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

Avi Kivity
2018-01-18T17:51:16Z

On 01/18/2018 05:46 PM, Jeff Moyer wrote:
> 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.

FWIW, I'm running an older version of this patchset on my desktop with
no problems so far.

> -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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