Re: [PATCH] SUNRPC: restore fair scheduling to priority queues.

[Trond Myklebust]

On Tue, Dec 15, 2015 at 6:44 PM, NeilBrown <neilb@xxxxxxxx> wrote:
>
> Commit: c05eecf63610 ("SUNRPC: Don't allow low priority tasks to pre-empt higher priority ones")
>
> removed the 'fair scheduling' feature from SUNRPC priority queues.
> This feature caused problems for some queues (send queue and session slot queue)
> but is still needed for others, particularly the tcp slot queue.
>
> Without fairness, reads (priority 1) can starve background writes
> (priority 0) so a streaming read can cause writeback to block
> indefinitely.  This is not easy to measure with default settings as
> the current slot table size is much larger than the read-ahead size.
> However if the slot-table size is reduced (seen when backporting to
> older kernels with a limited size) the problem is easily demonstrated.
>
> This patch conditionally restores fair scheduling.  It is now the
> default unless rpc_sleep_on_priority() is called directly.  Then the
> queue switches to strict priority observance.
>
> As that function is called for both the send queue and the session
> slot queue and not for any others, this has exactly the desired
> effect.
>
> The "count" field that was removed by the previous patch is restored.
> A value for '255' means "strict priority queuing, no fair queuing".
> Any other value is a could of owners to be processed before switching
> to a different priority level, just like before.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
> ---
>
> It is quite possible that you won't like the overloading of
> rpc_sleep_on_priority() to disable fair-scheduling and would prefer an
> extra arg to rpc_init_priority_wait_queue().  I can do it that way if
> you like.
> NeilBrown
>
>
>  include/linux/sunrpc/sched.h |  1 +
>  net/sunrpc/sched.c           | 12 +++++++++---
>  2 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/sunrpc/sched.h b/include/linux/sunrpc/sched.h
> index d703f0ef37d8..985efe8d7e26 100644
> --- a/include/linux/sunrpc/sched.h
> +++ b/include/linux/sunrpc/sched.h
> @@ -184,6 +184,7 @@ struct rpc_wait_queue {
>         pid_t                   owner;                  /* process id of last task serviced */
>         unsigned char           maxpriority;            /* maximum priority (0 if queue is not a priority queue) */
>         unsigned char           priority;               /* current priority */
> +       unsigned char           count;                  /* # task groups remaining to be serviced */
>         unsigned char           nr;                     /* # tasks remaining for cookie */
>         unsigned short          qlen;                   /* total # tasks waiting in queue */
>         struct rpc_timer        timer_list;
> diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
> index 73ad57a59989..e8fcd4f098bb 100644
> --- a/net/sunrpc/sched.c
> +++ b/net/sunrpc/sched.c
> @@ -117,6 +117,8 @@ static void rpc_set_waitqueue_priority(struct rpc_wait_queue *queue, int priorit
>                 rpc_rotate_queue_owner(queue);
>                 queue->priority = priority;
>         }
> +       if (queue->count != 255)
> +               queue->count = 1 << (priority * 2);
>  }
>
>  static void rpc_set_waitqueue_owner(struct rpc_wait_queue *queue, pid_t pid)
> @@ -144,8 +146,10 @@ static void __rpc_add_wait_queue_priority(struct rpc_wait_queue *queue,
>         INIT_LIST_HEAD(&task->u.tk_wait.links);
>         if (unlikely(queue_priority > queue->maxpriority))
>                 queue_priority = queue->maxpriority;
> -       if (queue_priority > queue->priority)
> -               rpc_set_waitqueue_priority(queue, queue_priority);
> +       if (queue->count == 255) {
> +               if (queue_priority > queue->priority)
> +                       rpc_set_waitqueue_priority(queue, queue_priority);
> +       }
>         q = &queue->tasks[queue_priority];
>         list_for_each_entry(t, q, u.tk_wait.list) {
>                 if (t->tk_owner == task->tk_owner) {
> @@ -401,6 +405,7 @@ void rpc_sleep_on_priority(struct rpc_wait_queue *q, struct rpc_task *task,
>          * Protect the queue operations.
>          */
>         spin_lock_bh(&q->lock);
> +       q->count = 255;
>         __rpc_sleep_on_priority(q, task, action, priority - RPC_PRIORITY_LOW);
>         spin_unlock_bh(&q->lock);
>  }
> @@ -478,7 +483,8 @@ static struct rpc_task *__rpc_find_next_queued_priority(struct rpc_wait_queue *q
>                 /*
>                  * Check if we need to switch queues.
>                  */
> -               goto new_owner;
> +               if (queue->count == 255 || --queue->count)
> +                       goto new_owner;
>         }
>
>         /*
>

Are we sure there is value in keeping FLUSH_LOWPRI for background writes?

Cheers
  Trond
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