2022-08-16 05:37:02

by Christophe JAILLET

Subject: [PATCH v2] scsi: target: Save a few cycles in transport_lookup_[cmd|tmr]_lun()

Use percpu_ref_tryget_live_rcu() instead of percpu_ref_tryget_live() to
save a few cycles when it is known that the rcu lock is already
taken/released.

Signed-off-by: Christophe JAILLET <[email protected]>
---
Change in v2:
* sync with -next

v1:
* https://lore.kernel.org/all/e4a21bc607c39935cb98d4825cd63ba349820550.1635974637.git.christophe.jaillet@wanadoo.fr/
---
drivers/target/target_core_device.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/target/target_core_device.c b/drivers/target/target_core_device.c
index b7f16ee8aa0e..76c6613a1da7 100644
--- a/drivers/target/target_core_device.c
+++ b/drivers/target/target_core_device.c
@@ -77,7 +77,7 @@ transport_lookup_cmd_lun(struct se_cmd *se_cmd)

se_lun = deve->se_lun;

- if (!percpu_ref_tryget_live(&se_lun->lun_ref)) {
+ if (!percpu_ref_tryget_live_rcu(&se_lun->lun_ref)) {
se_lun = NULL;
goto out_unlock;
}
@@ -154,7 +154,7 @@ int transport_lookup_tmr_lun(struct se_cmd *se_cmd)
if (deve) {
se_lun = deve->se_lun;

- if (!percpu_ref_tryget_live(&se_lun->lun_ref)) {
+ if (!percpu_ref_tryget_live_rcu(&se_lun->lun_ref)) {
se_lun = NULL;
goto out_unlock;
}
--
2.34.1

2022-08-16 08:26:18

by Chaitanya Kulkarni

Subject: Re: [PATCH v2] scsi: target: Save a few cycles in transport_lookup_[cmd|tmr]_lun()

On 8/15/22 13:52, Christophe JAILLET wrote:
> Use percpu_ref_tryget_live_rcu() instead of percpu_ref_tryget_live() to
> save a few cycles when it is known that the rcu lock is already
> taken/released.
>
> Signed-off-by: Christophe JAILLET <[email protected]>
> ---

do you have a quantitative data that shows actual savings of cycles?

-ck

2022-08-16 10:19:58

by Christophe JAILLET

Subject: Re: [PATCH v2] scsi: target: Save a few cycles in transport_lookup_[cmd|tmr]_lun()

Le 16/08/2022 à 08:06, Chaitanya Kulkarni a écrit :
> On 8/15/22 13:52, Christophe JAILLET wrote:
>> Use percpu_ref_tryget_live_rcu() instead of percpu_ref_tryget_live() to
>> save a few cycles when it is known that the rcu lock is already
>> taken/released.
>>
>> Signed-off-by: Christophe JAILLET <[email protected]>
>> ---
> do you have a quantitative data that shows actual savings of cycles?
>
> -ck
>

Some numbers were given for io_uring by the one who introduced
percpu_ref_tryget_live_rcu().

See [1].

I don't have specific numbers for the patch against scsi.

CJ


[1]:
https://lore.kernel.org/linux-kernel/[email protected]/