Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes

[Paul E. McKenney]

On Thu, Jul 14, 2016 at 03:18:09PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 13, 2016 at 10:51:02PM +0200, Peter Zijlstra wrote:
> > So, IIRC, the trade-off is a full memory barrier in read_lock and
> > read_unlock() vs sync_sched() in write.
> > 
> > Full memory barriers are expensive and while the combined cost might
> > well exceed the cost of the sync_sched() it doesn't suffer the latency
> > issues.
> > 
> > Not sure if we can frob the two in a single codebase, but I can have a
> > poke if Oleg or Paul doesn't beat me to it.
> 
> OK, not too horrible if I say so myself :-)
> 
> The below is a compile tested only first draft so far. I'll go give it
> some runtime next.


Hmmm...  How does this handle the following sequence of events for
the case where we are not biased towards the reader?

o	The per-CPU rwsem is set up with RCU_NONE and readers_slow
	(as opposed to readers_block).  The rcu_sync ->gp_state is
	GP_PENDING, which means that rcu_sync_is_idle() will always
	return true.

o	Task A on CPU 0 runs percpu_down_read() to completion, and remains
	within the critical section.  CPU 0's ->refcount is therefore 1.

o	Task B on CPU 1 does percpu_down_write(), which write-acquires
	->rw_sem, does rcu_sync_enter() (which is a no-op due to
	RCU_NONE), sets ->state to readers_block, and is just now going
	to wait for readers, which first invokes readers_active_check(),
	which starts summing the ->refcount for CPUs 0, 1, and 2,
	obtaining the value 1 so far.

o	Task C CPU 2 enters percpu_down_read(), disables preemption,
	increments CPU 2's ->refcount, sees rcu_sync_is_idle() return
	true (so skips __percpu_down_read()), enables preemption, and
	enters its critical section.

o	Task C migrates to CPU 3 and invokes percpu_up_read(), which
	disables preemption, sees rcu_sync_is_idle() return true, calls
	__this_cpu_dec() on CPU 3's ->refcount, and enables preemption.
	The value of CPU 3's ->refcount is thus (unsigned int)-1.

o	Task B on CPU 1 continues execution in readers_active_check(), with
	the full sum being zero.

So it looks to me like we have Task A as a writer at the same time that
Task A is a reader, which would not be so good.

So what am I missing here?

And a couple of checkpatch nits below.  Yes, I had to apply the patch to
figure out what it was doing.  ;-)

							Thanx, Paul

> ---
>  fs/super.c                    |   3 +-
>  include/linux/percpu-rwsem.h  |  96 +++++++++++++++--
>  include/linux/rcu_sync.h      |   2 +-
>  kernel/locking/percpu-rwsem.c | 243 +++++++++++++++++++++++++-----------------
>  kernel/rcu/sync.c             |  15 +++
>  5 files changed, 249 insertions(+), 110 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index d78b9847e6cb..8ff18af7703f 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -195,7 +195,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
>  	for (i = 0; i < SB_FREEZE_LEVELS; i++) {
>  		if (__percpu_init_rwsem(&s->s_writers.rw_sem[i],
>  					sb_writers_name[i],
> -					&type->s_writers_key[i]))
> +					&type->s_writers_key[i],
> +					PERCPU_RWSEM_READER))
>  			goto fail;
>  	}
>  	init_waitqueue_head(&s->s_writers.wait_unfrozen);
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index c2fa3ecb0dce..5e1c2b029e3a 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -10,29 +10,107 @@
> 
>  struct percpu_rw_semaphore {
>  	struct rcu_sync		rss;
> -	unsigned int __percpu	*fast_read_ctr;
> +	unsigned int __percpu	*refcount;
>  	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->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 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->refcount);
> +	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->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 *);
> 
> +enum percpu_rwsem_bias { PERCPU_RWSEM_READER, PERCPU_RWSEM_WRITER };
> +
>  extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
> -				const char *, struct lock_class_key *);
> +				const char *, struct lock_class_key *,
> +				enum percpu_rwsem_bias bias);
> +
>  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,		\
> +			    PERCPU_RWSEM_READER);		\
>  })
> 
> +#define percpu_init_rwsem_writer(sem)				\
> +({								\
> +	static struct lock_class_key rwsem_key;			\
> +	__percpu_init_rwsem(sem, #sem, &rwsem_key,i		\
> +			    PERCPU_RWSEM_WRITER);		\
> +})
> 
>  #define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)
> 
> diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
> index a63a33e6196e..e556baaf785e 100644
> --- a/include/linux/rcu_sync.h
> +++ b/include/linux/rcu_sync.h
> @@ -26,7 +26,7 @@
>  #include <linux/wait.h>
>  #include <linux/rcupdate.h>
> 
> -enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
> +enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC, RCU_NONE };
> 
>  /* Structure to mediate between updaters and fastpath-using readers.  */
>  struct rcu_sync {
> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
> index bec0b647f9cc..be37c7732b54 100644
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -8,152 +8,197 @@
>  #include <linux/sched.h>
>  #include <linux/errno.h>
> 
> -int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
> -			const char *name, struct lock_class_key *rwsem_key)
> +enum { readers_slow, readers_block };
> +
> +int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
> +			const char *name, struct lock_class_key *rwsem_key,
> +			enum percpu_rwsem_bias bias)
>  {
> -	brw->fast_read_ctr = alloc_percpu(int);
> -	if (unlikely(!brw->fast_read_ctr))
> +	sem->refcount = alloc_percpu(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);
> -	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, bias == PERCPU_RWSEM_READER ? 
> +				 RCU_SCHED_SYNC : 

