Ocfs2 devel - Mar 2020 - [RFC PATCH v1 29/50] fs/ocfs2/journal: Use prandom_u32() and not /dev/random for timeout

get_random_bytes() is expensive crypto-quality random numbers.
If we're just doing random backoff, prandom_u32() is plenty.

(Not to mention fetching 8 bytes of seed material only to
reduce it modulo 5000 is a huge waste.)

Also, convert timeouts to jiffies at compile time; convert
milliseconds to jiffies before picking a random number in the
range to take advantage of compile-time constant folding.

Signed-off-by: George Spelvin <lkml at sdf.org>
Cc: Mark Fasheh <mark at fasheh.com>
Cc: Joel Becker <jlbec at evilplan.org>
Cc: Joseph Qi <joseph.qi at linux.alibaba.com>
Cc: ocfs2-devel at oss.oracle.com
---
 fs/ocfs2/journal.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
index 68ba354cf3610..939a12e57fa8b 100644
--- a/fs/ocfs2/journal.c
+++ b/fs/ocfs2/journal.c
@@ -1884,11 +1884,8 @@ int ocfs2_mark_dead_nodes(struct ocfs2_super *osb)
  */
 static inline unsigned long ocfs2_orphan_scan_timeout(void)
 {
-	unsigned long time;
-
-	get_random_bytes(&time, sizeof(time));
-	time = ORPHAN_SCAN_SCHEDULE_TIMEOUT + (time % 5000);
-	return msecs_to_jiffies(time);
+	return msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT) +
+		prandom_u32_max(5 * HZ);
 }
 
 /*
-- 
2.26.0

Sorry for the late reply since I might miss this mail.

On 2019/3/21 11:07, George Spelvin wrote:
> get_random_bytes() is expensive crypto-quality random numbers.
> If we're just doing random backoff, prandom_u32() is plenty.
> 
> (Not to mention fetching 8 bytes of seed material only to
> reduce it modulo 5000 is a huge waste.)
> 
> Also, convert timeouts to jiffies at compile time; convert
> milliseconds to jiffies before picking a random number in the
> range to take advantage of compile-time constant folding.
> 
> Signed-off-by: George Spelvin <lkml at sdf.org>
> Cc: Mark Fasheh <mark at fasheh.com>
> Cc: Joel Becker <jlbec at evilplan.org>
> Cc: Joseph Qi <joseph.qi at linux.alibaba.com>
> Cc: ocfs2-devel at oss.oracle.com
> ---
>  fs/ocfs2/journal.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
> index 68ba354cf3610..939a12e57fa8b 100644
> --- a/fs/ocfs2/journal.c
> +++ b/fs/ocfs2/journal.c
> @@ -1884,11 +1884,8 @@ int ocfs2_mark_dead_nodes(struct ocfs2_super *osb)
>   */
>  static inline unsigned long ocfs2_orphan_scan_timeout(void)
>  {
> -	unsigned long time;
> -
> -	get_random_bytes(&time, sizeof(time));
> -	time = ORPHAN_SCAN_SCHEDULE_TIMEOUT + (time % 5000);
> -	return msecs_to_jiffies(time);
> +	return msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT) +
> +		prandom_u32_max(5 * HZ);
Seems better include the prandom_u32_max() into msecs_to_jiffies()?

Thanks,
Joseph
>  }
>  
>  /*
>