sched: Fix smp_call_function_single_async() usage for ILB

The recent commit: 90b5363acd47 ("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: 90b5363acd47 ("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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 95e457d..2cacc1e 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -637,41 +637,25 @@ void wake_up_nohz_cpu(int cpu)
                wake_up_idle_cpu(cpu);
 }
 
-static inline bool got_nohz_idle_kick(void)
+static void nohz_csd_func(void *info)
 {
-       int cpu = smp_processor_id();
-
-       if (!(atomic_read(nohz_flags(cpu)) & NOHZ_KICK_MASK))
-               return false;
-
-       if (idle_cpu(cpu) && !need_resched())
-               return true;
+       struct rq *rq = info;
+       int cpu = cpu_of(rq);
+       unsigned int flags;
 
        /*
-        * We can't run Idle Load Balance on this CPU for this time so we
-        * cancel it and clear NOHZ_BALANCE_KICK
+        * Release the rq::nohz_csd.
         */
-       atomic_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
-       return false;
-}
+       flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
+       WARN_ON(!(flags & NOHZ_KICK_MASK));
 
-static void nohz_csd_func(void *info)
-{
-       struct rq *rq = info;
-
-       if (got_nohz_idle_kick()) {
-               rq->idle_balance = 1;
+       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

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dda9b19..2890bd5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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 @@ abort:
  */
 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);

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4b32cff..3c163cb 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -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;