Re: [PATCH 1/1] SUNRPC: increase size of rpc_wait_queue.qlen from unsigned short to unsigned int

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2024-01-30 at 11:38 -0800, Dai Ngo wrote:
> When the NFS client is under extreme load the rpc_wait_queue.qlen counter
> can be overflowed. Here is an instant of the backlog queue overflow in a
> real world environment shown by drgn helper:
> 
> rpc_task_stats(rpc_clnt):
> -------------------------
> rpc_clnt: 0xffff92b65d2bae00
> rpc_xprt: 0xffff9275db64f000
>   Queue:  sending[64887] pending[524] backlog[30441] binding[0]
> XMIT task: 0xffff925c6b1d8e98
>      WRITE: 750654
>         __dta_call_status_580: 65463
>         __dta_call_transmit_status_579: 1
>         call_reserveresult: 685189
>         nfs_client_init_is_complete: 1
>     COMMIT: 584
>         call_reserveresult: 573
>         __dta_call_status_580: 11
>     ACCESS: 1
>         __dta_call_status_580: 1
>    GETATTR: 10
>         __dta_call_status_580: 4
>         call_reserveresult: 6
> 751249 tasks for server 111.222.333.444
> Total tasks: 751249
> 
> count_rpc_wait_queues(xprt):
> ----------------------------
> **** rpc_xprt: 0xffff9275db64f000 num_reqs: 65511
> wait_queue: xprt_binding[0] cnt: 0
> wait_queue: xprt_binding[1] cnt: 0
> wait_queue: xprt_binding[2] cnt: 0
> wait_queue: xprt_binding[3] cnt: 0
> rpc_wait_queue[xprt_binding].qlen: 0 maxpriority: 0
> wait_queue: xprt_sending[0] cnt: 0
> wait_queue: xprt_sending[1] cnt: 64887
> wait_queue: xprt_sending[2] cnt: 0
> wait_queue: xprt_sending[3] cnt: 0
> rpc_wait_queue[xprt_sending].qlen: 64887 maxpriority: 3
> wait_queue: xprt_pending[0] cnt: 524
> wait_queue: xprt_pending[1] cnt: 0
> wait_queue: xprt_pending[2] cnt: 0
> wait_queue: xprt_pending[3] cnt: 0
> rpc_wait_queue[xprt_pending].qlen: 524 maxpriority: 0
> wait_queue: xprt_backlog[0] cnt: 0
> wait_queue: xprt_backlog[1] cnt: 685801
> wait_queue: xprt_backlog[2] cnt: 0
> wait_queue: xprt_backlog[3] cnt: 0
> rpc_wait_queue[xprt_backlog].qlen: 30441 maxpriority: 3 [task cnt mismatch]
> 
> There is no effect on operations when this overflow occurs. However
> it causes confusion when trying to diagnose the performance problem.
> 
> Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
> ---
>  include/linux/sunrpc/sched.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/sunrpc/sched.h b/include/linux/sunrpc/sched.h
> index 8ada7dc802d3..8f9bee0e21c3 100644
> --- a/include/linux/sunrpc/sched.h
> +++ b/include/linux/sunrpc/sched.h
> @@ -186,7 +186,7 @@ struct rpc_wait_queue {
>  	unsigned char		maxpriority;		/* maximum priority (0 if queue is not a priority queue) */
>  	unsigned char		priority;		/* current priority */
>  	unsigned char		nr;			/* # tasks remaining for cookie */
> -	unsigned short		qlen;			/* total # tasks waiting in queue */
> +	unsigned int		qlen;			/* total # tasks waiting in queue */
>  	struct rpc_timer	timer_list;
>  #if IS_ENABLED(CONFIG_SUNRPC_DEBUG) || IS_ENABLED(CONFIG_TRACEPOINTS)
>  	const char *		name;

Seems fine to me. In fact, on x86_64 there is a 2 byte hole after the
existing 2 byte qlen, so this probably won't even result in the
structure growing.

Reviewed-by: Jeff Layton <jlayton@kernel.org>