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Subject: Re: aio poll, io_pgetevents and a new in-kernel poll API V3
To:     Jeff Moyer <jmoyer@redhat.com>, Christoph Hellwig <hch@lst.de>
Cc:     viro@zeniv.linux.org.uk, linux-aio@kvack.org,
        linux-fsdevel@vger.kernel.org, netdev@vger.kernel.org,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20180117192742.710-1-hch@lst.de>
 <x49bmhruq3k.fsf@segfault.boston.devel.redhat.com>
From:   Avi Kivity <avi@scylladb.com>
Message-ID: <7f66bc5a-e9dd-fe4f-c07a-f3de3882365b@scylladb.com>
Date:   Thu, 18 Jan 2018 19:51:09 +0200
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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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