From: Coly Li <[email protected]>
[ Upstream commit 9fcc34b1a6dd4b8e5337e2b6ef45e428897eca6b ]
In bch_mca_scan(), the number of shrinking btree node is calculated
by code like this,
unsigned long nr = sc->nr_to_scan;
nr /= c->btree_pages;
nr = min_t(unsigned long, nr, mca_can_free(c));
variable sc->nr_to_scan is number of objects (here is bcache B+tree
nodes' number) to shrink, and pointer variable sc is sent from memory
management code as parametr of a callback.
If sc->nr_to_scan is smaller than c->btree_pages, after the above
calculation, variable 'nr' will be 0 and nothing will be shrunk. It is
frequeently observed that only 1 or 2 is set to sc->nr_to_scan and make
nr to be zero. Then bch_mca_scan() will do nothing more then acquiring
and releasing mutex c->bucket_lock.
This patch checkes whether nr is 0 after the above calculation, if 0
is the result then set 1 to variable 'n'. Then at least bch_mca_scan()
will try to shrink a single B+tree node.
Signed-off-by: Coly Li <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---
drivers/md/bcache/btree.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 9406326216f17..96a6583e7b522 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -685,6 +685,8 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
* IO can always make forward progress:
*/
nr /= c->btree_pages;
+ if (nr == 0)
+ nr = 1;
nr = min_t(unsigned long, nr, mca_can_free(c));
i = 0;
--
2.20.1
>>>>> "Sasha" == Sasha Levin <[email protected]> writes:
Sasha> From: Coly Li <[email protected]>
Sasha> [ Upstream commit 9fcc34b1a6dd4b8e5337e2b6ef45e428897eca6b ]
Sasha> In bch_mca_scan(), the number of shrinking btree node is calculated
Sasha> by code like this,
Sasha> unsigned long nr = sc->nr_to_scan;
Sasha> nr /= c->btree_pages;
Sasha> nr = min_t(unsigned long, nr, mca_can_free(c));
Sasha> variable sc->nr_to_scan is number of objects (here is bcache B+tree
Sasha> nodes' number) to shrink, and pointer variable sc is sent from memory
Sasha> management code as parametr of a callback.
Sasha> If sc->nr_to_scan is smaller than c->btree_pages, after the above
Sasha> calculation, variable 'nr' will be 0 and nothing will be shrunk. It is
Sasha> frequeently observed that only 1 or 2 is set to sc->nr_to_scan and make
Sasha> nr to be zero. Then bch_mca_scan() will do nothing more then acquiring
Sasha> and releasing mutex c->bucket_lock.
Sasha> This patch checkes whether nr is 0 after the above calculation, if 0
Sasha> is the result then set 1 to variable 'n'. Then at least bch_mca_scan()
Sasha> will try to shrink a single B+tree node.
Sasha> nr /= c->btree_pages;
Sasha> + if (nr == 0)
Sasha> + nr = 1;
Wouldn't it be even more clear with:
nr /= c->bree_pages || 1;
instead?
John
On 2019/12/12 11:48 上午, John Stoffel wrote:
>>>>>> "Sasha" == Sasha Levin <[email protected]> writes:
>
> Sasha> From: Coly Li <[email protected]>
> Sasha> [ Upstream commit 9fcc34b1a6dd4b8e5337e2b6ef45e428897eca6b ]
>
> Sasha> In bch_mca_scan(), the number of shrinking btree node is calculated
> Sasha> by code like this,
> Sasha> unsigned long nr = sc->nr_to_scan;
>
> Sasha> nr /= c->btree_pages;
> Sasha> nr = min_t(unsigned long, nr, mca_can_free(c));
> Sasha> variable sc->nr_to_scan is number of objects (here is bcache B+tree
> Sasha> nodes' number) to shrink, and pointer variable sc is sent from memory
> Sasha> management code as parametr of a callback.
>
> Sasha> If sc->nr_to_scan is smaller than c->btree_pages, after the above
> Sasha> calculation, variable 'nr' will be 0 and nothing will be shrunk. It is
> Sasha> frequeently observed that only 1 or 2 is set to sc->nr_to_scan and make
> Sasha> nr to be zero. Then bch_mca_scan() will do nothing more then acquiring
> Sasha> and releasing mutex c->bucket_lock.
>
> Sasha> This patch checkes whether nr is 0 after the above calculation, if 0
> Sasha> is the result then set 1 to variable 'n'. Then at least bch_mca_scan()
> Sasha> will try to shrink a single B+tree node.
>
> Sasha> nr /= c->btree_pages;
> Sasha> + if (nr == 0)
> Sasha> + nr = 1;
>
>
> Wouldn't it be even more clear with:
>
> nr /= c->bree_pages || 1;
>
> instead?
No, it is not more clear. At least to me, I may confuse does it mean,
- nr = (nr / c->btree_pages) || 1;
- or nr = nr / (c->btree_pages || 1)
If I don't check C manual, I am not able to tell the correct calculate
at first time.
Thanks.
--
Coly Li
>>>>> "Coly" == Coly Li <[email protected]> writes:
Coly> On 2019/12/12 11:48 上午, John Stoffel wrote:
>>>>>>> "Sasha" == Sasha Levin <[email protected]> writes:
>>
Sasha> From: Coly Li <[email protected]>
Sasha> [ Upstream commit 9fcc34b1a6dd4b8e5337e2b6ef45e428897eca6b ]
>>
Sasha> In bch_mca_scan(), the number of shrinking btree node is calculated
Sasha> by code like this,
Sasha> unsigned long nr = sc->nr_to_scan;
>>
Sasha> nr /= c->btree_pages;
Sasha> nr = min_t(unsigned long, nr, mca_can_free(c));
Sasha> variable sc->nr_to_scan is number of objects (here is bcache B+tree
Sasha> nodes' number) to shrink, and pointer variable sc is sent from memory
Sasha> management code as parametr of a callback.
>>
Sasha> If sc->nr_to_scan is smaller than c->btree_pages, after the above
Sasha> calculation, variable 'nr' will be 0 and nothing will be shrunk. It is
Sasha> frequeently observed that only 1 or 2 is set to sc->nr_to_scan and make
Sasha> nr to be zero. Then bch_mca_scan() will do nothing more then acquiring
Sasha> and releasing mutex c->bucket_lock.
>>
Sasha> This patch checkes whether nr is 0 after the above calculation, if 0
Sasha> is the result then set 1 to variable 'n'. Then at least bch_mca_scan()
Sasha> will try to shrink a single B+tree node.
>>
Sasha> nr /= c->btree_pages;
Sasha> + if (nr == 0)
Sasha> + nr = 1;
>>
>>
>> Wouldn't it be even more clear with:
>>
>> nr /= c->bree_pages || 1;
>>
>> instead?
Coly> No, it is not more clear. At least to me, I may confuse does it mean,
Coly> - nr = (nr / c->btree_pages) || 1;
Coly> - or nr = nr / (c->btree_pages || 1)
Coly> If I don't check C manual, I am not able to tell the correct
Coly> calculate at first time.
You're right, it's not quite as clear, it needs proper parenthesis.
But maybe instead of a (possibly) expensive division all the time, why
not just shift and assume you have it shrink a node, or try to.
I honestly haven't looked closely enough at the code to figure out the
best shift to use here. But isn't this calculation wrong anyway? If
you have lots of c->bree_pages, wouldn't you want to do more freeing?
I'd need to read the code better, but I'm heading to bed now. Sorry.
John