Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

Subhra Mazumdar
2019-03-20T02:32:18Z

On 3/18/19 8:41 AM, Julien Desfossez wrote:
> The case where we try to acquire the lock on 2 runqueues belonging to 2
> different cores requires the rq_lockp wrapper as well otherwise we
> frequently deadlock in there.
>
> This fixes the crash reported in
> 1552577311-8218-1-git-send-email-jdesfossez@digitalocean.com
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 76fee56..71bb71f 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2078,7 +2078,7 @@ static inline void double_rq_lock(struct rq *rq1, struct rq *rq2)
> raw_spin_lock(rq_lockp(rq1));
> __acquire(rq2->lock); /* Fake it out ;) */
> } else {
> - if (rq1 < rq2) {
> + if (rq_lockp(rq1) < rq_lockp(rq2)) {
> raw_spin_lock(rq_lockp(rq1));
> raw_spin_lock_nested(rq_lockp(rq2), SINGLE_DEPTH_NESTING);
> } else {
With this fix and my previous NULL pointer fix my stress tests are
surviving. I
re-ran my 2 DB instance setup on 44 core 2 socket system by putting each DB
instance in separate core scheduling group. The numbers look much worse
now.

users  baseline  %stdev  %idle  core_sched  %stdev %idle
16     1         0.3     66     -73.4%      136.8 82
24     1         1.6     54     -95.8%      133.2 81
32     1         1.5     42     -97.5%      124.3 89

I also notice that if I enable a bunch of debug configs related to
mutexes, spin
locks, lockdep etc. (which I did earlier to debug the dead lock), it
opens up a
can of worms with multiple crashes.