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From: Jens Axboe <axboe@kernel.dk>
To: Roman Penyaev <rpenyaev@suse.de>
Cc: Azat Khuzhin <azat@libevent.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 00/13] epoll: support pollable epoll from userspace
Date: Fri, 31 May 2019 10:54:27 -0600
Message-ID: <e552262b-2069-075e-f7db-cec19a12a363@kernel.dk> (raw)
In-Reply-To: <1d47ee76735f25ae5e91e691195f7aa5@suse.de>

On 5/31/19 10:02 AM, Roman Penyaev wrote:
> On 2019-05-31 16:48, Jens Axboe wrote:
>> On 5/16/19 2:57 AM, Roman Penyaev wrote:
>>> Hi all,
>>>
>>> This is v3 which introduces pollable epoll from userspace.
>>>
>>> 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.
>>
>> Why aren't we just adding support to io_uring for this instead? Then we
>> don't need yet another entirely new ring, that's is just a little
>> different from what we have.
>>
>> I haven't looked into the details of your implementation, just curious
>> if there's anything that makes using io_uring a non-starter for this
>> purpose?
> 
> Afaict the main difference is that you do not need to recharge an fd
> (submit new poll request in terms of io_uring): once fd has been added
> to
> epoll with epoll_ctl() - we get events.  When you have thousands of fds
> -
> that should matter.
> 
> Also interesting question is how difficult to modify existing event
> loops
> in event libraries in order to support recharging (EPOLLONESHOT in terms
> of epoll).
> 
> Maybe Azat who maintains libevent can shed light on this (currently I
> see
> that libevent does not support "EPOLLONESHOT" logic).

In terms of existing io_uring poll support, which is what I'm guessing
you're referring to, it is indeed just one-shot. But there's no reason
why we can't have it persist until explicitly canceled with POLL_REMOVE.

-- 
Jens Axboe


  reply index

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16  8:57 Roman Penyaev
2019-05-16  8:57 ` [PATCH v3 01/13] epoll: move private helpers from a header to the source Roman Penyaev
2019-05-16  8:57 ` [PATCH v3 02/13] epoll: introduce user structures for polling from userspace Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 03/13] epoll: allocate user header and user events ring " Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 04/13] epoll: some sanity flags checks for epoll syscalls " Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 05/13] epoll: offload polling to a work in case of epfd polled " Roman Penyaev
2019-05-21  7:51   ` Eric Wong
2019-05-22 12:54     ` Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring Roman Penyaev
2019-05-31  9:55   ` Peter Zijlstra
2019-05-31 11:24     ` Roman Penyaev
2019-05-31 13:11       ` Peter Zijlstra
2019-05-31  9:56   ` Peter Zijlstra
2019-05-31 11:15     ` Roman Penyaev
2019-05-31 12:53       ` Peter Zijlstra
2019-05-31 14:28         ` Roman Penyaev
2019-05-31 16:53           ` Peter Zijlstra
2019-05-31 12:56       ` Peter Zijlstra
2019-05-31 14:21         ` Roman Penyaev
2019-05-31 16:51           ` Peter Zijlstra
2019-05-31 18:58             ` Roman Penyaev
2019-06-03  9:09               ` Peter Zijlstra
2019-06-03 10:02                 ` Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback() Roman Penyaev
2019-05-31  9:56   ` Peter Zijlstra
2019-05-31 11:22     ` Roman Penyaev
2019-05-31 13:05       ` Peter Zijlstra
2019-05-31 15:05         ` Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 08/13] epoll: support polling from userspace for ep_insert() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 09/13] epoll: support polling from userspace for ep_remove() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 10/13] epoll: support polling from userspace for ep_modify() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 11/13] epoll: support polling from userspace for ep_poll() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 12/13] epoll: support mapping for epfd when polled from userspace Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 13/13] epoll: implement epoll_create2() syscall Roman Penyaev
2019-05-16 10:03   ` Arnd Bergmann
2019-05-16 10:20     ` Roman Penyaev
2019-05-16 10:57       ` Arnd Bergmann
2019-05-22  2:33       ` Andrew Morton
2019-05-22  9:11         ` Roman Penyaev
2019-05-22 11:14         ` Arnd Bergmann
2019-05-22 18:36           ` Andrew Morton
2019-05-31  9:55 ` [PATCH v3 00/13] epoll: support pollable epoll from userspace Peter Zijlstra
2019-05-31 14:48 ` Jens Axboe
2019-05-31 16:02   ` Roman Penyaev
2019-05-31 16:54     ` Jens Axboe [this message]
2019-05-31 19:45       ` Roman Penyaev
2019-05-31 21:09         ` Jens Axboe
2019-06-05  6:17           ` Roman Penyaev
2019-05-31 16:33 ` Peter Zijlstra
2019-05-31 18:50   ` Roman Penyaev

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