Re: [RFC, PATCH 1/2] qrcu: "quick" srcu implementation

[Paul E. McKenney]

On Thu, Nov 30, 2006 at 04:57:14AM +0300, Oleg Nesterov wrote:
> (the same patch + comments from Paul)
> 
> [RFC, PATCH 1/2] qrcu: "quick" srcu implementation
> 
> Very much based on ideas, corrections, and patient explanations from
> Alan and Paul.
> 
> The current srcu implementation is very good for readers, lock/unlock
> are extremely cheap. But for that reason it is not possible to avoid
> synchronize_sched() and polling in synchronize_srcu().
> 
> Jens Axboe wrote:
> >
> > It works for me, but the overhead is still large. Before it would take
> > 8-12 jiffies for a synchronize_srcu() to complete without there actually
> > being any reader locks active, now it takes 2-3 jiffies. So it's
> > definitely faster, and as suspected the loss of two of three
> > synchronize_sched() cut down the overhead to a third.
> 
> 'qrcu' behaves the same as srcu but optimized for writers. The fast path
> for synchronize_qrcu() is mutex_lock() + atomic_read() + mutex_unlock().
> The slow path is __wait_event(), no polling. However, the reader does
> atomic inc/dec on lock/unlock, and the counters are not per-cpu.
> 
> Also, unlike srcu, qrcu read lock/unlock can be used in interrupt context,
> and 'qrcu_struct' can be compile-time initialized.
> 
> See also (a long) discussion:
> 	http://marc.theaimsgroup.com/?t=116370857600003

With the addition of a comment for the smp_mb() at the beginning of
synchronize_qrcu(), shown below:

