Messages in this thread | | | Date | Tue, 30 Jun 2020 18:55:02 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH v6 2/2] sched/uclamp: Protect uclamp fast path code with static key |
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On 06/30/20 19:07, Peter Zijlstra wrote: > On Tue, Jun 30, 2020 at 12:21:23PM +0100, Qais Yousef wrote: > > @@ -993,10 +1013,38 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p, > > > > lockdep_assert_held(&rq->lock); > > > > + /* > > + * If sched_uclamp_used was enabled after task @p was enqueued, > > + * we could end up with unbalanced call to uclamp_rq_dec_id(). > > + * > > + * In this case the uc_se->active flag should be false since no uclamp > > + * accounting was performed at enqueue time and we can just return > > + * here. > > + * > > + * Need to be careful of the following enqeueue/dequeue ordering > > + * problem too > > + * > > + * enqueue(taskA) > > + * // sched_uclamp_used gets enabled > > + * enqueue(taskB) > > + * dequeue(taskA) > > + * // Must not decrement bukcet->tasks here > > + * dequeue(taskB) > > + * > > + * where we could end up with stale data in uc_se and > > + * bucket[uc_se->bucket_id]. > > + * > > + * The following check here eliminates the possibility of such race. > > + */ > > + if (unlikely(!uc_se->active)) > > + return; > > + > > bucket = &uc_rq->bucket[uc_se->bucket_id]; > > + > > SCHED_WARN_ON(!bucket->tasks); > > if (likely(bucket->tasks)) > > bucket->tasks--; > > + > > uc_se->active = false; > > > > /* > > > @@ -1221,6 +1289,8 @@ static void __setscheduler_uclamp(struct task_struct *p, > > if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP))) > > return; > > > > + static_branch_enable(&sched_uclamp_used); > > + > > if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) { > > uclamp_se_set(&p->uclamp_req[UCLAMP_MIN], > > attr->sched_util_min, true); > > @@ -7387,6 +7457,8 @@ static ssize_t cpu_uclamp_write(struct kernfs_open_file *of, char *buf, > > if (req.ret) > > return req.ret; > > > > + static_branch_enable(&sched_uclamp_used); > > + > > mutex_lock(&uclamp_mutex); > > rcu_read_lock(); > > > > There's a fun race described in 9107c89e269d ("perf: Fix race between > event install and jump_labels"), are we sure this isn't also susceptible > to something similar? > > I suspect not, but I just wanted to make sure.
IIUC, the worry is that not all CPUs might have observed the change in the static key state; hence could not be running the patched enqueue/dequeue_task(), so we could end up with some CPUs accounting for uclamp in the enqueue/dequeue path but not others?
I was hoping this synchronization is guaranteed by the static_branch_*() call.
aarch64_insn_patch_text_nosync() in arch/arm64/kernel/insn.c performs __flush_icache_range() after writing the new instruction.
I need to dig into what __flush_icache_range() do, but I think it'll just flush the icache of the calling CPU. Need to dig into upper layers too.
It'd be good to know if I got you correctly first.
Thanks
-- Qais Yousef
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