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Message-Id: <20221001205102.2319658-1-eric.dumazet@gmail.com>
Date: Sat, 1 Oct 2022 13:51:02 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: "David S . Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Cc: netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Eric Dumazet <eric.dumazet@...il.com>,
Christophe Leroy <christophe.leroy@...roup.eu>,
Willy Tarreau <w@....eu>
Subject: [PATCH net-next] once: add DO_ONCE_SLOW() for sleepable contexts
From: Eric Dumazet <edumazet@...gle.com>
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 4c2c8f03a5ab ("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: 4c2c8f03a5ab ("tcp: increase source port perturb table to 2^16")
Fixes: 190cc82489f4 ("tcp: change source port randomizarion at connect() time")
Reported-by: Christophe Leroy <christophe.leroy@...roup.eu>
Link: https://lore.kernel.org/netdev/CANn89iLAEYBaoYajy0Y9UmGFff5GPxDUoG-ErVB2jDdRNQ5Tug@mail.gmail.com/T/#t
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Willy Tarreau <w@....eu>
---
include/linux/once.h | 28 ++++++++++++++++++++++++++++
lib/once.c | 30 ++++++++++++++++++++++++++++++
net/ipv4/inet_hashtables.c | 4 ++--
3 files changed, 60 insertions(+), 2 deletions(-)
diff --git a/include/linux/once.h b/include/linux/once.h
index b14d8b309d52b198bb144689fe67d9ed235c2b3e..176ab75b42df740a738d04d8480821a0b3b65ba9 100644
--- 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
--- 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
--- 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);
--
2.38.0.rc1.362.ged0d419d3c-goog
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