2007-08-03 18:45:00

by Paul E. McKenney

Subject: [PATCH RFC -rt] Allow rcutorture to handle synchronize_sched()

The rcutorture module uses a random-number generator that periodically
calls get_random_bytes() to add entropy. This random-number generator
is invoked from within the RCU read-side critical section, which, in the
case of the synchronize_sched() variant of RCU, has preeemption disabled.
Unfortunately, get_random_bytes() ends up acquiring normal spinlocks,
which can block in -rt, resulting in very large numbers of "scheduling
while atomic" messages.

This patch takes a very crude approach, simply substituting the time
of day for get_random_bytes().

Signed-off-by: Paul E. McKenney <[email protected]>
---

rcutorture.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

diff -urpNa -X dontdiff linux-2.6.22.1-rt4/kernel/rcutorture.c linux-2.6.22.1-rt4-rcutorturesched/kernel/rcutorture.c
--- linux-2.6.22.1-rt4/kernel/rcutorture.c 2007-07-21 16:58:22.000000000 -0700
+++ linux-2.6.22.1-rt4-rcutorturesched/kernel/rcutorture.c 2007-08-02 16:38:09.000000000 -0700
@@ -165,16 +165,13 @@ struct rcu_random_state {

/*
* Crude but fast random-number generator. Uses a linear congruential
- * generator, with occasional help from get_random_bytes().
+ * generator, with occasional help from the time-of-day clock.
*/
static unsigned long
rcu_random(struct rcu_random_state *rrsp)
{
- long refresh;
-
if (--rrsp->rrs_count < 0) {
- get_random_bytes(&refresh, sizeof(refresh));
- rrsp->rrs_state += refresh;
+ rrsp->rrs_state += xtime.tv_nsec;
rrsp->rrs_count = RCU_RANDOM_REFRESH;
}
rrsp->rrs_state = rrsp->rrs_state * RCU_RANDOM_MULT + RCU_RANDOM_ADD;

2007-08-03 22:38:19

by Josh Triplett

Subject: Re: [PATCH RFC -rt] Allow rcutorture to handle synchronize_sched()

On Fri, 2007-08-03 at 11:44 -0700, Paul E. McKenney wrote:
> The rcutorture module uses a random-number generator that periodically
> calls get_random_bytes() to add entropy. This random-number generator
> is invoked from within the RCU read-side critical section, which, in the
> case of the synchronize_sched() variant of RCU, has preeemption disabled.
> Unfortunately, get_random_bytes() ends up acquiring normal spinlocks,
> which can block in -rt, resulting in very large numbers of "scheduling
> while atomic" messages.
>
> This patch takes a very crude approach, simply substituting the time
> of day for get_random_bytes().
>
> Signed-off-by: Paul E. McKenney <[email protected]>

Acked-by: Josh Triplett <[email protected]>