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

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Message-ID: <x497esfunfw.fsf@segfault.boston.devel.redhat.com>
Date:   Thu, 18 Jan 2018 11:44:03 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     viro@...iv.linux.org.uk, Avi Kivity <avi@...lladb.com>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        netdev@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: aio poll, io_pgetevents and a new in-kernel poll API V3

Jeff Moyer <jmoyer@...hat.com> writes:

> 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.

A bisect lands on: eventfd: switch to ->poll_mask.  That's not super
helpful, though.  I did run the ltp eventfd2 tests, and they all pass.

The actual issue I get on boot is that several services don't start:

[FAILED] Failed to start Modem Manager.
See 'systemctl status ModemManager.service' for details.
[FAILED] Failed to start Authorization Manager.
See 'systemctl status polkit.service' for details.
[DEPEND] Dependency failed for Dynamic System Tuning Daemon.
[FAILED] Failed to start Network Manager.
See 'systemctl status NetworkManager.service' for details.
[DEPEND] Dependency failed for Network Manager Wait Online.
...

Christoph, are you able to reproduce this?

-Jeff

> Christoph Hellwig <hch@....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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>
> --
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