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Subject[PATCH 1/2] locking/percpu-rwsem: Optimize readers and reduce global impact
Currently the percpu-rwsem switches to (global) atomic ops while a
writer is waiting; which could be quite a while and slows down
releasing the readers.

This patch cures this problem by ordering the reader-state vs
reader-count (see the comments in __percpu_down_read() and
percpu_down_write()). This changes a global atomic op into a full
memory barrier, which doesn't have the global cacheline contention.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/percpu-rwsem.h | 85 +++++++++++++--
kernel/locking/percpu-rwsem.c | 236 ++++++++++++++++++++++++------------------
2 files changed, 216 insertions(+), 105 deletions(-)

--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -10,30 +10,97 @@

struct percpu_rw_semaphore {
struct rcu_sync rss;
- unsigned int __percpu *fast_read_ctr;
+ unsigned int __percpu *read_count;
struct rw_semaphore rw_sem;
- atomic_t slow_read_ctr;
- wait_queue_head_t write_waitq;
+ wait_queue_head_t writer;
+ int state;
};

-extern void percpu_down_read(struct percpu_rw_semaphore *);
-extern int percpu_down_read_trylock(struct percpu_rw_semaphore *);
-extern void percpu_up_read(struct percpu_rw_semaphore *);
+extern void __percpu_down_read(struct percpu_rw_semaphore *);
+extern int __percpu_down_read_trylock(struct percpu_rw_semaphore *);
+extern void __percpu_up_read(struct percpu_rw_semaphore *);
+
+static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
+{
+ might_sleep();
+
+ rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
+
+ preempt_disable();
+ /*
+ * We are in an RCU-sched read-side critical section, so the writer
+ * cannot both change sem->state from readers_fast and start checking
+ * counters while we are here. So if we see !sem->state, we know that
+ * the writer won't be checking until we're past the preempt_enable()
+ * and that one the synchronize_sched() is done, the writer will see
+ * anything we did within this RCU-sched read-size critical section.
+ */
+ __this_cpu_inc(*sem->read_count);
+ if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+ __percpu_down_read(sem); /* Unconditional memory barrier */
+ preempt_enable();
+ /*
+ * The barrier() from preempt_enable() prevents the compiler from
+ * bleeding the critical section out.
+ */
+}
+
+static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
+{
+ int ret = 1;
+
+ preempt_disable();
+ /*
+ * Same as in percpu_down_read().
+ */
+ __this_cpu_inc(*sem->read_count);
+ if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+ ret = __percpu_down_read_trylock(sem);
+ preempt_enable();
+ /*
+ * The barrier() from preempt_enable() prevents the compiler from
+ * bleeding the critical section out.
+ */
+
+ if (ret)
+ rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 1, _RET_IP_);
+
+ return ret;
+}
+
+static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
+{
+ /*
+ * The barrier() in preempt_disable() prevents the compiler from
+ * bleeding the critical section out.
+ */
+ preempt_disable();
+ /*
+ * Same as in percpu_down_read().
+ */
+ if (likely(rcu_sync_is_idle(&sem->rss)))
+ __this_cpu_dec(*sem->read_count);
+ else
+ __percpu_up_read(sem); /* Unconditional memory barrier */
+ preempt_enable();
+
+ rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
+}

extern void percpu_down_write(struct percpu_rw_semaphore *);
extern void percpu_up_write(struct percpu_rw_semaphore *);

extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
const char *, struct lock_class_key *);
+
extern void percpu_free_rwsem(struct percpu_rw_semaphore *);

-#define percpu_init_rwsem(brw) \
+#define percpu_init_rwsem(sem) \
({ \
static struct lock_class_key rwsem_key; \
- __percpu_init_rwsem(brw, #brw, &rwsem_key); \
+ __percpu_init_rwsem(sem, #sem, &rwsem_key); \
})

-
#define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)

static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -8,152 +8,196 @@
#include <linux/sched.h>
#include <linux/errno.h>

-int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
+enum { readers_slow, readers_block };
+
+int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
const char *name, struct lock_class_key *rwsem_key)
{
- brw->fast_read_ctr = alloc_percpu(int);
- if (unlikely(!brw->fast_read_ctr))
+ sem->read_count = alloc_percpu(int);
+ if (unlikely(!sem->read_count))
return -ENOMEM;

/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
- __init_rwsem(&brw->rw_sem, name, rwsem_key);
- rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
- atomic_set(&brw->slow_read_ctr, 0);
- init_waitqueue_head(&brw->write_waitq);
+ rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
+ __init_rwsem(&sem->rw_sem, name, rwsem_key);
+ init_waitqueue_head(&sem->writer);
+ sem->state = readers_slow;
return 0;
}
EXPORT_SYMBOL_GPL(__percpu_init_rwsem);

