Re: [PATCH RFC] rds: ib: Reduce the contention caused by the asynchronous workers to flush the mr pool

[Santosh Shilimkar <santosh.shilimkar@oracle.com>]

> On Jan 18, 2022, at 6:47 AM, Praveen Kannoju <praveen.kannoju@oracle.com> wrote:
> 
> This patch aims to reduce the number of asynchronous workers being spawned
> to execute the function "rds_ib_flush_mr_pool" during the high I/O
> situations. Synchronous call path's to this function "rds_ib_flush_mr_pool"
> will be executed without being disturbed. By reducing the number of
> processes contending to flush the mr pool, the total number of D state
> processes waiting to acquire the mutex lock will be greatly reduced, which
> otherwise were causing DB instance crash as the corresponding processes
> were not progressing while waiting to acquire the mutex lock.
> 
> Signed-off-by: Praveen Kumar Kannoju <praveen.kannoju@oracle.com>
> —
> 
[…]

> diff --git a/net/rds/ib_rdma.c b/net/rds/ib_rdma.c
> index 8f070ee..6b640b5 100644
> --- a/net/rds/ib_rdma.c
> +++ b/net/rds/ib_rdma.c
> @@ -393,6 +393,8 @@ int rds_ib_flush_mr_pool(struct rds_ib_mr_pool *pool,
> 	 */
> 	dirty_to_clean = llist_append_to_list(&pool->drop_list, &unmap_list);
> 	dirty_to_clean += llist_append_to_list(&pool->free_list, &unmap_list);
> +	WRITE_ONCE(pool->flush_ongoing, true);
> +	smp_wmb();
> 	if (free_all) {
> 		unsigned long flags;
> 
> @@ -430,6 +432,8 @@ int rds_ib_flush_mr_pool(struct rds_ib_mr_pool *pool,
> 	atomic_sub(nfreed, &pool->item_count);
> 
> out:
> +	WRITE_ONCE(pool->flush_ongoing, false);
> +	smp_wmb();
> 	mutex_unlock(&pool->flush_lock);
> 	if (waitqueue_active(&pool->flush_wait))
> 		wake_up(&pool->flush_wait);
> @@ -507,8 +511,17 @@ void rds_ib_free_mr(void *trans_private, int invalidate)
> 
> 	/* If we've pinned too many pages, request a flush */
> 	if (atomic_read(&pool->free_pinned) >= pool->max_free_pinned ||
> -	    atomic_read(&pool->dirty_count) >= pool->max_items / 5)
> -		queue_delayed_work(rds_ib_mr_wq, &pool->flush_worker, 10);
> +	    atomic_read(&pool->dirty_count) >= pool->max_items / 5) {
> +		smp_rmb();
You won’t need these explicit barriers since above atomic and write once already
issue them.

Other than that, it seems a good idea to me. With that fixed, feel free to add my
ack and repost.

Regards,
Santosh