[PATCH v4 00/14] epoll: support pollable epoll from userspace

From: Roman Penyaev
Date: Tue Jun 11 2019 - 10:59:37 EST

Hi all,

This is v4 which introduces pollable epoll from userspace.

Changes based on Peter Zijlstra remarks:

- remove wmb() which was used incorrectly on event path
- remove "atomic_" prefix from some local helpers in order not to
mix them with the atomic_t API.
- store and read all member shared with userspace using WRITE/READ_ONCE
in order to avoid store/load tearing.
- do xchg(&epi->event.events, event->events) in ep_modify() instead of
plain write to avoid value corruption on archs (parisc, sparc32 and
arc-eznps) where atomic ops support is limited.
- change lockless algorithm which adds events to the uring, mainly
get rid of busy loop on user side, which can last uncontrollably
long. The drawbacks are additional 2 cmpxchg ops on hot event path
and complexity of the add_event_to_uring() logic.
- no gcc atomic builtins are used

- add epoll_create2 syscall to all syscall*.tbl files (Arnd Bergmann)
- add uepoll kselftests: testing/selftests/uepoll/* (Andrew Morton)
- reduce size of epitem structure for the original epoll case (Eric Wong)

- queue work to system high priority workqueue

- Measurements made, represented below.

- Fix alignment for epoll_uitem structure on all 64-bit archs except
x86-64. epoll_uitem should be always 16 bit, proper BUILD_BUG_ON
is added. (Linus)

- Check pollflags explicitly on 0 inside work callback, and do nothing
if 0.

- No reallocations, the max number of items (thus size of the user ring)
is specified by the caller.

- Interface is simplified: -ENOSPC is returned on attempt to add a new
epoll item if number is reached the max, nothing more.

- Alloced pages are accounted using user->locked_vm and limited to

- EPOLLONESHOT is handled.

This series introduces pollable epoll from userspace, i.e. user creates
epfd with a new EPOLL_USERPOLL flag, mmaps epoll descriptor, gets header
and ring pointers and then consumes ready events from a ring, avoiding
epoll_wait() call. When ring is empty, user has to call epoll_wait()
in order to wait for new events. epoll_wait() returns -ESTALE if user
ring has events in the ring (kind of indication, that user has to consume
events from the user ring first, I could not invent anything better than
returning -ESTALE).

For user header and user ring allocation I used vmalloc_user(). I found
that it is much easy to reuse remap_vmalloc_range_partial() instead of
dealing with page cache (like aio.c does). What is also nice is that
virtual address is properly aligned on SHMLBA, thus there should not be
any d-cache aliasing problems on archs with vivt or vipt caches.

** Measurements

In order to measure polling from userspace libevent was modified [1] and
bench_http benchmark (client and server) was used:

o EPOLLET, original epoll:

20000 requests in 0.551306 sec. (36277.49 throughput)
Each took about 5.54 msec latency
1600000bytes read. 0 errors.

o EPOLLET + polling from userspace:

20000 requests in 0.475585 sec. (42053.47 throughput)
Each took about 4.78 msec latency
1600000bytes read. 0 errors.

So harvesting events from userspace gives 15% gain. Though bench_http
is not ideal benchmark, but at least it is the part of libevent and was
easy to modify.

Worth to mention that uepoll is very sensible to CPU, e.g. the gain above
is observed on desktop "Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz", but on
"Intel(R) Xeon(R) Silver 4110 CPU @ 2.10GHz" measurements are almost the
same for both runs.

** Limitations

1. Expect always EPOLLET flag for new epoll items (Edge Triggered behavior)
obviously we can't call vfs_epoll() from userpace to have level
triggered behaviour.

2. No support for EPOLLWAKEUP
events are consumed from userspace, thus no way to call __pm_relax()

3. No support for EPOLLEXCLUSIVE
If device does not pass pollflags to wake_up() there is no way to
call poll() from the context under spinlock, thus special work is
scheduled to offload polling. In this specific case we can't
support exclusive wakeups, because we do not know actual result
of scheduled work and have to wake up every waiter.

