[RFC PATCH 1/3 v3] futex: introduce FUTEX_SWAP operation

From: Peter Oskolkov
Date: Wed Jun 24 2020 - 14:53:37 EST

From: Peter Oskolkov <posk@xxxxxxxxxx>

This is an RFC!

As Paul Turner presented at LPC in 2013 ...
- pdf: http://pdxplumbers.osuosl.org/2013/ocw//system/presentations/1653/original/LPC%20-%20User%20Threading.pdf
- video: https://www.youtube.com/watch?v=KXuZi9aeGTw

... Google has developed an M:N userspace threading subsystem backed
by Google-private SwitchTo Linux Kernel API (page 17 in the pdf referenced
above). This subsystem provides latency-sensitive services at Google with
fine-grained user-space control/scheduling over what is running when,
and this subsystem is used widely internally (called schedulers or fibers).

This RFC patchset is the first step to open-source this work. As explained
in the linked pdf and video, SwitchTo API has three core operations: wait,
resume, and swap (=switch). So this patchset adds a FUTEX_SWAP operation
that, in addition to FUTEX_WAIT and FUTEX_WAKE, will provide a foundation
on top of which user-space threading libraries can be built.

Another common use case for FUTEX_SWAP is message passing a-la RPC
between tasks: task/thread T1 prepares a message,
wakes T2 to work on it, and waits for the results; when T2 is done, it
wakes T1 and waits for more work to arrive. Currently the simplest
way to implement this is

a. T1: futex-wake T2, futex-wait
b. T2: wakes, does what it has been woken to do
c. T2: futex-wake T1, futex-wait

With FUTEX_SWAP, steps a and c above can be reduced to one futex operation
that runs 5-10 times faster.

Patches in this patchset:

Patch 1: (this patch) introduce FUTEX_SWAP futex operation that,
internally, does wake + wait. The purpose of this patch is
to work out the API.
Patch 2: a first rough attempt to make FUTEX_SWAP faster than
what wake + wait can do.
Patch 3: a selftest that can also be used to benchmark FUTEX_SWAP vs
FUTEX_WAKE + FUTEX_WAIT.

Tested: see patch 3 in this patchset.

Signed-off-by: Peter Oskolkov <posk@xxxxxxxxxx>
---
include/uapi/linux/futex.h | 2 +
kernel/futex.c | 97 +++++++++++++++++++++++++++++++-------
2 files changed, 83 insertions(+), 16 deletions(-)

diff --git a/include/uapi/linux/futex.h b/include/uapi/linux/futex.h
index a89eb0accd5e..c1d151d97dea 100644
--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -21,6 +21,7 @@
#define FUTEX_WAKE_BITSET 10
#define FUTEX_WAIT_REQUEUE_PI 11
#define FUTEX_CMP_REQUEUE_PI 12
+#define FUTEX_SWAP 13

#define FUTEX_PRIVATE_FLAG 128
#define FUTEX_CLOCK_REALTIME 256
@@ -40,6 +41,7 @@
FUTEX_PRIVATE_FLAG)
#define FUTEX_CMP_REQUEUE_PI_PRIVATE (FUTEX_CMP_REQUEUE_PI | \
FUTEX_PRIVATE_FLAG)
+#define FUTEX_SWAP_PRIVATE (FUTEX_SWAP | FUTEX_PRIVATE_FLAG)

/*
* Support for robust futexes: the kernel cleans up held futexes at
diff --git a/kernel/futex.c b/kernel/futex.c
index e646661f6282..670d6d113561 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1595,16 +1595,16 @@ double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
}

/*
- * Wake up waiters matching bitset queued on this futex (uaddr).
+ * Prepare wake queue matching bitset queued on this futex (uaddr).
*/
static int
-futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
+prepare_wake_q(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset,
+ struct wake_q_head *wake_q)
{
struct futex_hash_bucket *hb;
struct futex_q *this, *next;
union futex_key key = FUTEX_KEY_INIT;
int ret;
- DEFINE_WAKE_Q(wake_q);

if (!bitset)
return -EINVAL;
@@ -1632,20 +1632,34 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
if (!(this->bitset & bitset))
continue;

- mark_wake_futex(&wake_q, this);
+ mark_wake_futex(wake_q, this);
if (++ret >= nr_wake)
break;
}
}

spin_unlock(&hb->lock);
- wake_up_q(&wake_q);
out_put_key:
put_futex_key(&key);
out:
return ret;
}

