linux-kernel - Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

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Date:	Tue, 27 Feb 2007 11:20:48 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
cc:	Ingo Molnar <mingo@...e.hu>, Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Zach Brown <zach.brown@...cle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

On Tue, 27 Feb 2007, Evgeniy Polyakov wrote:

> I probably selected wrong words to desribe, here is in details how
> kevent differs from epoll.
> 
> Polling case need to perform additional check before event can be copied
> to userspace, that check must be done for each even being copied.
> Kevent does not need that (it needs it for poll emulation) - if event is
> ready, then it is ready.

That could be changed too. The "void *key" doesn't need to be NULL. Wake 
ups to f_op->poll() waiters can use that to send ready events directly, 
avoiding an extra f_op->poll() to fetch them.
Infrastructure is already there, just need a big patch to do it everywhere ;)



> Kevent works slightly different - it does not perform additional check
> for readiness (although it can, and it does in poll notifications), if
> event is marked as ready, parked in waiting syscal thread is awakened
> and event is copied to userspace.
> Also waiting syscall is awakened through one queue - event is added
> and wake_up() is called, while in epoll() there are two queues.

The really ancient version of epoll (called /dev/epoll at that time) was 
doing a very similar thing. Was adding custom plugs is all over the places 
where we wanted to get events from, and was collecting them w/out 
resorting to extra f_op->poll(). Event masks going straight through an 
event buffer.
The reason why the current design of epoll was chosen, was because:

o Was not requiring custom plus all over the places
o Was working with the current kernel abstractions as-is (though f_op->poll)




- Davide


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