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

[Avi Kivity]

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@xxxxxx> 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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