[PATCHSET RFC v2 0/5] Add support for epoll min_wait

[PATCHSET RFC v2 0/5] Add support for epoll min_wait

[Jens Axboe]

Hi,

tldr - we saw a 6-7% CPU reduction with this patch. See patch 4 for
full numbers.

This adds support for EPOLL_CTL_MIN_WAIT, which allows setting a minimum
time that epoll_wait() should wait for events on a given epoll context.
Some justification and numbers are in patch 5, patches 1-4 are really
just prep patches.

Sending this as an RFC to hash out the API, basically. This is
obviously a per-context type of operation in this patchset, which isn't
necessarily ideal for any use case. Questions to be debated:

1) Would we want this to be available through epoll_wait() directly?
   That would allow this to be done on a per-epoll_wait() basis, rather
   than be tied to the specific context.

2) If the answer to #1 is yes, would we still want EPOLL_CTL_MIN_WAIT?

I think there are pros and cons to both, and perhaps the answer to both
is "yes". There are some benefits to doing this at epoll setup time,
for example - it nicely isolates it to that part rather than needing
to be done dynamically everytime epoll_wait() is called. This also
helps the application code, as it can turn off any busy'ness tracking
based on if the setup accepted EPOLL_CTL_MIN_WAIT or not.

Anyway, tossing this out there as it yielded quite good results in
some initial testing, we're running more of it. Not a lot of changes
since v1, but it was posted in the middle of the merge window. Hoping
to get some more discussion this time around, or at least some...

Also available here:

https://git.kernel.dk/cgit/linux-block/log/?h=epoll-min_ts

Since v1:
- Split patch 4 a bit, to make the meat of the changes smaller
- Get rid of EPOLL_DEF_MIN_WAIT
- Rebase on current -git master

-- 
Jens Axboe

[PATCH 1/5] eventpoll: cleanup branches around sleeping for events

[Jens Axboe]

Rather than have two separate branches here, collapse them into a single
one instead. No functional changes here, just a cleanup in preparation
for changes in this area.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/eventpoll.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 52954d4637b5..3061bdde6cba 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1869,14 +1869,15 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * important.
 		 */
 		eavail = ep_events_available(ep);
-		if (!eavail)
+		if (!eavail) {
 			__add_wait_queue_exclusive(&ep->wq, &wait);
-
-		write_unlock_irq(&ep->lock);
-
-		if (!eavail)
+			write_unlock_irq(&ep->lock);
 			timed_out = !schedule_hrtimeout_range(to, slack,
 							      HRTIMER_MODE_ABS);
+		} else {
+			write_unlock_irq(&ep->lock);
+		}
+
 		__set_current_state(TASK_RUNNING);
 
 		/*
-- 
2.35.1

[PATCH 2/5] eventpoll: split out wait handling

[Jens Axboe]

In preparation for making changes to how wakeups and sleeps are done,
move the timeout scheduling into a helper and manage it rather than
rely on schedule_hrtimeout_range().

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/eventpoll.c | 70 ++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 56 insertions(+), 14 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 3061bdde6cba..f53bb4ec9e91 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1762,6 +1762,47 @@ static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
 	return ret;
 }
 
+struct epoll_wq {
+	wait_queue_entry_t wait;
+	struct hrtimer timer;
+	bool timed_out;
+};
+
+static enum hrtimer_restart ep_timer(struct hrtimer *timer)
+{
+	struct epoll_wq *ewq = container_of(timer, struct epoll_wq, timer);
+	struct task_struct *task = ewq->wait.private;
+
+	ewq->timed_out = true;
+	wake_up_process(task);
+	return HRTIMER_NORESTART;
+}
+
+static void ep_schedule(struct eventpoll *ep, struct epoll_wq *ewq, ktime_t *to,
+			u64 slack)
+{
+	if (ewq->timed_out)
+		return;
+	if (to && *to == 0) {
+		ewq->timed_out = true;
+		return;
+	}
+	if (!to) {
+		schedule();
+		return;
+	}
+
+	hrtimer_init_on_stack(&ewq->timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
+	ewq->timer.function = ep_timer;
+	hrtimer_set_expires_range_ns(&ewq->timer, *to, slack);
+	hrtimer_start_expires(&ewq->timer, HRTIMER_MODE_ABS);
+
+	schedule();
+
+	hrtimer_cancel(&ewq->timer);
+	destroy_hrtimer_on_stack(&ewq->timer);
+}
+
 /**
  * ep_poll - Retrieves ready events, and delivers them to the caller-supplied
  *           event buffer.
@@ -1782,13 +1823,15 @@ static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		   int maxevents, struct timespec64 *timeout)
 {
-	int res, eavail, timed_out = 0;
+	int res, eavail;
 	u64 slack = 0;
-	wait_queue_entry_t wait;
 	ktime_t expires, *to = NULL;
+	struct epoll_wq ewq;
 
