[PATCH 07/12] sunrpc/cache: allow thread to block while waiting for cache update.

NeilBrown
2009-08-04T05:24:57Z

The current practice of waiting for cache updates by queueing the
whole request to be retried has (at least) two problems.

1/ We NFSv4, requests can be quite complex and re-trying a whole
request when a later part fails should only be a list-resort, not a
normal practice.

2/ Large requests, and in particular any 'write' request, will not be
queued by the current code and doing so would be undesirable.

In many cases only a very sort wait is needed before the cache gets
valid data.

So, providing the underlying transport permits it by setting
->thread_wait,
arrange to wait briefly for an upcall to be completed (as reflected in
the clearing of CACHE_PENDING).
If the short wait was not long enough and CACHE_PENDING is still set,
fall back on the old approach.

The 'thread_wait' value is set to 5 seconds when there are spare
threads, and 1 second when there are no spare threads.

These values are probably much higher than needed, but will ensure
some forward progress.

Signed-off-by: NeilBrown <neilb@suse.de>
---

include/linux/sunrpc/cache.h | 3 +++
net/sunrpc/cache.c | 44 +++++++++++++++++++++++++++++++++++++++++-
net/sunrpc/svc_xprt.c | 11 +++++++++++
3 files changed, 57 insertions(+), 1 deletions(-)

diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.h
index 2d8b211..ad62e8d 100644
--- a/include/linux/sunrpc/cache.h
+++ b/include/linux/sunrpc/cache.h
@@ -112,6 +112,9 @@ struct cache_detail {
*/
struct cache_req {
struct cache_deferred_req *(*defer)(struct cache_req *req);
+ int thread_wait; /* How long (jiffies) we can block the
+ * current thread to wait for updates.
+ */
};
/* this must be embedded in a deferred_request that is being
* delayed awaiting cache-fill
diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index 4892c5c..ba2e113 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -559,10 +559,22 @@ static LIST_HEAD(cache_defer_list);
static struct list_head cache_defer_hash[DFR_HASHSIZE];
static int cache_defer_cnt;

+struct thread_deferred_req {
+ struct cache_deferred_req handle;
+ wait_queue_head_t wait;
+};
+static void cache_restart_thread(struct cache_deferred_req *dreq, int too_many)
+{
+ struct thread_deferred_req *dr =
+ container_of(dreq, struct thread_deferred_req, handle);
+ wake_up(&dr->wait);
+}
+
static int cache_defer_req(struct cache_req *req, struct cache_head *item)
{
struct cache_deferred_req *dreq, *discard;
int hash = DFR_HASH(item);
+ struct thread_deferred_req sleeper;

if (cache_defer_cnt >= DFR_MAX) {
/* too much in the cache, randomly drop this one,
@@ -571,7 +583,14 @@ static int cache_defer_req(struct cache_req *req, struct cache_head *item)
if (net_random()&1)
return 0;
}
- dreq = req->defer(req);
+ if (req->thread_wait) {
+ dreq = &sleeper.handle;
+ init_waitqueue_head(&sleeper.wait);
+ dreq->revisit = cache_restart_thread;
+ } else
+ dreq = req->defer(req);
+
+ retry:
if (dreq == NULL)
return 0;

@@ -605,6 +624,29 @@ static int cache_defer_req(struct cache_req *req, struct cache_head *item)
cache_revisit_request(item);
return 0;
}
+
+ if (dreq == &sleeper.handle) {
+ wait_event_interruptible_timeout(
+ sleeper.wait,
+ !test_bit(CACHE_PENDING, &item->flags)
+ || list_empty(&sleeper.handle.hash),
+ req->thread_wait);
+ spin_lock(&cache_defer_lock);
+ if (!list_empty(&sleeper.handle.hash)) {
+ list_del_init(&sleeper.handle.recent);
+ list_del_init(&sleeper.handle.hash);
+ cache_defer_cnt--;
+ }
+ spin_unlock(&cache_defer_lock);
+ if (test_bit(CACHE_PENDING, &item->flags)) {
+ /* item is still pending, try request
+ * deferral
+ */
+ dreq = req->defer(req);
+ goto retry;
+ }
+ return 0;
+ }
return 1;
}

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 27d4433..465443c 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -655,12 +655,23 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
pool->sp_nwaking--;
BUG_ON(pool->sp_nwaking < 0);
}
+
+ /* Normally we will wait up to 5 seconds for any required
+ * cache information to be provided.
+ */
+ rqstp->rq_chandle.thread_wait = 5*HZ;
xprt = svc_xprt_dequeue(pool);
if (xprt) {
rqstp->rq_xprt = xprt;
svc_xprt_get(xprt);
rqstp->rq_reserved = serv->sv_max_mesg;
atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
+
+ /* As there is a shortage of threads and this request
+ * had to be queue, don't allow the thread to wait so
+ * long for cache updates.
+ */
+ rqstp->rq_chandle.thread_wait = 1*HZ;
} else {
/* No data pending. Go to sleep */
svc_thread_enqueue(pool, rqstp);