** Principle of operation

* Basic structures shared with userspace:

In order to consume events from userspace all inserted items should be
stored in items array, which has original epoll_event field and u32
field for keeping ready events, i.e. each item has the following struct:

struct epoll_uitem {
__poll_t ready_events;
__poll_t events;
__u64 data;
BUILD_BUG_ON(sizeof(struct epoll_uitem) != 16);

And the following is a header, which is seen by userspace:

struct epoll_uheader {
u32 magic; /* epoll user header magic */
u32 header_length; /* length of the header + items */
u32 index_length; /* length of the index ring, always pow2 */
u32 max_items_nr; /* max num of items */
u32 head; /* updated by userland */
u32 int tail; /* updated by kernel */

struct epoll_uitem items[] __aligned(128);

/* Header is 128 bytes, thus items are aligned on CPU cache */
BUILD_BUG_ON(sizeof(struct epoll_uheader) != 128);

In order to poll epfd from userspace application has to call:

epoll_create2(EPOLL_USERPOLL, max_items_nr);

Ready events are kept in a ring buffer, which is simply an index table,
where each element points to an item in a header:

unsinged int *user_index;

* How is new event accounted on kernel side? Hot it is consumed from
* userspace?

When new event comes for some epoll item kernel does the following:

struct epoll_uitem *uitem;

/* Each item has a bit (index in user items array), discussed later */
uitem = user_header->items[epi->bit];

if (!atomic_fetch_or(uitem->ready_events, pollflags)) {
/* Increase all three subcounters at once */
cnt = atomic64_add_return_acquire(0x100010001, &ep->shadow_cnt);

idx = cnt_to_monotonic(cnt) - 1;
item_idx = &ep->user_index[idx & index_mask];

/* Add a bit to the uring */
WRITE_ONCE(*item_idx, uepi->bit);

do {
old = cnt;
if (cnt_to_refs(cnt) == 1) {
/* We are the last, we will advance the tail */
advance = cnt_to_advance(cnt);
/* Zero out all fields except monotonic counter */
} else {
/* Someone else will advance, only drop the ref */
advance = 0;
cnt -= 1;
} while ((cnt = atomic64_cmpxchg_release(&ep->shadow_cnt,
old, cnt)) != old);

if (advance) {
* Advance the tail. Tail is shared with userspace, thus we
* can't use kernel atomic_t for just atomic add, so use
* cmpxchg(). Sigh.
* We can race here with another cpu which also advances the
* tail. This is absolutely ok, since the tail is advanced
* in one direction and eventually addition is commutative.
unsigned int old, tail = READ_ONCE(ep->user_header->tail);
do {
old = tail;
} while ((tail = cmpxchg(&ep->user_header->tail,
old, old + advance)) != old);

The most complicated part is how new event is added to the ring locklessly.
To achieve that ->shadow_cnt is split on 3 subcounters and the whole
layout of the counter can be represented as follows:

struct counter_t {
unsigned long long monotonic :32;
unsigned long long advance :16;
unsigned long long refs :16;

'monotonic' - Monotonically increases on each event insertion,
never decreases. Used as an index for an event
in the uring.

'advance' - Represents number of events on which user ->tail
has to be advanced. Monotonically increases if
events are coming in parallel from different cpus
while reference number keeps > 1.

'refs' - Represents reference number, i.e. number of cpus
inserting events in parallel. Once there is a
last inserter (the reference is 1), it should
zero out 'advance' member and advance the tail
for the userspace.

What this is all about? The main problem is that since event can be
inserted from many cpus in parallel, we can't advance the tail if
previous insertion has not been fully completed. The idea to solve
this is simple: the last one advances the tail. Who is exactly the
last? Who detects the reference number is equal to 1.