-void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
+void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
{
/*
* XXX: temporary kludge. The error path in alloc_super()
* assumes that percpu_free_rwsem() is safe after kzalloc().
*/
- if (!brw->fast_read_ctr)
+ if (!sem->read_count)
return;

- rcu_sync_dtor(&brw->rss);
- free_percpu(brw->fast_read_ctr);
- brw->fast_read_ctr = NULL; /* catch use after free bugs */
+ rcu_sync_dtor(&sem->rss);
+ free_percpu(sem->read_count);
+ sem->read_count = NULL; /* catch use after free bugs */
}
EXPORT_SYMBOL_GPL(percpu_free_rwsem);

-/*
- * This is the fast-path for down_read/up_read. If it succeeds we rely
- * on the barriers provided by rcu_sync_enter/exit; see the comments in
- * percpu_down_write() and percpu_up_write().
- *
- * If this helper fails the callers rely on the normal rw_semaphore and
- * atomic_dec_and_test(), so in this case we have the necessary barriers.
- */
-static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
+void __percpu_down_read(struct percpu_rw_semaphore *sem)
{
- bool success;
+ /*
+ * Due to having preemption disabled the decrement happens on
+ * the same CPU as the increment, avoiding the
+ * increment-on-one-CPU-and-decrement-on-another problem.
+ *
+ * And yes, if the reader misses the writer's assignment of
+ * readers_block to sem->state, then the writer is
+ * guaranteed to see the reader's increment. Conversely, any
+ * readers that increment their sem->read_count after the
+ * writer looks are guaranteed to see the readers_block value,
+ * which in turn means that they are guaranteed to immediately
+ * decrement their sem->read_count, so that it doesn't matter
+ * that the writer missed them.
+ */

- preempt_disable();
- success = rcu_sync_is_idle(&brw->rss);
- if (likely(success))
- __this_cpu_add(*brw->fast_read_ctr, val);
- preempt_enable();
+ smp_mb(); /* A matches D */

- return success;
-}
+ /*
+ * If !readers_block the critical section starts here, matched by the
+ * release in percpu_up_write().
+ */
+ if (likely(smp_load_acquire(&sem->state) != readers_block))
+ return;

-/*
- * Like the normal down_read() this is not recursive, the writer can
- * come after the first percpu_down_read() and create the deadlock.
- *
- * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
- * percpu_up_read() does rwsem_release(). This pairs with the usage
- * of ->rw_sem in percpu_down/up_write().
- */
-void percpu_down_read(struct percpu_rw_semaphore *brw)
-{
- might_sleep();
- rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
+ /*
+ * Per the above comment; we still have preemption disabled and
+ * will thus decrement on the same CPU as we incremented.
+ */
+ __percpu_up_read(sem);

- if (likely(update_fast_ctr(brw, +1)))
- return;
+ /*
+ * We either call schedule() in the wait, or we'll fall through
+ * and reschedule on the preempt_enable() in percpu_down_read().
+ */
+ preempt_enable_no_resched();
+
+ /*
+ * Avoid lockdep for the down/up_read() we already have them.
+ */
+ __down_read(&sem->rw_sem);
+ __this_cpu_inc(*sem->read_count);
+ __up_read(&sem->rw_sem);

- /* Avoid rwsem_acquire_read() and rwsem_release() */
- __down_read(&brw->rw_sem);
- atomic_inc(&brw->slow_read_ctr);
- __up_read(&brw->rw_sem);
+ preempt_disable();
}
-EXPORT_SYMBOL_GPL(percpu_down_read);
+EXPORT_SYMBOL_GPL(__percpu_down_read);

-int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
+int __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
- if (unlikely(!update_fast_ctr(brw, +1))) {
- if (!__down_read_trylock(&brw->rw_sem))
- return 0;
- atomic_inc(&brw->slow_read_ctr);
- __up_read(&brw->rw_sem);
- }
+ smp_mb(); /* A matches D */
+
+ if (likely(smp_load_acquire(&sem->state) != readers_block))
+ return 1;

- rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
- return 1;
+ __percpu_up_read(sem);
+ return 0;
}
+EXPORT_SYMBOL_GPL(__percpu_down_read_trylock);

