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Subject[RFC][PATCH 5/5] percpu-rwsem: Optimize readers and reduce global impact
Currently the percpu-rwsem has two issues:

- it switches to (global) atomic ops while a writer is waiting;
which could be quite a while and slows down releasing the readers.

- it employs synchronize_sched_expedited() _twice_ which is evil and
should die -- it shoots IPIs around the machine.

This patch cures the first 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.

It cures the second problem by employing the rcu-sync primitives by
Oleg which reduces to no sync_sched() calls in the 'normal' case of
no write contention -- global locks had better be rare, and has a
maximum of one sync_sched() call in case of contention.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/percpu-rwsem.h | 62 +++++++++-
kernel/locking/percpu-rwsem.c | 238 +++++++++++++++++++++---------------------
2 files changed, 176 insertions(+), 124 deletions(-)

--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -5,18 +5,64 @@
#include <linux/rwsem.h>
#include <linux/percpu.h>
#include <linux/wait.h>
+#include <linux/rcusync.h>
#include <linux/lockdep.h>

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

-extern void percpu_down_read(struct percpu_rw_semaphore *);
-extern void percpu_up_read(struct percpu_rw_semaphore *);
+extern void __percpu_down_read(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 past the
+ * preempt_enable() and that once the synchronize_sched() is done, the
+ * writer will see anything we did within this RCU-sched read-side
+ * critical section.
+ */
+ __this_cpu_inc(*sem->refcount);
+ 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 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->refcount);
+ 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 *);
@@ -25,10 +71,10 @@ extern int __percpu_init_rwsem(struct pe
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); \
})

#endif
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -8,158 +8,164 @@
#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->refcount = alloc_percpu(unsigned int);
+ if (unlikely(!sem->refcount))
return -ENOMEM;

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

-void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
+void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
{
- free_percpu(brw->fast_read_ctr);
- brw->fast_read_ctr = NULL; /* catch use after free bugs */
+ rcu_sync_dtor(&sem->rss);
+ free_percpu(sem->refcount);
+ sem->refcount = NULL; /* catch use after free bugs */
}

-/*
- * This is the fast-path for down_read/up_read, it only needs to ensure
- * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
- * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
- * serialize with the preempt-disabled section below.
- *
- * The nontrivial part is that we should guarantee acquire/release semantics
- * in case when
- *
- * R_W: down_write() comes after up_read(), the writer should see all
- * changes done by the reader
- * or
- * W_R: down_read() comes after up_write(), the reader should see all
- * changes done by the writer
- *
- * 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.
- *
- * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
- * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
- * reader inside the critical section. See the comments in down_write and
- * up_write below.
- */
-static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
+void __percpu_down_read(struct percpu_rw_semaphore *sem)
{
- bool success = false;
+ /*
+ * 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->refcount 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->refcount, so that it doesn't matter
+ * that the writer missed them.
+ */
+
+ smp_mb(); /* A matches D */
+
+ if (likely(sem->state != readers_block))
+ return;
+
+ /*
+ * Per the above comment; we still have preemption disabled and
+ * will thus decrement on the same CPU as we incremented.
+ */
+ __percpu_up_read(sem);
+
+ /*
+ * 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->refcount);
+ __up_read(&sem->rw_sem);

preempt_disable();
- if (likely(!atomic_read(&brw->write_ctr))) {
- __this_cpu_add(*brw->fast_read_ctr, val);
- success = true;
- }
- preempt_enable();
+}
+
+void __percpu_up_read(struct percpu_rw_semaphore *sem)
+{
+ 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->refcount);

- return success;
+ /* Prod writer to recheck readers_active */
+ wake_up(&sem->writer);
}

