net: avoid calling sched_clock when LLS is off

Change Low Latency Sockets code for select and poll so that
when LLS is disabled sched_clock() is never called.

Also, avoid sending POLL_LL to sockets if disabled.

Reported-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 22 insertions, 6 deletions

diff --git a/fs/select.c b/fs/select.c
index 79b876eb91da..36540754bad7 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -402,7 +402,7 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
         poll_table *wait;
         int retval, i, timed_out = 0;
         unsigned long slack = 0;
-        unsigned int ll_flag = POLL_LL;
+        unsigned int ll_flag = ll_get_flag();
         u64 ll_time = ll_end_time();
 
         rcu_read_lock();
@@ -497,7 +497,8 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
                         break;
                 }
 
-                if (can_ll && can_poll_ll(ll_time))
+                /* only if on, have sockets with POLL_LL and not out of time */
+                if (ll_flag && can_ll && can_poll_ll(ll_time))
                         continue;
 
                 /*
@@ -768,7 +769,7 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
         ktime_t expire, *to = NULL;
         int timed_out = 0, count = 0;
         unsigned long slack = 0;
-        unsigned int ll_flag = POLL_LL;
+        unsigned int ll_flag = ll_get_flag();
         u64 ll_time = ll_end_time();
 
         /* Optimise the no-wait case */
@@ -817,8 +818,10 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
                 if (count || timed_out)
                         break;
 
-                if (can_ll && can_poll_ll(ll_time))
+                /* only if on, have sockets with POLL_LL and not out of time */
+                if (ll_flag && can_ll && can_poll_ll(ll_time))
                         continue;
+
                 /*
                  * If this is the first loop and we have a timeout
                  * given, then we convert to ktime_t and set the to
diff --git a/include/net/ll_poll.h b/include/net/ll_poll.h
index 6d45e6f6ff4c..6c06f7c0a22d 100644
--- a/include/net/ll_poll.h
+++ b/include/net/ll_poll.h
@@ -37,6 +37,11 @@ extern unsigned int sysctl_net_ll_poll __read_mostly;
 #define LL_FLUSH_FAILED         -1
 #define LL_FLUSH_BUSY           -2
 
+static inline unsigned int ll_get_flag(void)
+{
+        return sysctl_net_ll_poll ? POLL_LL : 0;
+}
+
 /* a wrapper to make debug_smp_processor_id() happy
  * we can use sched_clock() because we don't care much about precision
  * we only care that the average is bounded
@@ -67,10 +72,14 @@ static inline u64 ll_sk_end_time(struct sock *sk)
         return ((u64)ACCESS_ONCE(sk->sk_ll_usec) << 10) + ll_sched_clock();
 }
 
-/* in poll/select we use the global sysctl_net_ll_poll value */
+/* in poll/select we use the global sysctl_net_ll_poll value
+ * only call sched_clock() if enabled
+ */
 static inline u64 ll_end_time(void)
 {
-        return ((u64)ACCESS_ONCE(sysctl_net_ll_poll) << 10) + ll_sched_clock();
+        u64 end_time = ACCESS_ONCE(sysctl_net_ll_poll);
+
+        return end_time ? (end_time << 10) + ll_sched_clock() : 0;
 }
 
 static inline bool sk_valid_ll(struct sock *sk)
@@ -141,6 +150,10 @@ static inline void sk_mark_ll(struct sock *sk, struct sk_buff *skb)
 }
 
 #else /* CONFIG_NET_LL_RX_POLL */
+static inline unsigned long ll_get_flag(void)
+{
+        return 0;
+}
 
 static inline u64 sk_ll_end_time(struct sock *sk)
 {