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    Subject[PATCH 6.0 344/862] once: add DO_ONCE_SLOW() for sleepable contexts
    Date
    From: Eric Dumazet <edumazet@google.com>

    [ Upstream commit 62c07983bef9d3e78e71189441e1a470f0d1e653 ]

    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@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>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 b14d8b309d52..176ab75b42df 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 59149bf3bfb4..351f66aad310 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 b9d995b5ce24..f5950a7172d6 100644
    --- a/net/ipv4/inet_hashtables.c
    +++ b/net/ipv4/inet_hashtables.c
    @@ -729,8 +729,8 @@ 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);
    --
    2.35.1


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