Whitespace complaint on prior two lines.

> +				 RCU_NONE);
> +	__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->refcount)
>  		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->refcount);
> +	sem->refcount = 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->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.
> +	 */
> 
> -	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->refcount);
> +	__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);
> -	}
> -
> -	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
> -	return 1;
> +	smp_mb(); /* A matches D */
> +
> +	if (likely(smp_load_acquire(&sem->state) != readers_block))
> +		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->refcount);
> 
> -	/* 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->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.
> + */
> +static bool readers_active_check(struct percpu_rw_semaphore *sem)
>  {
> -	unsigned int sum = 0;
> -	int cpu;
> +	if (per_cpu_sum(*sem->refcount) != 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);
> +	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->refcount 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);
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> index be922c9f3d37..48055bf629af 100644
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -55,6 +55,7 @@ static const struct {
>  		.wait = rcu_barrier_bh,
>  		__INIT_HELD(rcu_read_lock_bh_held)
>  	},
> +	[RCU_NONE] = { },
>  };
> 
>  enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
> @@ -65,6 +66,9 @@ enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
>  #ifdef CONFIG_PROVE_RCU
>  void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
>  {
> +	if (rsp->gp_type == RCU_NONE)
> +		return;
> +
>  	RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
>  			 "suspicious rcu_sync_is_idle() usage");
>  }
> @@ -80,6 +84,8 @@ void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
>  	memset(rsp, 0, sizeof(*rsp));
>  	init_waitqueue_head(&rsp->gp_wait);
>  	rsp->gp_type = type;
> +	if (rsp->gp_type == RCU_NONE)
> +		rsp->gp_state = GP_PENDING; /* anything !0 */
>  }
> 
>  /**
> @@ -101,6 +107,9 @@ void rcu_sync_enter(struct rcu_sync *rsp)
>  {
>  	bool need_wait, need_sync;
> 
> +	if (rsp->gp_type == RCU_NONE)
> +		return;
> +
>  	spin_lock_irq(&rsp->rss_lock);
>  	need_wait = rsp->gp_count++;
>  	need_sync = rsp->gp_state == GP_IDLE;
> @@ -188,6 +197,9 @@ static void rcu_sync_func(struct rcu_head *rcu)
>   */
>  void rcu_sync_exit(struct rcu_sync *rsp)
>  {
> +	if (rsp->gp_type == RCU_NONE)
> +		return;
> +
>  	spin_lock_irq(&rsp->rss_lock);
>  	if (!--rsp->gp_count) {
>  		if (rsp->cb_state == CB_IDLE) {
> @@ -208,6 +220,9 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
>  {
>  	int cb_state;
> 
> +	if (rsp->gp_type == RCU_NONE)
> +		return;
> +
>  	BUG_ON(rsp->gp_count);
> 
>  	spin_lock_irq(&rsp->rss_lock);
>