[PATCH] scsi: libfc: fix seconds_since_last_reset calculation

[PATCH] scsi: libfc: fix seconds_since_last_reset calculation

[Arnd Bergmann]

The fc_get_host_stats() function contains a complex conversion
from jiffies to timespec to seconds. As we try to get rid of
uses of struct timespec, we can clean this up and replace it
with a simpler computation.

Simply dividing the difference in jiffies by HZ is not only
much more efficient, it also avoids a problem that causes the
seconds_since_last_reset value to be incorrect if jiffies has
overrun since the 'boot_time' value was recorded.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/scsi/libfc/fc_lport.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/scsi/libfc/fc_lport.c b/drivers/scsi/libfc/fc_lport.c
index e01a29863c38..99c6d9251404 100644
--- a/drivers/scsi/libfc/fc_lport.c
+++ b/drivers/scsi/libfc/fc_lport.c
@@ -301,7 +301,6 @@ struct fc_host_statistics *fc_get_host_stats(struct Scsi_Host *shost)
 {
 	struct fc_host_statistics *fc_stats;
 	struct fc_lport *lport = shost_priv(shost);
-	struct timespec v0, v1;
 	unsigned int cpu;
 	u64 fcp_in_bytes = 0;
 	u64 fcp_out_bytes = 0;
@@ -309,9 +308,7 @@ struct fc_host_statistics *fc_get_host_stats(struct Scsi_Host *shost)
 	fc_stats = &lport->host_stats;
 	memset(fc_stats, 0, sizeof(struct fc_host_statistics));
 
-	jiffies_to_timespec(jiffies, &v0);
-	jiffies_to_timespec(lport->boot_time, &v1);
-	fc_stats->seconds_since_last_reset = (v0.tv_sec - v1.tv_sec);
+	fc_stats->seconds_since_last_reset = (lport->boot_time - jiffies) / HZ;
 
 	for_each_possible_cpu(cpu) {
 		struct fc_stats *stats;
-- 
2.9.0

Re: [PATCH] scsi: libfc: fix seconds_since_last_reset calculation

[Bart Van Assche]

On 06/17/2016 05:47 PM, Arnd Bergmann wrote:
> -	jiffies_to_timespec(jiffies, &v0);
> -	jiffies_to_timespec(lport->boot_time, &v1);
> -	fc_stats->seconds_since_last_reset = (v0.tv_sec - v1.tv_sec);
> +	fc_stats->seconds_since_last_reset = (lport->boot_time - jiffies) / HZ;

Does this patch introduce a 64-bit division? There might still be 32-bit 
users of this code.

Bart.

Re: [PATCH] scsi: libfc: fix seconds_since_last_reset calculation

[Arnd Bergmann]

On Monday, June 20, 2016 3:54:06 PM CEST Bart Van Assche wrote:
> On 06/17/2016 05:47 PM, Arnd Bergmann wrote:
> > -     jiffies_to_timespec(jiffies, &v0);
> > -     jiffies_to_timespec(lport->boot_time, &v1);
> > -     fc_stats->seconds_since_last_reset = (v0.tv_sec - v1.tv_sec);
> > +     fc_stats->seconds_since_last_reset = (lport->boot_time - jiffies) / HZ;
> 
> Does this patch introduce a 64-bit division? There might still be 32-bit 
> users of this code.

No, both lport->boot_time and jiffies are 'unsigned long'.

	Arnd

Re: [PATCH] scsi: libfc: fix seconds_since_last_reset calculation

[Bart Van Assche]

On 06/20/2016 04:48 PM, Arnd Bergmann wrote:
> On Monday, June 20, 2016 3:54:06 PM CEST Bart Van Assche wrote:
>> On 06/17/2016 05:47 PM, Arnd Bergmann wrote:
>>> -     jiffies_to_timespec(jiffies, &v0);
>>> -     jiffies_to_timespec(lport->boot_time, &v1);
>>> -     fc_stats->seconds_since_last_reset = (v0.tv_sec - v1.tv_sec);
>>> +     fc_stats->seconds_since_last_reset = (lport->boot_time - jiffies) / HZ;
>>
>> Does this patch introduce a 64-bit division? There might still be 32-bit
>> users of this code.
>
> No, both lport->boot_time and jiffies are 'unsigned long'.

Ah, you're right. Hence:

Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>

Re: [PATCH] scsi: libfc: fix seconds_since_last_reset calculation

[Martin K. Petersen]

>>>>> "Arnd" == Arnd Bergmann <arnd@arndb.de> writes:

Arnd> The fc_get_host_stats() function contains a complex conversion
Arnd> from jiffies to timespec to seconds. As we try to get rid of uses
Arnd> of struct timespec, we can clean this up and replace it with a
Arnd> simpler computation.

Arnd> Simply dividing the difference in jiffies by HZ is not only much
Arnd> more efficient, it also avoids a problem that causes the
Arnd> seconds_since_last_reset value to be incorrect if jiffies has
Arnd> overrun since the 'boot_time' value was recorded.

Applied to 4.8/scsi-queue.

-- 
Martin K. Petersen	Oracle Linux Engineering