The other thing is worth to mention is that the ring can't infinitely
grow and corrupt other elements, because kernel always checks that item
was marked as ready, so userspace has to clear ready_events field.

On userside events the following code should be used in order to consume

tail = READ_ONCE(header->tail);
for (i = 0; header->head != tail; header->head++) {
item_idx_ptr = &index[idx & indeces_mask];

/* Load index */
idx = __atomic_load_n(item_idx_ptr, __ATOMIC_ACQUIRE);

item = &header->items[idx];

* Fetch data first, if event is cleared by the kernel we drop the data
* returning false.
event->data = item->event.data;
event->events = __atomic_exchange_n(&item->ready_events, 0,


* How new epoll item gets its index inside user items array?

Kernel has a bitmap for that and gets free bit on attempt to insert a new
epoll item. When bitmap is full -ENOSPC is returned.

* Is there any testing app available?

There is a small app [2] which starts many threads with many event fds and
produces many events, while single consumer fetches them from userspace
and goes to kernel from time to time in order to wait.

Also libevent modification [1] is available, see "measurements" section

[1] https://github.com/libevent/libevent/pull/801
[2] https://github.com/rouming/test-tools/blob/master/userpolled-epoll.c

Roman Penyaev (14):
epoll: move private helpers from a header to the source
epoll: introduce user structures for polling from userspace
epoll: allocate user header and user events ring for polling from
epoll: some sanity flags checks for epoll syscalls for polling from
epoll: offload polling to a work in case of epfd polled from userspace
epoll: introduce helpers for adding/removing events to uring
epoll: call ep_add_event_to_uring() from ep_poll_callback()
epoll: support polling from userspace for ep_insert()
epoll: support polling from userspace for ep_remove()
epoll: support polling from userspace for ep_modify()
epoll: support polling from userspace for ep_poll()
epoll: support mapping for epfd when polled from userspace
epoll: implement epoll_create2() syscall
kselftest: add uepoll-test which tests polling from userspace

arch/alpha/kernel/syscalls/syscall.tbl | 2 +
arch/arm/tools/syscall.tbl | 1 +
arch/arm64/include/asm/unistd.h | 2 +-
arch/arm64/include/asm/unistd32.h | 2 +
arch/ia64/kernel/syscalls/syscall.tbl | 2 +
arch/m68k/kernel/syscalls/syscall.tbl | 2 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n32.tbl | 2 +
arch/mips/kernel/syscalls/syscall_n64.tbl | 2 +
arch/mips/kernel/syscalls/syscall_o32.tbl | 2 +
arch/parisc/kernel/syscalls/syscall.tbl | 2 +
arch/powerpc/kernel/syscalls/syscall.tbl | 2 +
arch/s390/kernel/syscalls/syscall.tbl | 2 +
arch/sh/kernel/syscalls/syscall.tbl | 2 +
arch/sparc/kernel/syscalls/syscall.tbl | 2 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
fs/eventpoll.c | 925 ++++++++++++++++--
include/linux/syscalls.h | 1 +
include/uapi/asm-generic/unistd.h | 4 +-
include/uapi/linux/eventpoll.h | 47 +-
kernel/sys_ni.c | 1 +
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/uepoll/.gitignore | 1 +
tools/testing/selftests/uepoll/Makefile | 16 +
.../uepoll/atomic-builtins-support.c | 13 +
tools/testing/selftests/uepoll/uepoll-test.c | 603 ++++++++++++
28 files changed, 1540 insertions(+), 103 deletions(-)
create mode 100644 tools/testing/selftests/uepoll/.gitignore
create mode 100644 tools/testing/selftests/uepoll/Makefile
create mode 100644 tools/testing/selftests/uepoll/atomic-builtins-support.c
create mode 100644 tools/testing/selftests/uepoll/uepoll-test.c

Signed-off-by: Roman Penyaev <rpenyaev@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Azat Khuzhin <azat@xxxxxxxxxxxx>
Cc: Eric Wong <e@xxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx