| Message ID | 20190624144151.22688-1-rpenyaev@suse.de (mailing list archive) |
|---|---|
| Headers | show |
| Series | epoll: support pollable epoll from userspace | expand |
On Mon, Jun 24, 2019 at 10:42 PM Roman Penyaev <rpenyaev@suse.de> wrote: > > 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. Hmm. 15% may be big in a big picture thing, but when it comes to what is pretty much a micro-benchmark, I'm not sure how meaningful it is. And the CPU sensitivity thing worries me. Did you check _why_ it doesn't seem to make any difference on the Xeon 4110? Is it just because at that point the machine has enough cores that you might as well just sit in epoll() in the kernel and uepoll doesn't give you much? Or is there something else going on? Because this is a big enough change and UAPI thing that it's a bit concerning if there isn't a clear and unambiguous win. Linus
Roman Penyaev <rpenyaev@suse.de> wrote: > Hi all, +cc Jason Baron > ** Limitations <snip> > 4. 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. Lacking EPOLLEXCLUSIVE support is probably a showstopper for common applications using per-task epoll combined with non-blocking accept4() (e.g. nginx). Fwiw, I'm still a weirdo who prefers a dedicated thread doing blocking accept4 for distribution between tasks (so epoll never sees a listen socket). But, depending on what runtime/language I'm using, I can't always dedicate a blocking thread, so I recently started using EPOLLEXCLUSIVE from Perl5 where I couldn't rely on threads being available. If I could dedicate time to improving epoll; I'd probably add writev() support for batching epoll_ctl modifications to reduce syscall traffic, or pick-up the kevent()-like interface started long ago: https://lore.kernel.org/lkml/1393206162-18151-1-git-send-email-n1ght.4nd.d4y@gmail.com/ (but I'm not sure I want to increase the size of the syscall table).
On 2019-06-25 02:24, Eric Wong wrote: > Roman Penyaev <rpenyaev@suse.de> wrote: >> Hi all, > > +cc Jason Baron > >> ** Limitations > > <snip> > >> 4. 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. > > Lacking EPOLLEXCLUSIVE support is probably a showstopper for > common applications using per-task epoll combined with > non-blocking accept4() (e.g. nginx). For the 'accept' case it seems SO_REUSEPORT can be used: https://lwn.net/Articles/542629/ Although I've never tried it in O_NONBLOCK + epoll scenario. But I've just again dived into this add-wait-exclusive logic and it seems possible to support EPOLLEXCLUSIVE by iterating over all "epis" for a particular fd, which has been woken up. For now I want to leave it as is just not to overcomplicate the code. > Fwiw, I'm still a weirdo who prefers a dedicated thread doing > blocking accept4 for distribution between tasks (so epoll never > sees a listen socket). But, depending on what runtime/language > I'm using, I can't always dedicate a blocking thread, so I > recently started using EPOLLEXCLUSIVE from Perl5 where I > couldn't rely on threads being available. > > > If I could dedicate time to improving epoll; I'd probably > add writev() support for batching epoll_ctl modifications > to reduce syscall traffic, or pick-up the kevent()-like interface > started long ago: > https://lore.kernel.org/lkml/1393206162-18151-1-git-send-email-n1ght.4nd.d4y@gmail.com/ > (but I'm not sure I want to increase the size of the syscall table). There is also fresh fs/io_uring.c thingy, which supports polling and batching (among other IO things). But polling there acts only as a single-shot, so it might make sense to support there event subscription instead of resurrecting kevent and co. -- Roman
On 2019-06-24 22:38, Linus Torvalds wrote: > On Mon, Jun 24, 2019 at 10:42 PM Roman Penyaev <rpenyaev@suse.de> > wrote: >> >> 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. > > Hmm. 15% may be big in a big picture thing, but when it comes to what > is pretty much a micro-benchmark, I'm not sure how meaningful it is. > > And the CPU sensitivity thing worries me. Did you check _why_ it > doesn't seem to make any difference on the Xeon 4110? Is it just > because at that point the machine has enough cores that you might as > well just sit in epoll() in the kernel and uepoll doesn't give you > much? Or is there something else going on? This http tool is a singlethreaded test, i.e. client and server work as a standalone processes and each has a single event thread for everything. According to what I saw there, is that events come slowly (or event loop acts faster?), so when time has come to harvest events there is nothing, we take a slow path and go to kernel in order to sleep. That does not explain the main "why", unfortunately. I would like to retest that adding more clients to the server, thus server is more likely to observe events in a ring, avoiding sleep. -- Roman
Hi all, This is v5 which introduces pollable epoll from userspace. v5: - Do not support architectures with reduced set of atomic ops, namely arc-plat-eznps, sparc32, parisc. Return -EOPNOTSUP on attempt to create uepoll on these archs. - Explicitly hint compiler, that return value of 'xchg()' call is not used, this fixes compilation on ia64. Reported-by: kbuild test robot <lkp@intel.com> v4: 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 the system's high priority workqueue v3: - 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. v2: - 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 RLIMIT_MEMLOCK value. - 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. No support on architectures with reduced set of atomic ops, namely arc-plat-eznps, sparc32, parisc. For these archs -EOPNOTSUP is returned. 2. 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. 3. No support for EPOLLWAKEUP events are consumed from userspace, thus no way to call __pm_relax() 4. 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); WARN_ON(!advance); /* Zero out all fields except monotonic counter */ cnt &= ~MONOTONIC_MASK; } 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 events: 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, __ATOMIC_RELEASE); } * 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 above. [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 userspace epoll: some sanity flags checks for epoll syscalls for polling from userspace 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 | 951 ++++++++++++++++-- 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, 1566 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@suse.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Azat Khuzhin <azat@libevent.org> Cc: Eric Wong <e@80x24.org> Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org