Xunlei Pang <[email protected]> writes:
> The current condition to judge clock drift in expire_cfs_rq_runtime()
> is wrong, the two runtime_expires are actually the same when clock
> drift happens, so this condtion can never hit. The orginal design was
> correctly done by commit a9cf55b28610 ("sched: Expire invalid runtime"),
> but was changed to be the current one due to its locking issue.
>
> This patch introduces another way, it adds a new field in both structure
> cfs_rq and cfs_bandwidth to record the expiration update sequence, and
> use them to figure out if clock drift happens(true if they equal).
It might just be simplest to revert the comparison change - if we read a
torn value, the worst that happens is we extend incorrectly, and that
is exactly what happens if we just read the old value.
An extra int isn't exactly the worst thing though, so whichever.
>
> This fix is also needed by the following patch.
>
> Fixes: 51f2176d74ac ("sched/fair: Fix unlocked reads of some
> cfs_b->quota/period")
> Cc: Ben Segall <[email protected]>
> Signed-off-by: Xunlei Pang <[email protected]>
> ---
> kernel/sched/fair.c | 14 ++++++++------
> kernel/sched/sched.h | 6 ++++--
> 2 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e497c05aab7f..9f384264e832 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4590,6 +4590,7 @@ void __refill_cfs_bandwidth_runtime(struct
> cfs_bandwidth *cfs_b)
> now = sched_clock_cpu(smp_processor_id());
> cfs_b->runtime = cfs_b->quota;
> cfs_b->runtime_expires = now + ktime_to_ns(cfs_b->period);
> + cfs_b->expires_seq++;
> }
>
> static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> @@ -4612,6 +4613,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> struct task_group *tg = cfs_rq->tg;
> struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(tg);
> u64 amount = 0, min_amount, expires;
> + int expires_seq;
>
> /* note: this is a positive sum as runtime_remaining <= 0 */
> min_amount = sched_cfs_bandwidth_slice() - cfs_rq->runtime_remaining;
> @@ -4629,6 +4631,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> }
> }
> expires = cfs_b->runtime_expires;
> + expires_seq = cfs_b->expires_seq;
> raw_spin_unlock(&cfs_b->lock);
>
> cfs_rq->runtime_remaining += amount;
> @@ -4637,8 +4640,10 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> * spread between our sched_clock and the one on which runtime was
> * issued.
> */
> - if ((s64)(expires - cfs_rq->runtime_expires) > 0)
> + if ((s64)(expires - cfs_rq->runtime_expires) > 0) {
> cfs_rq->runtime_expires = expires;
> + cfs_rq->expires_seq = expires_seq;
> + }
>
> return cfs_rq->runtime_remaining > 0;
> }
> @@ -4664,12 +4669,9 @@ static void expire_cfs_rq_runtime(struct cfs_rq
> *cfs_rq)
> * has not truly expired.
> *
> * Fortunately we can check determine whether this the case by checking
> - * whether the global deadline has advanced. It is valid to compare
> - * cfs_b->runtime_expires without any locks since we only care about
> - * exact equality, so a partial write will still work.
> + * whether the global deadline(cfs_b->expires_seq) has advanced.
> */
> -
> - if (cfs_rq->runtime_expires != cfs_b->runtime_expires) {
> + if (cfs_rq->expires_seq == cfs_b->expires_seq) {
> /* extend local deadline, drift is bounded above by 2 ticks */
> cfs_rq->runtime_expires += TICK_NSEC;
> } else {
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 6601baf2361c..e977e04f8daf 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -334,9 +334,10 @@ struct cfs_bandwidth {
> u64 runtime;
> s64 hierarchical_quota;
> u64 runtime_expires;
> + int expires_seq;
>
> - int idle;
> - int period_active;
> + short idle;
> + short period_active;
> struct hrtimer period_timer;
> struct hrtimer slack_timer;
> struct list_head throttled_cfs_rq;
> @@ -551,6 +552,7 @@ struct cfs_rq {
>
> #ifdef CONFIG_CFS_BANDWIDTH
> int runtime_enabled;
> + int expires_seq;
> u64 runtime_expires;
> s64 runtime_remaining;