 	lockdep_assert_irqs_enabled();
 
+	ewq.timed_out = false;
+
 	if (timeout && (timeout->tv_sec | timeout->tv_nsec)) {
 		slack = select_estimate_accuracy(timeout);
 		to = &expires;
@@ -1798,7 +1841,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * Avoid the unnecessary trip to the wait queue loop, if the
 		 * caller specified a non blocking operation.
 		 */
-		timed_out = 1;
+		ewq.timed_out = 1;
 	}
 
 	/*
@@ -1823,10 +1866,10 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 				return res;
 		}
 
-		if (timed_out)
+		if (ewq.timed_out)
 			return 0;
 
-		eavail = ep_busy_loop(ep, timed_out);
+		eavail = ep_busy_loop(ep, ewq.timed_out);
 		if (eavail)
 			continue;
 
@@ -1850,8 +1893,8 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * performance issue if a process is killed, causing all of its
 		 * threads to wake up without being removed normally.
 		 */
-		init_wait(&wait);
-		wait.func = ep_autoremove_wake_function;
+		init_wait(&ewq.wait);
+		ewq.wait.func = ep_autoremove_wake_function;
 
 		write_lock_irq(&ep->lock);
 		/*
@@ -1870,10 +1913,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 */
 		eavail = ep_events_available(ep);
 		if (!eavail) {
-			__add_wait_queue_exclusive(&ep->wq, &wait);
+			__add_wait_queue_exclusive(&ep->wq, &ewq.wait);
 			write_unlock_irq(&ep->lock);
-			timed_out = !schedule_hrtimeout_range(to, slack,
-							      HRTIMER_MODE_ABS);
+			ep_schedule(ep, &ewq, to, slack);
 		} else {
 			write_unlock_irq(&ep->lock);
 		}
@@ -1887,7 +1929,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 */
 		eavail = 1;
 
-		if (!list_empty_careful(&wait.entry)) {
+		if (!list_empty_careful(&ewq.wait.entry)) {
 			write_lock_irq(&ep->lock);
 			/*
 			 * If the thread timed out and is not on the wait queue,
@@ -1896,9 +1938,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 			 * Thus, when wait.entry is empty, it needs to harvest
 			 * events.
 			 */
-			if (timed_out)
-				eavail = list_empty(&wait.entry);
-			__remove_wait_queue(&ep->wq, &wait);
+			if (ewq.timed_out)
+				eavail = list_empty(&ewq.wait.entry);
+			__remove_wait_queue(&ep->wq, &ewq.wait);
 			write_unlock_irq(&ep->lock);
 		}
 	}
-- 
2.35.1

[PATCH 3/5] eventpoll: move expires to epoll_wq

[Jens Axboe]

This makes the expiration available to the wakeup handler. No functional
changes expected in this patch, purely in preparation for being able to
use the timeout on the wakeup side.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/eventpoll.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index f53bb4ec9e91..8b3c94ab7762 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1765,6 +1765,7 @@ static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
 struct epoll_wq {
 	wait_queue_entry_t wait;
 	struct hrtimer timer;
+	ktime_t timeout_ts;
 	bool timed_out;
 };
 
@@ -1825,7 +1826,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 {
 	int res, eavail;
 	u64 slack = 0;
-	ktime_t expires, *to = NULL;
+	ktime_t *to = NULL;
 	struct epoll_wq ewq;
 
 	lockdep_assert_irqs_enabled();
@@ -1834,7 +1835,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 
 	if (timeout && (timeout->tv_sec | timeout->tv_nsec)) {
 		slack = select_estimate_accuracy(timeout);
-		to = &expires;
+		to = &ewq.timeout_ts;
 		*to = timespec64_to_ktime(*timeout);
 	} else if (timeout) {
 		/*
-- 
2.35.1

