Messages in this thread | | | Date | Mon, 30 Mar 2020 16:34:12 +0000 | From | George Spelvin <> | Subject | Re: [RFC PATCH v1 29/50] fs/ocfs2/journal: Use prandom_u32() and not /dev/random for timeout |
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On Mon, Mar 30, 2020 at 08:09:33PM +0800, Joseph Qi wrote: > Sorry for the late reply since I might miss this mail.
You're hardly late; I expect replies to dribble in for a week.
> On 2019/3/21 11:07, George Spelvin wrote: >> 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()?
What I'm trying to take advantage of here is constant propagation.
msecs_to_jiffies is zero cost (it's evaluated entirely at compile time) if its argument is a compile-time constant. It's a function call and a few instructions if its argument is variable.
msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT + prandom_u32_max(5000)) would be forced to use the expensive version.
The compiler does't know, but *I* know, that msecs_to_jiffies() is a linear function, and prandom_u32_max() is a sort-of linear function.
(It's a linear function for a given PRNG starting state, so each individual call is linear, but multiple calls mess things up.)
Modulo a bit of rounding, we have:
msecs_to_jiffies(a + b) = msecs_to_jiffies(a) + msecs_to_jiffies(b) msecs_to_jiffies(a) * b = msecs_to_jiffies(a * b) prandom_u32_max(a) * b = prandom_u32_max(a * b) prandom_u32_max(msecs_to_jiffies(a)) = msecs_to_jiffies(prandom_u32_max(a))
By doing the addition in jiffies rather than milliseconds, we get the cheap code. It's not a huge big deal, but it's definitely smaller and faster.
Admittedly, I happen to be using HZ = 300, which requires a multiply to convert, and makes the resultant random numbers slightly non-uniform. The default HZ = 250 makes it just a divide by 4, which is pretty simple.
(When HZ = 300, you get 1..3 ms -> 1 jiffy, 4..6 ms -> 2 jiffies, and 7..10 ms -> 3 jiffies. Multiples of 3 are 33% more likely to be chosen.)
It just seems silly and wasteful to pick a random number between 0 and 4999 (plus 30000), only to convert it to a random number between 0 and 1249 (plus 7500).
And if HZ = 2000 ever happens, the timeout won't be artificially limited to integer milliseconds.
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