linux-kernel - Re: kdbus: add documentation

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Message-ID: <CANq1E4QJhbypbs1PueKdW7AmBPiiYEg1dN-fsAewDJap82feyg@mail.gmail.com>
Date:	Sun, 30 Nov 2014 18:12:39 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Florian Weimer <fw@...eb.enyo.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Tom Gundersen <teg@...m.no>, Jiri Kosina <jkosina@...e.cz>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: kdbus: add documentation

Hi

On Sun, Nov 30, 2014 at 10:08 AM, Florian Weimer <fw@...eb.enyo.de> wrote:
> * Andy Lutomirski:
>
>> At the risk of opening a can of worms, wouldn't this be much more
>> useful if you could share a pool between multiple connections?
>
> They would also be useful to reduce context switches when receiving
> data from all kinds of descriptors.  At present, when polling, you
> receive notification, and then you have to call into the kernel,
> again, to actually fetch the data and associated information.

poll(2) and friends cannot return data for changed descriptors. I
think a single trap for each KDBUS_CMD_MSG_RECV is acceptable. If this
turns out to be a bottleneck, we can provide bulk-operations in the
future. Anyway, I don't see how a _shared_ pool would change any of
this?

> kernel could also queue the data for one specific recipient,
> addressing the same issue that SO_REUSEPORT tries to solve (on poll
> notification, the kernel does not know which recipient will eventually
> retrieve the data, so it has to notify and wake up all of them).

We already queue data only for the addressed recipients. We *do* know
all recipients of a message at poll-notification time. We only wake up
recipients that actually got a message queued.

Not sure how this is related to SO_REUSEPORT. Can you elaborate on
your optimizations?

Thanks
David
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