once: add DO_ONCE_SLOW() for sleepable contexts

Christophe Leroy reported a ~80ms latency spike
happening at first TCP connect() time.

This is because __inet_hash_connect() uses get_random_once()
to populate a perturbation table which became quite big
after commit 85bf0dc6b58d ("tcp: increase source port perturb table to 2^16")

get_random_once() uses DO_ONCE(), which block hard irqs for the duration
of the operation.

This patch adds DO_ONCE_SLOW() which uses a mutex instead of a spinlock
for operations where we prefer to stay in process context.

Then __inet_hash_connect() can use get_random_slow_once()
to populate its perturbation table.

Fixes: 85bf0dc6b58d ("tcp: increase source port perturb table to 2^16")
Fixes: 57c2c1517858 ("tcp: change source port randomizarion at connect() time")
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/netdev/CANn89iLAEYBaoYajy0Y9UmGFff5GPxDUoG-ErVB2jDdRNQ5Tug@mail.gmail.com/T/#t
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willy Tarreau <w@1wt.eu>
Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/once.h b/include/linux/once.h
index b14d8b309d52b198bb144689fe67d9ed235c2b3e..176ab75b42df740a738d04d8480821a0b3b65ba9 100644 (file)
--- a/include/linux/once.h
+++ b/include/linux/once.h
@@ -5,10 +5,18 @@
 #include <linux/types.h>
 #include <linux/jump_label.h>
 
+/* Helpers used from arbitrary contexts.
+ * Hard irqs are blocked, be cautious.
+ */
 bool __do_once_start(bool *done, unsigned long *flags);
 void __do_once_done(bool *done, struct static_key_true *once_key,
                    unsigned long *flags, struct module *mod);
 
+/* Variant for process contexts only. */
+bool __do_once_slow_start(bool *done);
+void __do_once_slow_done(bool *done, struct static_key_true *once_key,
+                        struct module *mod);
+
 /* Call a function exactly once. The idea of DO_ONCE() is to perform
  * a function call such as initialization of random seeds, etc, only
  * once, where DO_ONCE() can live in the fast-path. After @func has
@@ -52,7 +60,27 @@ void __do_once_done(bool *done, struct static_key_true *once_key,
                ___ret;                                                      \
        })
 
+/* Variant of DO_ONCE() for process/sleepable contexts. */
+#define DO_ONCE_SLOW(func, ...)                                                     \
+       ({                                                                   \
+               bool ___ret = false;                                         \
+               static bool __section(".data.once") ___done = false;         \
+               static DEFINE_STATIC_KEY_TRUE(___once_key);                  \
+               if (static_branch_unlikely(&___once_key)) {                  \
+                       ___ret = __do_once_slow_start(&___done);             \
+                       if (unlikely(___ret)) {                              \
+                               func(__VA_ARGS__);                           \
+                               __do_once_slow_done(&___done, &___once_key,  \
+                                                   THIS_MODULE);            \
+                       }                                                    \
+               }                                                            \
+               ___ret;                                                      \
+       })
+
 #define get_random_once(buf, nbytes)                                        \
        DO_ONCE(get_random_bytes, (buf), (nbytes))
 
+#define get_random_slow_once(buf, nbytes)                                   \
+       DO_ONCE_SLOW(get_random_bytes, (buf), (nbytes))
+
 #endif /* _LINUX_ONCE_H */

diff --git a/lib/once.c b/lib/once.c
index 59149bf3bfb4a97e4fa7febee737155d700bae48..351f66aad310a47f17d0636da0ed5b2b4460522d 100644 (file)
--- a/lib/once.c
+++ b/lib/once.c
@@ -66,3 +66,33 @@ void __do_once_done(bool *done, struct static_key_true *once_key,
        once_disable_jump(once_key, mod);
 }
 EXPORT_SYMBOL(__do_once_done);
+
+static DEFINE_MUTEX(once_mutex);
+
+bool __do_once_slow_start(bool *done)
+       __acquires(once_mutex)
+{
+       mutex_lock(&once_mutex);
+       if (*done) {
+               mutex_unlock(&once_mutex);
+               /* Keep sparse happy by restoring an even lock count on
+                * this mutex. In case we return here, we don't call into
+                * __do_once_done but return early in the DO_ONCE_SLOW() macro.
+                */
+               __acquire(once_mutex);
+               return false;
+       }
+
+       return true;
+}
+EXPORT_SYMBOL(__do_once_slow_start);
+
+void __do_once_slow_done(bool *done, struct static_key_true *once_key,
+                        struct module *mod)
+       __releases(once_mutex)
+{
+       *done = true;
+       mutex_unlock(&once_mutex);
+       once_disable_jump(once_key, mod);
+}
+EXPORT_SYMBOL(__do_once_slow_done);

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 49db8c597eea83a27e91edc429c2c4779b0a5cd7..dc1c5629cd0d61716d6d99131c57b49717785709 100644 (file)
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -958,8 +958,8 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
        if (likely(remaining > 1))
                remaining &= ~1U;
 
-       net_get_random_once(table_perturb,
-                           INET_TABLE_PERTURB_SIZE * sizeof(*table_perturb));
+       get_random_slow_once(table_perturb,
+                            INET_TABLE_PERTURB_SIZE * sizeof(*table_perturb));
        index = port_offset & (INET_TABLE_PERTURB_SIZE - 1);
 
        offset = READ_ONCE(table_perturb[index]) + (port_offset >> 32);