-void percpu_up_read(struct percpu_rw_semaphore *brw)
+void __percpu_up_read(struct percpu_rw_semaphore *sem)
{
- rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
-
- if (likely(update_fast_ctr(brw, -1)))
- return;
+ smp_mb(); /* B matches C */
+ /*
+ * In other words, if they see our decrement (presumably to aggregate
+ * zero, as that is the only time it matters) they will also see our
+ * critical section.
+ */
+ __this_cpu_dec(*sem->read_count);

- /* false-positive is possible but harmless */
- if (atomic_dec_and_test(&brw->slow_read_ctr))
- wake_up_all(&brw->write_waitq);
+ /* Prod writer to recheck readers_active */
+ wake_up(&sem->writer);
}
-EXPORT_SYMBOL_GPL(percpu_up_read);
+EXPORT_SYMBOL_GPL(__percpu_up_read);
+
+#define per_cpu_sum(var) \
+({ \
+ typeof(var) __sum = 0; \
+ int cpu; \
+ compiletime_assert_atomic_type(__sum); \
+ for_each_possible_cpu(cpu) \
+ __sum += per_cpu(var, cpu); \
+ __sum; \
+})

-static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
+/*
+ * Return true if the modular sum of the sem->read_count per-CPU variable is
+ * zero. If this sum is zero, then it is stable due to the fact that if any
+ * newly arriving readers increment a given counter, they will immediately
+ * decrement that same counter.
+ */
+static bool readers_active_check(struct percpu_rw_semaphore *sem)
{
- unsigned int sum = 0;
- int cpu;
+ if (per_cpu_sum(*sem->read_count) != 0)
+ return false;
+
+ /*
+ * If we observed the decrement; ensure we see the entire critical
+ * section.
+ */

- for_each_possible_cpu(cpu) {
- sum += per_cpu(*brw->fast_read_ctr, cpu);
- per_cpu(*brw->fast_read_ctr, cpu) = 0;
- }
+ smp_mb(); /* C matches B */

- return sum;
+ return true;
}

-void percpu_down_write(struct percpu_rw_semaphore *brw)
+void percpu_down_write(struct percpu_rw_semaphore *sem)
{
+ down_write(&sem->rw_sem);
+
+ /* Notify readers to take the slow path. */
+ rcu_sync_enter(&sem->rss);
+
/*
- * Make rcu_sync_is_idle() == F and thus disable the fast-path in
- * percpu_down_read() and percpu_up_read(), and wait for gp pass.
- *
- * The latter synchronises us with the preceding readers which used
- * the fast-past, so we can not miss the result of __this_cpu_add()
- * or anything else inside their criticial sections.
+ * Notify new readers to block; up until now, and thus throughout the
+ * longish rcu_sync_enter() above, new readers could still come in.
*/
- rcu_sync_enter(&brw->rss);
+ WRITE_ONCE(sem->state, readers_block);

- /* exclude other writers, and block the new readers completely */
- down_write(&brw->rw_sem);
+ smp_mb(); /* D matches A */

- /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
- atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
+ /*
+ * If they don't see our writer of readers_block to sem->state,
+ * then we are guaranteed to see their sem->read_count increment, and
+ * therefore will wait for them.
+ */

- /* wait for all readers to complete their percpu_up_read() */
- wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
+ /* Wait for all now active readers to complete. */
+ wait_event(sem->writer, readers_active_check(sem));
}
EXPORT_SYMBOL_GPL(percpu_down_write);

-void percpu_up_write(struct percpu_rw_semaphore *brw)
+void percpu_up_write(struct percpu_rw_semaphore *sem)
{
- /* release the lock, but the readers can't use the fast-path */
- up_write(&brw->rw_sem);
/*
- * Enable the fast-path in percpu_down_read() and percpu_up_read()
- * but only after another gp pass; this adds the necessary barrier
- * to ensure the reader can't miss the changes done by us.
+ * Signal the writer is done, no fast path yet.
+ *
+ * One reason that we cannot just immediately flip to readers_fast is
+ * that new readers might fail to see the results of this writer's
+ * critical section.
+ *
+ * Therefore we force it through the slow path which guarantees an
+ * acquire and thereby guarantees the critical section's consistency.
+ */
+ smp_store_release(&sem->state, readers_slow);
+
+ /*
+ * Release the write lock, this will allow readers back in the game.
+ */
+ up_write(&sem->rw_sem);
+
+ /*
+ * Once this completes (at least one RCU grace period hence) the reader
+ * fast path will be available again. Safe to use outside the exclusive
+ * write lock because its counting.
*/
- rcu_sync_exit(&brw->rss);
+ rcu_sync_exit(&sem->rss);
}
-EXPORT_SYMBOL_GPL(percpu_up_write);
+EXPORT_SYMBOL(percpu_up_write);

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