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

Jeff Moyer
2018-01-18T15:55:52Z

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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