+/*
+ * Wake up waiters matching bitset queued on this futex (uaddr).
+ */
+static int
+futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
+{
+ int ret;
+ DEFINE_WAKE_Q(wake_q);
+
+ ret = prepare_wake_q(uaddr, flags, nr_wake, bitset, &wake_q);
+ wake_up_q(&wake_q);
+
+ return ret;
+}
+
static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
{
unsigned int op = (encoded_op & 0x70000000) >> 28;
@@ -2603,9 +2617,12 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
* @hb: the futex hash bucket, must be locked by the caller
* @q: the futex_q to queue up on
* @timeout: the prepared hrtimer_sleeper, or null for no timeout
+ * @next: if present, wake next and hint to the scheduler that we'd
+ * prefer to execute it locally.
*/
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
- struct hrtimer_sleeper *timeout)
+ struct hrtimer_sleeper *timeout,
+ struct task_struct *next)
{
/*
* The task state is guaranteed to be set before another task can
@@ -2630,10 +2647,27 @@ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
* flagged for rescheduling. Only call schedule if there
* is no timeout, or if it has yet to expire.
*/
- if (!timeout || timeout->task)
+ if (!timeout || timeout->task) {
+ if (next) {
+ /*
+ * wake_up_process() below will be replaced
+ * in the next patch with
+ * wake_up_process_prefer_current_cpu().
+ */
+ wake_up_process(next);
+ put_task_struct(next);
+ next = NULL;
+ }
freezable_schedule();
+ }
}
__set_current_state(TASK_RUNNING);
+
+ if (next) {
+ /* Maybe call wake_up_process_prefer_current_cpu()? */
+ wake_up_process(next);
+ put_task_struct(next);
+ }
}

/**
@@ -2713,7 +2747,7 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
}

static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
- ktime_t *abs_time, u32 bitset)
+ ktime_t *abs_time, u32 bitset, struct task_struct *next)
{
struct hrtimer_sleeper timeout, *to;
struct restart_block *restart;
@@ -2737,7 +2771,8 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
goto out;

/* queue_me and wait for wakeup, timeout, or a signal. */
- futex_wait_queue_me(hb, &q, to);
+ futex_wait_queue_me(hb, &q, to, next);
+ next = NULL;

/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
@@ -2770,6 +2805,10 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
ret = -ERESTART_RESTARTBLOCK;

out:
+ if (next) {
+ wake_up_process(next);
+ put_task_struct(next);
+ }
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
@@ -2777,7 +2816,6 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
return ret;
}

-
static long futex_wait_restart(struct restart_block *restart)
{
u32 __user *uaddr = restart->futex.uaddr;
@@ -2789,10 +2827,35 @@ static long futex_wait_restart(struct restart_block *restart)
}
restart->fn = do_no_restart_syscall;

- return (long)futex_wait(uaddr, restart->futex.flags,
- restart->futex.val, tp, restart->futex.bitset);
+ return (long)futex_wait(uaddr, restart->futex.flags, restart->futex.val,
+ tp, restart->futex.bitset, NULL);
}

+static int futex_swap(u32 __user *uaddr, unsigned int flags, u32 val,
+ ktime_t *abs_time, u32 __user *uaddr2)
+{
+ u32 bitset = FUTEX_BITSET_MATCH_ANY;
+ struct task_struct *next = NULL;
+ DEFINE_WAKE_Q(wake_q);
+ int ret;
+
+ ret = prepare_wake_q(uaddr2, flags, 1, bitset, &wake_q);
+ if (!wake_q_empty(&wake_q)) {
+ /* Pull the first wakee out of the queue to swap into. */
+ next = container_of(wake_q.first, struct task_struct, wake_q);
+ wake_q.first = wake_q.first->next;
+ next->wake_q.next = NULL;
+ /*
+ * Note that wake_up_q does not touch wake_q.last, so we
+ * do not bother with it here.
+ */
+ wake_up_q(&wake_q);
+ }
+ if (ret < 0)
+ return ret;
+
+ return futex_wait(uaddr, flags, val, abs_time, bitset, next);
+}

/*
* Userspace tried a 0 -> TID atomic transition of the futex value
@@ -3278,7 +3341,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
}

/* Queue the futex_q, drop the hb lock, wait for wakeup. */
- futex_wait_queue_me(hb, &q, to);
+ futex_wait_queue_me(hb, &q, to, NULL);

spin_lock(&hb->lock);
ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
@@ -3808,7 +3871,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
val3 = FUTEX_BITSET_MATCH_ANY;
/* fall through */
case FUTEX_WAIT_BITSET:
- return futex_wait(uaddr, flags, val, timeout, val3);
+ return futex_wait(uaddr, flags, val, timeout, val3, NULL);
case FUTEX_WAKE:
val3 = FUTEX_BITSET_MATCH_ANY;
/* fall through */
@@ -3832,6 +3895,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
uaddr2);
case FUTEX_CMP_REQUEUE_PI:
return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
+ case FUTEX_SWAP:
+ return futex_swap(uaddr, flags, val, timeout, uaddr2);
}
return -ENOSYS;
}
@@ -3848,7 +3913,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,

if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
cmd == FUTEX_WAIT_BITSET ||
- cmd == FUTEX_WAIT_REQUEUE_PI)) {
+ cmd == FUTEX_WAIT_REQUEUE_PI || cmd == FUTEX_SWAP)) {
if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG))))
return -EFAULT;
if (get_timespec64(&ts, utime))
@@ -3857,7 +3922,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
return -EINVAL;

t = timespec64_to_ktime(ts);
- if (cmd == FUTEX_WAIT)
+ if (cmd == FUTEX_WAIT || cmd == FUTEX_SWAP)
t = ktime_add_safe(ktime_get(), t);
tp = &t;
}
--
2.25.1