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Subject[ANNOUNCE] v5.9.1-rt20
Dear RT folks!

I'm pleased to announce the v5.9.1-rt20 patch set.

Changes since v5.9.1-rt19:

- Tiny update to the rtmutex patches (make __read_rt_trylock()
static).

- The test_lockup module failed to compile. Reported by Fernando
Lopez-Lezcano.

- The `kcompactd' daemon together with MEMCG could have accessed
per-CPU variables in preemtible context.

- The patch for the crash in the block layer (previously reported by
David Runge) has been replaced with another set of patches which
were submitted upstream.

Known issues
- It has been pointed out that due to changes to the printk code the
internal buffer representation changed. This is only an issue if tools
like `crash' are used to extract the printk buffer from a kernel memory
image.

The delta patch against v5.9.1-rt19 is appended below and can be found here:

https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.9/incr/patch-5.9.1-rt19-rt20.patch.xz

You can get this release via the git tree at:

git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git v5.9.1-rt20

The RT patch against v5.9.1 can be found here:

https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.9/older/patch-5.9.1-rt20.patch.xz

The split quilt queue is available at:

https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.9/older/patches-5.9.1-rt20.tar.xz

Sebastian
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 99d2fb51e0e84..1f9b312595a19 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -41,7 +41,7 @@
#include "blk-mq-sched.h"
#include "blk-rq-qos.h"

-static DEFINE_PER_CPU(struct list_head, blk_cpu_done);
+static DEFINE_PER_CPU(struct llist_head, blk_cpu_done);

static void blk_mq_poll_stats_start(struct request_queue *q);
static void blk_mq_poll_stats_fn(struct blk_stat_callback *cb);
@@ -565,91 +565,49 @@ void blk_mq_end_request(struct request *rq, blk_status_t error)
}
EXPORT_SYMBOL(blk_mq_end_request);

-/*
- * Softirq action handler - move entries to local list and loop over them
- * while passing them to the queue registered handler.
- */
-static __latent_entropy void blk_done_softirq(struct softirq_action *h)
+static void blk_complete_reqs(struct llist_head *cpu_list)
{
- struct list_head *cpu_list, local_list;
+ struct llist_node *entry;
+ struct request *rq, *rq_next;

- local_irq_disable();
- cpu_list = this_cpu_ptr(&blk_cpu_done);
- list_replace_init(cpu_list, &local_list);
- local_irq_enable();
+ entry = llist_del_all(cpu_list);
+ entry = llist_reverse_order(entry);

- while (!list_empty(&local_list)) {
- struct request *rq;
-
- rq = list_entry(local_list.next, struct request, ipi_list);
- list_del_init(&rq->ipi_list);
+ llist_for_each_entry_safe(rq, rq_next, entry, ipi_list)
rq->q->mq_ops->complete(rq);
- }
}

-static void blk_mq_trigger_softirq(struct request *rq)
+static __latent_entropy void blk_done_softirq(struct softirq_action *h)
{
- struct list_head *list;
- unsigned long flags;
-
- local_irq_save(flags);
- list = this_cpu_ptr(&blk_cpu_done);
- list_add_tail(&rq->ipi_list, list);
-
- /*
- * If the list only contains our just added request, signal a raise of
- * the softirq. If there are already entries there, someone already
- * raised the irq but it hasn't run yet.
- */
- if (list->next == &rq->ipi_list)
- raise_softirq_irqoff(BLOCK_SOFTIRQ);
- local_irq_restore(flags);
- preempt_check_resched_rt();
+ blk_complete_reqs(this_cpu_ptr(&blk_cpu_done));
}

static int blk_softirq_cpu_dead(unsigned int cpu)
{
- /*
- * If a CPU goes away, splice its entries to the current CPU
- * and trigger a run of the softirq
- */
- local_irq_disable();
- list_splice_init(&per_cpu(blk_cpu_done, cpu),
- this_cpu_ptr(&blk_cpu_done));
- raise_softirq_irqoff(BLOCK_SOFTIRQ);
- local_irq_enable();
- preempt_check_resched_rt();
-
+ blk_complete_reqs(&per_cpu(blk_cpu_done, cpu));
return 0;
}

-
static void __blk_mq_complete_request_remote(void *data)
{
- struct request *rq = data;
-
- /*
- * For most of single queue controllers, there is only one irq vector
- * for handling I/O completion, and the only irq's affinity is set
- * to all possible CPUs. On most of ARCHs, this affinity means the irq
- * is handled on one specific CPU.
- *
- * So complete I/O requests in softirq context in case of single queue
- * devices to avoid degrading I/O performance due to irqsoff latency.
- */
- if (rq->q->nr_hw_queues == 1)
- blk_mq_trigger_softirq(rq);
- else
- rq->q->mq_ops->complete(rq);
+ __raise_softirq_irqoff(BLOCK_SOFTIRQ);
}

static inline bool blk_mq_complete_need_ipi(struct request *rq)
{
int cpu = raw_smp_processor_id();

- if (!IS_ENABLED(CONFIG_SMP) || IS_ENABLED(CONFIG_PREEMPT_RT) ||
+ if (!IS_ENABLED(CONFIG_SMP) ||
!test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags))
return false;
+ /*
+ * With force threaded interrupts enabled, raising softirq from an SMP
+ * function call will always result in waking the ksoftirqd thread.
+ * This is probably worse than completing the request on a different
+ * cache domain.
+ */
+ if (force_irqthreads)
+ return false;