+
+#define per_cpu_sum(var) \
+({ \
+ typeof(var) __sum = 0; \
+ int cpu; \
+ for_each_possible_cpu(cpu) \
+ __sum += per_cpu(var, cpu); \
+ __sum; \
+})
+
/*
- * 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().
+ * Return true if the modular sum of the sem->refcount 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.
*/
-void percpu_down_read(struct percpu_rw_semaphore *brw)
+static bool readers_active_check(struct percpu_rw_semaphore *sem)
{
- might_sleep();
- if (likely(update_fast_ctr(brw, +1))) {
- rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
- return;
- }
+ if (per_cpu_sum(*sem->refcount) != 0)
+ return false;
+
+ /*
+ * If we observed the decrement; ensure we see the entire critical
+ * section.
+ */

- down_read(&brw->rw_sem);
- atomic_inc(&brw->slow_read_ctr);
- /* avoid up_read()->rwsem_release() */
- __up_read(&brw->rw_sem);
+ smp_mb(); /* C matches B */
+
+ return true;
}

-void percpu_up_read(struct percpu_rw_semaphore *brw)
+void percpu_down_write(struct percpu_rw_semaphore *sem)
{
- rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
+ down_write(&sem->rw_sem);

- if (likely(update_fast_ctr(brw, -1)))
- return;
+ /* Notify readers to take the slow path. */
+ rcu_sync_enter(&sem->rss);

- /* false-positive is possible but harmless */
- if (atomic_dec_and_test(&brw->slow_read_ctr))
- wake_up_all(&brw->write_waitq);
-}
+ /*
+ * Notify new readers to block; up until now, and thus throughout the
+ * longish rcu_sync_enter() above, new readers could still come in.
+ */
+ sem->state = readers_block;

-static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
-{
- unsigned int sum = 0;
- int cpu;
+ smp_mb(); /* D matches A */

- for_each_possible_cpu(cpu) {
- sum += per_cpu(*brw->fast_read_ctr, cpu);
- per_cpu(*brw->fast_read_ctr, cpu) = 0;
- }
+ /*
+ * If they don't see our writer of readers_block to sem->state,
+ * then we are guaranteed to see their sem->refcount increment, and
+ * therefore will wait for them.
+ */

- return sum;
+ /* Wait for all now active readers to complete. */
+ wait_event(sem->writer, readers_active_check(sem));
}

-/*
- * A writer increments ->write_ctr to force the readers to switch to the
- * slow mode, note the atomic_read() check in update_fast_ctr().
- *
- * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
- * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
- * counter it represents the number of active readers.
- *
- * Finally the writer takes ->rw_sem for writing and blocks the new readers,
- * then waits until the slow counter becomes zero.
- */
-void percpu_down_write(struct percpu_rw_semaphore *brw)
+void percpu_up_write(struct percpu_rw_semaphore *sem)
{
- /* tell update_fast_ctr() there is a pending writer */
- atomic_inc(&brw->write_ctr);
/*
- * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
- * so that update_fast_ctr() can't succeed.
- *
- * 2. Ensures we see the result of every previous this_cpu_add() in
- * update_fast_ctr().
+ * Signal the writer is done, no fast path yet.
*
- * 3. Ensures that if any reader has exited its critical section via
- * fast-path, it executes a full memory barrier before we return.
- * See R_W case in the comment above update_fast_ctr().
+ * 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.
*/
- synchronize_sched_expedited();
-
- /* exclude other writers, and block the new readers completely */
- down_write(&brw->rw_sem);
+ sem->state = readers_slow;

- /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
- atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
-
- /* wait for all readers to complete their percpu_up_read() */
- wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
-}
+ /*
+ * Release the write lock, this will allow readers back in the game.
+ */
+ up_write(&sem->rw_sem);

-void percpu_up_write(struct percpu_rw_semaphore *brw)
-{
- /* release the lock, but the readers can't use the fast-path */
- up_write(&brw->rw_sem);
/*
- * Insert the barrier before the next fast-path in down_read,
- * see W_R case in the comment above update_fast_ctr().
+ * 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.
*/
- synchronize_sched_expedited();
- /* the last writer unblocks update_fast_ctr() */
- atomic_dec(&brw->write_ctr);
+ rcu_sync_exit(&sem->rss);
}



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