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sched: Fix smp_call_function_single_async() usage for ILB

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The recent commit: 90b5363acd ("sched: Clean up scheduler_ipi()")
got smp_call_function_single_async() subtly wrong. Even though it will
return -EBUSY when trying to re-use a csd, that condition is not
atomic and still requires external serialization.

The change in kick_ilb() got this wrong.

While on first reading kick_ilb() has an atomic test-and-set that
appears to serialize the use, the matching 'release' is not in the
right place to actually guarantee this serialization.

Rework the nohz_idle_balance() trigger so that the release is in the
IPI callback and thus guarantees the required serialization for the
CSD.

Fixes: 90b5363acd ("sched: Clean up scheduler_ipi()")
Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: mgorman@techsingularity.net
Link: https://lore.kernel.org/r/20200526161907.778543557@infradead.org
This commit is contained in:
Peter Zijlstra 2020-05-26 18:10:58 +02:00 committed by Ingo Molnar
parent 58ef57b16d
commit 19a1f5ec69
3 changed files with 20 additions and 37 deletions
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38
kernel/sched/core.c
View File

@ -637,41 +637,25 @@ void wake_up_nohz_cpu(int cpu)
wake_up_idle_cpu(cpu);
}
static inline bool got_nohz_idle_kick(void)
{
int cpu = smp_processor_id();
if (!(atomic_read(nohz_flags(cpu)) & NOHZ_KICK_MASK))
return false;
if (idle_cpu(cpu) && !need_resched())
return true;
/*
* We can't run Idle Load Balance on this CPU for this time so we
* cancel it and clear NOHZ_BALANCE_KICK
*/
atomic_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
return false;
}
static void nohz_csd_func(void *info)
{
struct rq *rq = info;
int cpu = cpu_of(rq);
unsigned int flags;
if (got_nohz_idle_kick()) {
rq->idle_balance = 1;
/*
* Release the rq::nohz_csd.
*/
flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
WARN_ON(!(flags & NOHZ_KICK_MASK));
rq->idle_balance = idle_cpu(cpu);
if (rq->idle_balance && !need_resched()) {
rq->nohz_idle_balance = flags;
raise_softirq_irqoff(SCHED_SOFTIRQ);
}
}
#else /* CONFIG_NO_HZ_COMMON */
static inline bool got_nohz_idle_kick(void)
{
return false;
}
#endif /* CONFIG_NO_HZ_COMMON */
#ifdef CONFIG_NO_HZ_FULL

18
kernel/sched/fair.c
View File

@ -10024,6 +10024,10 @@ static void kick_ilb(unsigned int flags)
if (ilb_cpu >= nr_cpu_ids)
return;
/*
* Access to rq::nohz_csd is serialized by NOHZ_KICK_MASK; he who sets
* the first flag owns it; cleared by nohz_csd_func().
*/
flags = atomic_fetch_or(flags, nohz_flags(ilb_cpu));
if (flags & NOHZ_KICK_MASK)
return;
@ -10371,20 +10375,14 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
*/
static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
{
int this_cpu = this_rq->cpu;
unsigned int flags;
unsigned int flags = this_rq->nohz_idle_balance;
if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_KICK_MASK))
if (!flags)
return false;
if (idle != CPU_IDLE) {
atomic_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
return false;
}
this_rq->nohz_idle_balance = 0;
/* could be _relaxed() */
flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
if (!(flags & NOHZ_KICK_MASK))
if (idle != CPU_IDLE)
return false;
_nohz_idle_balance(this_rq, flags, idle);

1
kernel/sched/sched.h
View File

@ -951,6 +951,7 @@ struct rq {
struct callback_head *balance_callback;
unsigned char nohz_idle_balance;
unsigned char idle_balance;
unsigned long misfit_task_load;