Hi Mike---
>Hi Tang Junhui---
>
>I'm not really sure about this one. It changes the semantics of the
>amount of work done-- nr_to_scan now means number of things to free
>instead of the number to check.
>
The code seems to be designed as that, sc->nr_to_scan marks how much btree
nodes to scan in c->btree_cache_used, and it doesn't include the btree
nodes in c->btree_cache_freeable.
>If the system is under severe memory pressure, and most of the cache is
>essential/actively used, this could greatly increase the amount of work
>done.
>
Yes, I didn't relize this.
>As to the i < btree_cache_used, it's arguably a bug, but it only reduces
>the amount of work done if the cache is very, very small.
>
I think it is a bug, I'll send a new patch to add variable to record
c->btree_cache_used befor for(;;) loop.
>Mike
>
>On 01/30/2018 06:46 PM, tang.junhui@xxxxxxxxxx wrote:
>> From: Tang Junhui <tang.junhui@xxxxxxxxxx>
>>
>> In bch_mca_scan(), nr btree nodes are expected to shrink, so the for(;;)
>> loop need to satisfy the condition freed < nr.
>> And since c->btree_cache_used always decrease after mca_data_free() calling
>> in for(;;) loop, so we need a auto variable to record c->btree_cache_used
>> before the for(;;) loop.
>>
>> Signed-off-by: Tang Junhui <tang.junhui@xxxxxxxxxx>
>> ---
>> drivers/md/bcache/btree.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
>> index 81e8dc3..7e9ef3d 100644
>> --- a/drivers/md/bcache/btree.c
>> +++ b/drivers/md/bcache/btree.c
>> @@ -664,6 +664,7 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
>> struct btree *b, *t;
>> unsigned long i, nr = sc->nr_to_scan;
>> unsigned long freed = 0;
>> + unsigned int btree_cache_used;
>>
>> if (c->shrinker_disabled)
>> return SHRINK_STOP;
>> @@ -700,7 +701,8 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
>> }
>> }
>>
>> - for (i = 0; (nr--) && i < c->btree_cache_used; i++) {
>> + btree_cache_used = c->btree_cache_used;
>> + for (i = 0; freed < nr && i < btree_cache_used; i++) {
>> if (list_empty(&c->btree_cache))
>> goto out;
>>
>>
Thanks,
Tang Junhui
