Re: aio poll, io_pgetevents and a new in-kernel poll API V3

[Jeff Moyer <jmoyer@redhat.com>]

FYI, this kernel has issues.  It will boot up, but I don't have
networking, and even rebooting doesn't succeed.  I'm looking into it.

-Jeff

Christoph Hellwig <hch@lst.de> writes:

> Hi all,
>
> this series adds support for the IOCB_CMD_POLL operation to poll for the
> readyness of file descriptors using the aio subsystem.  The API is based
> on patches that existed in RHAS2.1 and RHEL3, which means it already is
> supported by libaio.  To implement the poll support efficiently new
> methods to poll are introduced in struct file_operations:  get_poll_head
> and poll_mask.  The first one returns a wait_queue_head to wait on
> (lifetime is bound by the file), and the second does a non-blocking
> check for the POLL* events.  This allows aio poll to work without
> any additional context switches, unlike epoll.
>
> To make the interface fully useful a new io_pgetevents system call is
> added, which atomically saves and restores the signal mask over the
> io_pgetevents system call.  It it the logical equivalent to pselect and
> ppoll for io_pgetevents.
>
> The corresponding libaio changes for io_pgetevents support and
> documentation, as well as a test case will be posted in a separate
> series.
>
> The changes were sponsored by Scylladb, and improve performance
> of the seastar framework up to 10%, while also removing the need
> for a privileged SCHED_FIFO epoll listener thread.
>
> The patches are on top of Als __poll_t annoations, so I've also
> prepared a git branch on top of those here:
>
>     git://git.infradead.org/users/hch/vfs.git aio-poll.3
>
> Gitweb:
>
>     http://git.infradead.org/users/hch/vfs.git/shortlog/refs/heads/aio-poll.3
>
> Libaio changes:
>
>     https://pagure.io/libaio.git io-poll
>
> Seastar changes (not updated for the new io_pgetevens ABI yet):
>
>     https://github.com/avikivity/seastar/commits/aio
>
> Changes since V2:
>  - removed a double initialization
>  - new vfs_get_poll_head helper
>  - document that ->get_poll_head can return NULL
>  - call ->poll_mask before sleeping
>  - various ACKs
>  - add conversion of random to ->poll_mask
>  - add conversion of af_alg to ->poll_mask
>  - lacking ->poll_mask support now returns -EINVAL for IOCB_CMD_POLL
>  - reshuffled the series so that prep patches and everything not
>    requiring the new in-kernel poll API is in the beginning
>
> Changes since V1:
>  - handle the NULL ->poll case in vfs_poll
>  - dropped the file argument to the ->poll_mask socket operation
>  - replace the ->pre_poll socket operation with ->get_poll_head as
>    in the file operations
>
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