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> 
> --- 19-rc6/include/linux/srcu.h~1_qrcu	2006-10-22 18:24:03.000000000 +0400
> +++ 19-rc6/include/linux/srcu.h	2006-11-30 04:32:42.000000000 +0300
> @@ -27,6 +27,8 @@
>  #ifndef _LINUX_SRCU_H
>  #define _LINUX_SRCU_H
> 
> +#include <linux/wait.h>
> +
>  struct srcu_struct_array {
>  	int c[2];
>  };
> @@ -50,4 +52,32 @@ void srcu_read_unlock(struct srcu_struct
>  void synchronize_srcu(struct srcu_struct *sp);
>  long srcu_batches_completed(struct srcu_struct *sp);
> 
> +/*
> + * fully compatible with srcu, but optimized for writers.
> + */
> +
> +struct qrcu_struct {
> +	int completed;
> +	atomic_t ctr[2];
> +	wait_queue_head_t wq;
> +	struct mutex mutex;
> +};
> +
> +int init_qrcu_struct(struct qrcu_struct *qp);
> +int qrcu_read_lock(struct qrcu_struct *qp);
> +void qrcu_read_unlock(struct qrcu_struct *qp, int idx);
> +void synchronize_qrcu(struct qrcu_struct *qp);
> +
> +/**
> + * cleanup_qrcu_struct - deconstruct a quick-RCU structure
> + * @qp: structure to clean up.
> + *
> + * Must invoke this after you are finished using a given qrcu_struct that
> + * was initialized via init_qrcu_struct().  We reserve the right to
> + * leak memory should you fail to do this!
> + */
> +static inline void cleanup_qrcu_struct(struct qrcu_struct *qp)
> +{
> +}
> +
>  #endif
> --- 19-rc6/kernel/srcu.c~1_qrcu	2006-10-22 18:24:03.000000000 +0400
> +++ 19-rc6/kernel/srcu.c	2006-11-30 04:39:53.000000000 +0300
> @@ -256,3 +256,94 @@ EXPORT_SYMBOL_GPL(srcu_read_unlock);
>  EXPORT_SYMBOL_GPL(synchronize_srcu);
>  EXPORT_SYMBOL_GPL(srcu_batches_completed);
>  EXPORT_SYMBOL_GPL(srcu_readers_active);
> +
> +/**
> + * init_qrcu_struct - initialize a quick-RCU structure.
> + * @qp: structure to initialize.
> + *
> + * Must invoke this on a given qrcu_struct before passing that qrcu_struct
> + * to any other function.  Each qrcu_struct represents a separate domain
> + * of QRCU protection.
> + */
> +int init_qrcu_struct(struct qrcu_struct *qp)
> +{
> +	qp->completed = 0;
> +	atomic_set(qp->ctr + 0, 1);
> +	atomic_set(qp->ctr + 1, 0);
> +	init_waitqueue_head(&qp->wq);
> +	mutex_init(&qp->mutex);
> +
> +	return 0;
> +}
> +
> +/**
> + * qrcu_read_lock - register a new reader for an QRCU-protected structure.
> + * @qp: qrcu_struct in which to register the new reader.
> + *
> + * Counts the new reader in the appropriate element of the qrcu_struct.
> + * Returns an index that must be passed to the matching qrcu_read_unlock().
> + */
> +int qrcu_read_lock(struct qrcu_struct *qp)
> +{
> +	for (;;) {
> +		int idx = qp->completed & 0x1;
> +		if (likely(atomic_inc_not_zero(qp->ctr + idx)))
> +			return idx;
> +	}
> +}
> +
> +/**
> + * qrcu_read_unlock - unregister a old reader from an QRCU-protected structure.
> + * @qp: qrcu_struct in which to unregister the old reader.
> + * @idx: return value from corresponding qrcu_read_lock().
> + *
> + * Removes the count for the old reader from the appropriate element of
> + * the qrcu_struct.
> + */
> +void qrcu_read_unlock(struct qrcu_struct *qp, int idx)
> +{
> +	if (atomic_dec_and_test(qp->ctr + idx))
> +		wake_up(&qp->wq);
> +}
> +
> +/**
> + * synchronize_qrcu - wait for prior QRCU read-side critical-section completion
> + * @qp: qrcu_struct with which to synchronize.
> + *
> + * Flip the completed counter, and wait for the old count to drain to zero.
> + * As with classic RCU, the updater must use some separate means of
> + * synchronizing concurrent updates.  Can block; must be called from
> + * process context.
> + *
> + * Note that it is illegal to call synchronize_qrcu() from the corresponding
> + * QRCU read-side critical section; doing so will result in deadlock.
> + * However, it is perfectly legal to call synchronize_qrcu() on one
> + * qrcu_struct from some other qrcu_struct's read-side critical section.
> + */
> +void synchronize_qrcu(struct qrcu_struct *qp)
> +{
> +	int idx;

	/*
	 * The following memory barrier is needed to ensure that
	 * any prior data-structure manipulation is seen by other
	 * CPUs to happen before picking up the value of
	 * qp->completed.
	 */

> +	smp_mb();
> +	mutex_lock(&qp->mutex);
> +
> +	idx = qp->completed & 0x1;
> +	if (atomic_read(qp->ctr + idx) == 1)
> +		goto out;
> +
> +	atomic_inc(qp->ctr + (idx ^ 0x1));
> +	/* Reduce the likelihood that qrcu_read_lock() will loop */
> +	smp_mb__after_atomic_inc();
> +	qp->completed++;
> +
> +	atomic_dec(qp->ctr + idx);
> +	__wait_event(qp->wq, !atomic_read(qp->ctr + idx));
> +out:
> +	mutex_unlock(&qp->mutex);

	/*
	 * The following memory barrier is needed to ensure that
	 * and subsequent freeing of data elements previously
	 * removed is seen by other CPUs after the wait completes.
	 */

Hmmm...  Now I am wondering if the memory barriers inherent in the
__wait_event() suffice for this last barrier...  :-/  Thoughts?

> +	smp_mb();
> +}
> +
> +EXPORT_SYMBOL_GPL(init_qrcu_struct);
> +EXPORT_SYMBOL_GPL(qrcu_read_lock);
> +EXPORT_SYMBOL_GPL(qrcu_read_unlock);
> +EXPORT_SYMBOL_GPL(synchronize_qrcu);
> 
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