Re: Who should set the priority of a select(2)ing thread being waken up?

From: Seigo Tanimura (tanimura_at_tanimura.dyndns.org)
Date: 11/03/03

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    Date: Mon, 03 Nov 2003 22:03:29 +0900
To: current@FreeBSD.org, arch@FreeBSD.org

    [posted to -current as well, because there were no replies in -arch]

    On Wed, 22 Oct 2003 19:21:46 +0900,
      Seigo Tanimura <tanimura@tanimura.dyndns.org> said:

    tanimura> In good old days, only a socket and a pipe were the major file
    tanimura> descriptors being select(2)ed. As select(2) was just a socket
    tanimura> operation, it was sufficient to set the priority of select(2)ing
    tanimura> process to PSOCK(*1), I suppose.

    tanimura> Nowadays, quite a few drivers support select(2) as well, including
    tanimura> sound, usb, scsi controllers, and so on. I am not convinced whether a
    tanimura> process should select(2) those devices at PSOCK as we do for a socket.

    tanimura> Suppose that a process select(2)s for a pcm device and a socket at
    tanimura> once. If the process is waken up by the pcm driver at PSOCK, another
    tanimura> process at a better priority may preempt the first one, which can
    tanimura> result in dropping some pcm data.

    tanimura> Maybe it would be better if the caller of selwakeup() could determine
    tanimura> the priority of a process or a thread. That would let us raise the
    tanimura> priority to PRIBIO if pcm data was ready, while the priority would
    tanimura> stay at PSOCK if the socket was ready.

    tanimura> (*1) I broke that in 5-CURRENT when I modified select(2) and poll(2)
    tanimura> to use a conditional variable.

    tanimura> The attached patch implements selwakeuppri(), which lets you set the
    tanimura> priority of a thread being waken up. Also in the patch is a small
    tanimura> test code to use selwakeuppri() in pcm(4).
    (patch snipped)

    The updated patch at:

    http://people.freebsd.org/~tanimura/selwakeuppri.diff.gz

    converts all selwakeup()s into selwakeuppri()s with appropriate
    priorities. Old selwakeup() is left, but I may axe it.

    Any objections if I commit the patch in a week? 

    -- 
Seigo Tanimura <tanimura@tanimura.dyndns.org> <tanimura@FreeBSD.org>
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