Re: [PATCH] SUNRPC: use congestion_wait() in svc_alloc_args()

[Chuck Lever III <chuck.lever@oracle.com>]

Hi Neil-

> On Sep 6, 2021, at 12:44 AM, NeilBrown <neilb@suse.de> wrote:
> 
> 
> Many places that need to wait before retrying a memory allocation use
> congestion_wait().  xfs_buf_alloc_pages() is a good example which
> follows a similar pattern to that in svc_alloc_args().
> 
> It make sense to do the same thing in svc_alloc_args(); This will allow
> the allocation to be retried sooner if some backing device becomes
> non-congested before the timeout.
> 
> Every call to congestion_wait() in the entire kernel passes BLK_RW_ASYNC
> as the first argument, so we should so.
> 
> The second argument - an upper limit for waiting - seem fairly
> arbitrary.  Many places use "HZ/50" or "HZ/10".  As there is no obvious
> choice, it seems reasonable to leave the maximum time unchanged.
> 
> If a service using svc_alloc_args() is terminated, it may now have to
> wait up to the full 500ms before termination completes as
> congestion_wait() cannot be interrupted.  I don't believe this will be a
> problem in practice, though it might be justification for using a
> smaller timeout.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> 
> I happened to notice this inconsistency between svc_alloc_args() and
> xfs_buf_alloc_pages() despite them doing very similar things, so thought
> I'd send a patch.
> 
> NeilBrown

When we first considered the alloc_pages_bulk() API, the SUNRPC
patch in that series replaced this schedule_timeout(). Mel
suggested we postpone that to a separate patch. Now is an ideal
time to consider this change again. I've added the MM folks for
expert commentary.

I would rather see a shorter timeout, since that will be less
disruptive in practice and today's systems shouldn't have to wait
that long for free memory to become available. DEFAULT_IO_TIMEOUT
might be a defensible choice -- it will slow down this loop
effectively without adding a significant delay.


> net/sunrpc/svc_xprt.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index 796eebf1787d..161433ae0fab 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -10,6 +10,7 @@
> #include <linux/freezer.h>
> #include <linux/kthread.h>
> #include <linux/slab.h>
> +#include <linux/backing-dev.h>
> #include <net/sock.h>
> #include <linux/sunrpc/addr.h>
> #include <linux/sunrpc/stats.h>
> @@ -682,12 +683,10 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
> 			/* Made progress, don't sleep yet */
> 			continue;
> 
> -		set_current_state(TASK_INTERRUPTIBLE);
> -		if (signalled() || kthread_should_stop()) {
> -			set_current_state(TASK_RUNNING);
> +		if (signalled() || kthread_should_stop())
> 			return -EINTR;
> -		}
> -		schedule_timeout(msecs_to_jiffies(500));
> +
> +		congestion_wait(BLK_RW_ASYNC, msecs_to_jiffies(500));
> 	}
> 	rqstp->rq_page_end = &rqstp->rq_pages[pages];
> 	rqstp->rq_pages[pages] = NULL; /* this might be seen in nfsd_splice_actor() */
> -- 
> 2.33.0
> 

--
Chuck Lever