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authorPeter Zijlstra <peterz@infradead.org>2016-07-14 20:08:46 +0200
committerIngo Molnar <mingo@kernel.org>2016-08-10 14:34:01 +0200
commit80127a39681bd68c959f0953f84a830cbd7c3b1c (patch)
tree223bcc2a5cbec5c0873f8fae85a98797f94e6c56 /kernel/locking/percpu-rwsem.c
parent08be8f63c40c030b5cf95b4368e314e563a86301 (diff)
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. This also enables using the percpu-rwsem with rcu_sync disabled in order to bias the implementation differently, reducing the writer latency by adding some cost to readers. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org [ Fixed modular build. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking/percpu-rwsem.c')
-rw-r--r--kernel/locking/percpu-rwsem.c228
1 files changed, 131 insertions, 97 deletions
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index bec0b647f9cc..ce182599cf2e 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -8,152 +8,186 @@
#include <linux/sched.h>
#include <linux/errno.h>
-int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
+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->readers_block = 0;
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)
+int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
{
- 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.
+ *
+ * If the reader misses the writer's assignment of readers_block, 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->readers_block)))
+ return 1;
-/*
- * 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;
+ if (try)
+ return 0;
- /* Avoid rwsem_acquire_read() and rwsem_release() */
- __down_read(&brw->rw_sem);
- atomic_inc(&brw->slow_read_ctr);
- __up_read(&brw->rw_sem);
-}
-EXPORT_SYMBOL_GPL(percpu_down_read);
+ /*
+ * 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();
-int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
-{
- 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);
- }
-
- rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
+ /*
+ * 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);
+
+ preempt_disable();
return 1;
}
+EXPORT_SYMBOL_GPL(__percpu_down_read);
-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)
{
+ /* Notify readers to take the slow path. */
+ rcu_sync_enter(&sem->rss);
+
+ down_write(&sem->rw_sem);
+
/*
- * 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->readers_block, 1);
- /* 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, 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->readers_block, 0);
+
+ /*
+ * Release the write lock, this will allow readers back in the game.
+ */
+ up_write(&sem->rw_sem);
+
+ /*
+ * Once this completes (at least one RCU-sched 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);