Re: aio poll, io_pgetevents and a new in-kernel poll API V5

[Christoph Hellwig]

ping?

On Mon, Mar 05, 2018 at 01:27:07PM -0800, Christoph Hellwig wrote:
> 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.
> 
>     git://git.infradead.org/users/hch/vfs.git aio-poll.5
> 
> Gitweb:
> 
>     http://git.infradead.org/users/hch/vfs.git/shortlog/refs/heads/aio-poll.5
> 
> 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 V4:
>  - rebased ontop of Linux 4.16-rc4
> 
> Changes since V3:
>  - remove the pre-sleep ->poll_mask call in vfs_poll,
>    allow ->get_poll_head to return POLL* values.
> 
> 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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