[PATCH 4/5] eventpoll: move file checking earlier for epoll_ctl()

[Jens Axboe]

This just cleans up the checking a bit, in preparation for a change
that will need access to 'ep' earlier.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/eventpoll.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 8b3c94ab7762..cd2138d02bda 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2111,6 +2111,20 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
 	if (!f.file)
 		goto error_return;
 
+	/*
+	 * We have to check that the file structure underneath the file
+	 * descriptor the user passed to us _is_ an eventpoll file.
+	 */
+	error = -EINVAL;
+	if (!is_file_epoll(f.file))
+		goto error_fput;
+
+	/*
+	 * At this point it is safe to assume that the "private_data" contains
+	 * our own data structure.
+	 */
+	ep = f.file->private_data;
+
 	/* Get the "struct file *" for the target file */
 	tf = fdget(fd);
 	if (!tf.file)
@@ -2126,12 +2140,10 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
 		ep_take_care_of_epollwakeup(epds);
 
 	/*
-	 * We have to check that the file structure underneath the file descriptor
-	 * the user passed to us _is_ an eventpoll file. And also we do not permit
-	 * adding an epoll file descriptor inside itself.
+	 * We do not permit adding an epoll file descriptor inside itself.
 	 */
 	error = -EINVAL;
-	if (f.file == tf.file || !is_file_epoll(f.file))
+	if (f.file == tf.file)
 		goto error_tgt_fput;
 
 	/*
@@ -2147,12 +2159,6 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
 			goto error_tgt_fput;
 	}
 
-	/*
-	 * At this point it is safe to assume that the "private_data" contains
-	 * our own data structure.
-	 */
-	ep = f.file->private_data;
-
 	/*
 	 * When we insert an epoll file descriptor inside another epoll file
 	 * descriptor, there is the chance of creating closed loops, which are
-- 
2.35.1

[PATCH 5/5] eventpoll: add support for min-wait

[Jens Axboe]

Rather than just have a timeout value for waiting on events, add
EPOLL_CTL_MIN_WAIT to allow setting a minimum time that epoll_wait()
should always wait for events to arrive.

For medium workload efficiencies, some production workloads inject
artificial timers or sleeps before calling epoll_wait() to get
better batching and higher efficiencies. While this does help, it's
not as efficient as it could be. By adding support for epoll_wait()
for this directly, we can avoids extra context switches and scheduler
and timer overhead.

As an example, running an AB test on an identical workload at about
~370K reqs/second, without this change and with the sleep hack
mentioned above (using 200 usec as the timeout), we're doing 310K-340K
non-voluntary context switches per second. Idle CPU on the host is 27-34%.
With the the sleep hack removed and epoll set to the same 200 usec
value, we're handling the exact same load but at 292K-315k non-voluntary
context switches and idle CPU of 33-41%, a substantial win.

Basic test case:

struct d {
        int p1, p2;
};

static void *fn(void *data)
{
        struct d *d = data;
        char b = 0x89;

	/* Generate 2 events 20 msec apart */
        usleep(10000);
        write(d->p1, &b, sizeof(b));
        usleep(10000);
        write(d->p2, &b, sizeof(b));

        return NULL;
}

int main(int argc, char *argv[])
{
        struct epoll_event ev, events[2];
        pthread_t thread;
        int p1[2], p2[2];
        struct d d;
        int efd, ret;

        efd = epoll_create1(0);
        if (efd < 0) {
                perror("epoll_create");
                return 1;
        }

        if (pipe(p1) < 0) {
                perror("pipe");
                return 1;
        }
        if (pipe(p2) < 0) {
                perror("pipe");
                return 1;
        }

        ev.events = EPOLLIN;
        ev.data.fd = p1[0];
        if (epoll_ctl(efd, EPOLL_CTL_ADD, p1[0], &ev) < 0) {
                perror("epoll add");
                return 1;
        }
        ev.events = EPOLLIN;
        ev.data.fd = p2[0];
        if (epoll_ctl(efd, EPOLL_CTL_ADD, p2[0], &ev) < 0) {
                perror("epoll add");
                return 1;
        }

