>> +                       if (zhdr->first_chunks == 0 ||
>>> +                           zhdr->middle_chunks == 0 ||
>>> +                           zhdr->last_chunks == 0)
>>> +                               atomic64_dec(&pool->unbuddied_nr);
>>> +
>>>                         /*
>>>                          * We need encode the handles before unlocking, since
>>>                          * we can race with free that will set
>>> @@ -569,7 +586,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>>>                          */
>>>                         clear_bit(PAGE_HEADLESS, &page->private);
>>>                         free_z3fold_page(zhdr);
>>> -                       pool->pages_nr--;
>>> +                       atomic64_dec(&pool->pages_nr);
>>>                         spin_unlock(&pool->lock);
>>>                         return 0;
>>>                 }  else if (!test_bit(PAGE_HEADLESS, &page->private)) {
>>> @@ -584,6 +601,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>>>                                 freechunks = num_free_chunks(zhdr);
>>>                                 list_add(&zhdr->buddy,
>>>                                          &pool->unbuddied[freechunks]);
>>> +                               atomic64_inc(&pool->unbuddied_nr);
>>>                         }
>>>                 }
>>>
>>> @@ -672,12 +690,11 @@ static void z3fold_unmap(struct z3fold_pool *pool, unsigned long handle)
>>>   * z3fold_get_pool_size() - gets the z3fold pool size in pages
>>>   * @pool:      pool whose size is being queried
>>>   *
>>> - * Returns: size in pages of the given pool.  The pool lock need not be
>>> - * taken to access pages_nr.
>>> + * Returns: size in pages of the given pool.
>>>   */
>>>  static u64 z3fold_get_pool_size(struct z3fold_pool *pool)
>>>  {
>>> -       return pool->pages_nr;
>>> +       return atomic64_read(&pool->pages_nr);
>>>  }
>>>
>>>  /*****************
>>> --
>>> 2.4.2

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