* Luiz Capitulino | 2016-05-09 10:50:37 [-0400]:
>diff --git a/include/linux/locallock.h b/include/linux/locallock.h
>index 6fe5928..f4cd691 100644
>--- a/include/linux/locallock.h
>+++ b/include/linux/locallock.h
>@@ -104,6 +104,17 @@ static inline void __local_unlock(struct local_irq_lock *lv)
> put_local_var(lvar); \
> } while (0)
>
>+#define local_lock_other_cpu(lvar, cpu) \
I would prefer to have local_lock_on() to be in sync with
local_lock_irq_on()
>+ do { \
>+ __local_lock(&per_cpu(lvar, cpu)); \
>+ } while (0)
>+
>+#define local_unlock_other_cpu(lvar, cpu) \
>+ do { \
>+ __local_unlock(&per_cpu(lvar, cpu)); \
>+ } while (0)
>+
>+
> static inline void __local_lock_irq(struct local_irq_lock *lv)
> {
> spin_lock_irqsave(&lv->lock, lv->flags);
>@@ -163,6 +174,22 @@ static inline int __local_lock_irqsave(struct local_irq_lock *lv)
> _flags = per_cpu(lvar, cpu).flags; \
> } while (0)
>
>+#define local_lock_irqsave_other_cpu(lvar, _flags, cpu) \
why not local_lock_irq_on()?
>+ do { \
>+ if (cpu == smp_processor_id()) \
>+ local_lock_irqsave(lvar, _flags); \
>+ else \
>+ local_lock_other_cpu(lvar, cpu); \
>+ } while (0)
>+
…
>diff --git a/mm/swap.c b/mm/swap.c
>index ca194ae..84c3c21 100644
>--- a/mm/swap.c
>+++ b/mm/swap.c
>@@ -821,9 +821,9 @@ void lru_add_drain_cpu(int cpu)
> unsigned long flags;
>
> /* No harm done if a racing interrupt already did this */
>- local_lock_irqsave(rotate_lock, flags);
>+ local_lock_irqsave_other_cpu(rotate_lock, flags, cpu);
> pagevec_move_tail(pvec);
>- local_unlock_irqrestore(rotate_lock, flags);
>+ local_unlock_irqrestore_other_cpu(rotate_lock, flags, cpu);
This piece might be required independently of this patch. It would be
nice to have it in page_alloc_cpu_notify() :) I take care of this…
> }
>
> pvec = &per_cpu(lru_deactivate_file_pvecs, cpu);
>@@ -866,12 +866,32 @@ void lru_add_drain(void)
> local_unlock_cpu(swapvec_lock);
> }
>
>+static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);
>+
>+#ifdef CONFIG_PREEMPT_RT_BASE
>+static inline void remote_lru_add_drain(int cpu, struct cpumask *has_work)
>+{
>+ local_lock_other_cpu(swapvec_lock, cpu);
>+ lru_add_drain_cpu(cpu);
>+ local_unlock_other_cpu(swapvec_lock, cpu);
>+}
>+#else
> static void lru_add_drain_per_cpu(struct work_struct *dummy)
> {
> lru_add_drain();
> }
>
>-static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);
>+static inline void remote_lru_add_drain(int cpu, struct cpumask *has_work)
>+{
>+ struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
>+
>+ INIT_WORK(work, lru_add_drain_per_cpu);
>+ schedule_work_on(cpu, work);
>+ cpumask_set_cpu(cpu, has_work);
>+
>+}
>+#endif
>+
So on RT you use the local locks to perform $work which should be done
on the remote CPU locally. RT's local locks help here. You can't do this
in the !RT case because we optimize the local locks away so you have to
stick with the schedule_work_on() way of dealing with it.
In your description you write
>lru_add_drain_all() works by scheduling lru_add_drain_cpu() to run
>on all CPUs that have non-empty LRU pagevecs and then waiting for
>the scheduled work to complete. However, workqueue threads may never
>have the chance to run on a CPU that's running a SCHED_FIFO task.
>This causes lru_add_drain_all() to block forever.
It might block forever but not necessarily. By default the scheduler
would push RT tasks off the CPU if they run for too long. So you have
disabled this (and I don't complain about it). And once this does not
happen, there is nothing to force the workqueue to run.
But this is not a RT-only problem, is it? You could run high prio RT
task on a CPU and the workqueue would never get on the CPU as well.
Sebastian
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