	/* always wait 200 msec for events */
        ev.data.u64 = 200000;
        if (epoll_ctl(efd, EPOLL_CTL_MIN_WAIT, -1, &ev) < 0) {
                perror("epoll add set timeout");
                return 1;
        }

        d.p1 = p1[1];
        d.p2 = p2[1];
        pthread_create(&thread, NULL, fn, &d);

	/* expect to get 2 events here rather than just 1 */
        ret = epoll_wait(efd, events, 2, -1);
        printf("epoll_wait=%d\n", ret);

        return 0;
}

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/eventpoll.c                 | 100 ++++++++++++++++++++++++++++-----
 include/linux/eventpoll.h      |   2 +-
 include/uapi/linux/eventpoll.h |   1 +
 3 files changed, 87 insertions(+), 16 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index cd2138d02bda..828e2b9771d6 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -117,6 +117,9 @@ struct eppoll_entry {
 	/* The "base" pointer is set to the container "struct epitem" */
 	struct epitem *base;
 
+	/* min wait time if (min_wait_ts) & 1 != 0 */
+	ktime_t min_wait_ts;
+
 	/*
 	 * Wait queue item that will be linked to the target file wait
 	 * queue head.
@@ -217,6 +220,9 @@ struct eventpoll {
 	u64 gen;
 	struct hlist_head refs;
 
+	/* min wait for epoll_wait() */
+	unsigned int min_wait_ts;
+
 #ifdef CONFIG_NET_RX_BUSY_POLL
 	/* used to track busy poll napi_id */
 	unsigned int napi_id;
@@ -1747,6 +1753,32 @@ static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms)
 	return to;
 }
 
+struct epoll_wq {
+	wait_queue_entry_t wait;
+	struct hrtimer timer;
+	ktime_t timeout_ts;
+	ktime_t min_wait_ts;
+	struct eventpoll *ep;
+	bool timed_out;
+	int maxevents;
+	int wakeups;
+};
+
+static bool ep_should_min_wait(struct epoll_wq *ewq)
+{
+	if (ewq->min_wait_ts & 1) {
+		/* just an approximation */
+		if (++ewq->wakeups >= ewq->maxevents)
+			goto stop_wait;
+		if (ktime_before(ktime_get_ns(), ewq->min_wait_ts))
+			return true;
+	}
+
+stop_wait:
+	ewq->min_wait_ts &= ~(u64) 1;
+	return false;
+}
+
 /*
  * autoremove_wake_function, but remove even on failure to wake up, because we
  * know that default_wake_function/ttwu will only fail if the thread is already
@@ -1756,27 +1788,37 @@ static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms)
 static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
 				       unsigned int mode, int sync, void *key)
 {
-	int ret = default_wake_function(wq_entry, mode, sync, key);
+	struct epoll_wq *ewq = container_of(wq_entry, struct epoll_wq, wait);
+	int ret;
+
+	/*
+	 * If min wait time hasn't been satisfied yet, keep waiting
+	 */
+	if (ep_should_min_wait(ewq))
+		return 0;
 
+	ret = default_wake_function(wq_entry, mode, sync, key);
 	list_del_init(&wq_entry->entry);
 	return ret;
 }
 
-struct epoll_wq {
-	wait_queue_entry_t wait;
-	struct hrtimer timer;
-	ktime_t timeout_ts;
-	bool timed_out;
-};
-
 static enum hrtimer_restart ep_timer(struct hrtimer *timer)
 {
 	struct epoll_wq *ewq = container_of(timer, struct epoll_wq, timer);
 	struct task_struct *task = ewq->wait.private;
+	const bool is_min_wait = ewq->min_wait_ts & 1;
+
+	if (!is_min_wait || ep_events_available(ewq->ep)) {
+		if (!is_min_wait)
+			ewq->timed_out = true;
+		ewq->min_wait_ts &= ~(u64) 1;
+		wake_up_process(task);
+		return HRTIMER_NORESTART;
+	}
 
