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

[Vitaly Wool <vitalywool@gmail.com>]

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.

> 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