/* same CPU or cache domain? Complete locally */
if (cpu == rq->mq_ctx->cpu ||
@@ -663,6 +621,7 @@ static inline bool blk_mq_complete_need_ipi(struct request *rq)

bool blk_mq_complete_request_remote(struct request *rq)
{
+ struct llist_head *cpu_list;
WRITE_ONCE(rq->state, MQ_RQ_COMPLETE);

/*
@@ -673,14 +632,21 @@ bool blk_mq_complete_request_remote(struct request *rq)
return false;

if (blk_mq_complete_need_ipi(rq)) {
- rq->csd.func = __blk_mq_complete_request_remote;
- rq->csd.info = rq;
- rq->csd.flags = 0;
- smp_call_function_single_async(rq->mq_ctx->cpu, &rq->csd);
+ unsigned int cpu;
+
+ cpu = rq->mq_ctx->cpu;
+ cpu_list = &per_cpu(blk_cpu_done, cpu);
+ if (llist_add(&rq->ipi_list, cpu_list)) {
+ rq->csd.func = __blk_mq_complete_request_remote;
+ rq->csd.flags = 0;
+ smp_call_function_single_async(cpu, &rq->csd);
+ }
} else {
if (rq->q->nr_hw_queues > 1)
return false;
- blk_mq_trigger_softirq(rq);
+ cpu_list = this_cpu_ptr(&blk_cpu_done);
+ if (llist_add(&rq->ipi_list, cpu_list))
+ raise_softirq(BLOCK_SOFTIRQ);
}

return true;
@@ -3883,7 +3849,7 @@ static int __init blk_mq_init(void)
int i;

for_each_possible_cpu(i)
- INIT_LIST_HEAD(&per_cpu(blk_cpu_done, i));
+ init_llist_head(&per_cpu(blk_cpu_done, i));
open_softirq(BLOCK_SOFTIRQ, blk_done_softirq);

cpuhp_setup_state_nocalls(CPUHP_BLOCK_SOFTIRQ_DEAD,
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 868e11face00e..28231798f1613 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -154,7 +154,7 @@ struct request {
*/
union {
struct hlist_node hash; /* merge hash */
- struct list_head ipi_list;
+ struct llist_node ipi_list;
};

/*
diff --git a/kernel/locking/rwlock-rt.c b/kernel/locking/rwlock-rt.c
index 4cd72a2968a6f..3d2d1f14b5135 100644
--- a/kernel/locking/rwlock-rt.c
+++ b/kernel/locking/rwlock-rt.c
@@ -61,7 +61,7 @@ void __rwlock_biased_rt_init(struct rt_rw_lock *lock, const char *name,
lock->rtmutex.save_state = 1;
}

-int __read_rt_trylock(struct rt_rw_lock *lock)
+static int __read_rt_trylock(struct rt_rw_lock *lock)
{
int r, old;

diff --git a/lib/test_lockup.c b/lib/test_lockup.c
index f1a020bcc763e..864554e769738 100644
--- a/lib/test_lockup.c
+++ b/lib/test_lockup.c
@@ -480,6 +480,21 @@ static int __init test_lockup_init(void)
return -EINVAL;

#ifdef CONFIG_DEBUG_SPINLOCK
+#ifdef CONFIG_PREEMPT_RT
+ if (test_magic(lock_spinlock_ptr,
+ offsetof(spinlock_t, lock.wait_lock.magic),
+ SPINLOCK_MAGIC) ||
+ test_magic(lock_rwlock_ptr,
+ offsetof(rwlock_t, rtmutex.wait_lock.magic),
+ SPINLOCK_MAGIC) ||
+ test_magic(lock_mutex_ptr,
+ offsetof(struct mutex, lock.wait_lock.magic),
+ SPINLOCK_MAGIC) ||
+ test_magic(lock_rwsem_ptr,
+ offsetof(struct rw_semaphore, rtmutex.wait_lock.magic),
+ SPINLOCK_MAGIC))
+ return -EINVAL;
+#else
if (test_magic(lock_spinlock_ptr,
offsetof(spinlock_t, rlock.magic),
SPINLOCK_MAGIC) ||
@@ -493,6 +508,7 @@ static int __init test_lockup_init(void)
offsetof(struct rw_semaphore, wait_lock.magic),
SPINLOCK_MAGIC))
return -EINVAL;
+#endif
#endif

if ((wait_state != TASK_RUNNING ||
diff --git a/localversion-rt b/localversion-rt
index 483ad771f201a..e095ab8197147 100644
--- a/localversion-rt
+++ b/localversion-rt
@@ -1 +1 @@
--rt19
+-rt20
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f35ffe52cc4f8..09fa87f7a8119 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -829,6 +829,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
memcg = pn->memcg;

+ preempt_disable_rt();
/* Update memcg */
__mod_memcg_state(memcg, idx, val);

@@ -848,6 +849,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
x = 0;
}
__this_cpu_write(pn->lruvec_stat_cpu->count[idx], x);
+ preempt_enable_rt();
}

/**
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