-	ewq->timed_out = true;
-	wake_up_process(task);
-	return HRTIMER_NORESTART;
+	ewq->min_wait_ts &= ~(u64) 1;
+	hrtimer_set_expires_range_ns(&ewq->timer, ewq->timeout_ts, 0);
+	return HRTIMER_RESTART;
 }
 
 static void ep_schedule(struct eventpoll *ep, struct epoll_wq *ewq, ktime_t *to,
@@ -1831,12 +1873,14 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 
 	lockdep_assert_irqs_enabled();
 
+	ewq.ep = ep;
 	ewq.timed_out = false;
+	ewq.maxevents = maxevents;
+	ewq.wakeups = 0;
 
 	if (timeout && (timeout->tv_sec | timeout->tv_nsec)) {
 		slack = select_estimate_accuracy(timeout);
-		to = &ewq.timeout_ts;
-		*to = timespec64_to_ktime(*timeout);
+		ewq.timeout_ts = timespec64_to_ktime(*timeout);
 	} else if (timeout) {
 		/*
 		 * Avoid the unnecessary trip to the wait queue loop, if the
@@ -1845,6 +1889,21 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		ewq.timed_out = 1;
 	}
 
+	/*
+	 * If min_wait is set for this epoll instance, note the min_wait
+	 * time. Ensure the lowest bit is set in ewq.min_wait_ts, that's
+	 * the state bit for whether or not min_wait is enabled.
+	 */
+	if (ep->min_wait_ts) {
+		ewq.min_wait_ts = ktime_add_us(ktime_get_ns(),
+						ep->min_wait_ts);
+		ewq.min_wait_ts |= (u64) 1;
+		to = &ewq.min_wait_ts;
+	} else {
+		ewq.min_wait_ts = 0;
+		to = &ewq.timeout_ts;
+	}
+
 	/*
 	 * This call is racy: We may or may not see events that are being added
 	 * to the ready list under the lock (e.g., in IRQ callbacks). For cases
@@ -1913,7 +1972,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * important.
 		 */
 		eavail = ep_events_available(ep);
-		if (!eavail) {
+		if (!eavail || ewq.min_wait_ts & 1) {
 			__add_wait_queue_exclusive(&ep->wq, &ewq.wait);
 			write_unlock_irq(&ep->lock);
 			ep_schedule(ep, &ewq, to, slack);
@@ -2125,6 +2184,17 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
 	 */
 	ep = f.file->private_data;
 
+	/*
+	 * Handle EPOLL_CTL_MIN_WAIT upfront as we don't need to care about
+	 * the fd being passed in.
+	 */
+	if (op == EPOLL_CTL_MIN_WAIT) {
+		/* return old value */
+		error = ep->min_wait_ts;
+		ep->min_wait_ts = epds->data;
+		goto error_fput;
+	}
+
 	/* Get the "struct file *" for the target file */
 	tf = fdget(fd);
 	if (!tf.file)
@@ -2257,7 +2327,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
 {
 	struct epoll_event epds;
 
-	if (ep_op_has_event(op) &&
+	if ((ep_op_has_event(op) || op == EPOLL_CTL_MIN_WAIT) &&
 	    copy_from_user(&epds, event, sizeof(struct epoll_event)))
 		return -EFAULT;
 
diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
index 3337745d81bd..cbef635cb7e4 100644
--- a/include/linux/eventpoll.h
+++ b/include/linux/eventpoll.h
@@ -59,7 +59,7 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
 /* Tells if the epoll_ctl(2) operation needs an event copy from userspace */
 static inline int ep_op_has_event(int op)
 {
-	return op != EPOLL_CTL_DEL;
+	return op != EPOLL_CTL_DEL && op != EPOLL_CTL_MIN_WAIT;
 }
 
 #else
diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
index 8a3432d0f0dc..81ecb1ca36e0 100644
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -26,6 +26,7 @@
 #define EPOLL_CTL_ADD 1
 #define EPOLL_CTL_DEL 2
 #define EPOLL_CTL_MOD 3
+#define EPOLL_CTL_MIN_WAIT	4
 
 /* Epoll event masks */
 #define EPOLLIN		(__force __poll_t)0x00000001
-- 
2.35.1