Date: Wed, 22 Oct 2003 19:21:46 +0900 From: Seigo Tanimura <tanimura@tanimura.dyndns.org> To: arch@FreeBSD.org Cc: Seigo Tanimura <tanimura@tanimura.dyndns.org> Subject: Re: Who should set the priority of a select(2)ing thread being waken up? Message-ID: <200310221021.h9MALkUM023905@urban> In-Reply-To: <200310220319.h9M3JGI1005225@urban> References: <200310220319.h9M3JGI1005225@urban>
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--Multipart_Wed_Oct_22_19:21:46_2003-1
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On Wed, 22 Oct 2003 12:19:16 +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.
The attached patch implements selwakeuppri(), which lets you set the
priority of a thread being waken up. Also in the patch is a small
test code to use selwakeuppri() in pcm(4).
Comments are welcome, thanks in advance.
--
Seigo Tanimura <tanimura@tanimura.dyndns.org> <tanimura@FreeBSD.org>
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diff -urN --exclude-from=/home/urban/tanimura/selwakeuppri.exclude.txt freebsd/sys/dev/sound/pcm/channel.c handoffpri/dev/sound/pcm/channel.c
--- freebsd/sys/dev/sound/pcm/channel.c Fri Oct 17 14:53:58 2003
+++ handoffpri/dev/sound/pcm/channel.c Wed Oct 22 18:34:09 2003
@@ -116,7 +116,7 @@
CHN_LOCKASSERT(c);
if (SEL_WAITING(sndbuf_getsel(bs)) && chn_polltrigger(c))
- selwakeup(sndbuf_getsel(bs));
+ selwakeuppri(sndbuf_getsel(bs), PRIBIO);
wakeup(bs);
}
diff -urN --exclude-from=/home/urban/tanimura/selwakeuppri.exclude.txt freebsd/sys/kern/sys_generic.c handoffpri/kern/sys_generic.c
--- freebsd/sys/kern/sys_generic.c Fri Oct 17 14:54:25 2003
+++ handoffpri/kern/sys_generic.c Wed Oct 22 18:34:09 2003
@@ -82,6 +82,7 @@
size_t, off_t, int);
static int dofilewrite(struct thread *, struct file *, int,
const void *, size_t, off_t, int);
+static void doselwakeup(struct selinfo *, int);
/*
* Read system call.
@@ -1162,12 +1163,30 @@
mtx_unlock(&sellock);
}
+/* Wake up a selecting thread. */
+void
+selwakeup(sip)
+ struct selinfo *sip;
+{
+ doselwakeup(sip, -1);
+}
+
+/* Wake up a selecting thread, and set its priority. */
+void
+selwakeuppri(sip, pri)
+ struct selinfo *sip;
+ int pri;
+{
+ doselwakeup(sip, pri);
+}
+
/*
* Do a wakeup when a selectable event occurs.
*/
-void
-selwakeup(sip)
+static void
+doselwakeup(sip, pri)
struct selinfo *sip;
+ int pri;
{
struct thread *td;
@@ -1188,6 +1207,8 @@
if (td->td_wchan == &selwait) {
cv_waitq_remove(td);
TD_CLR_SLEEPING(td);
+ if (pri >= PRI_MIN && pri <= PRI_MAX)
+ td->td_priority = pri;
setrunnable(td);
} else
td->td_flags &= ~TDF_SELECT;
diff -urN --exclude-from=/home/urban/tanimura/selwakeuppri.exclude.txt freebsd/sys/sys/selinfo.h handoffpri/sys/selinfo.h
--- freebsd/sys/sys/selinfo.h Fri May 2 11:05:02 2003
+++ handoffpri/sys/selinfo.h Wed Oct 22 18:34:09 2003
@@ -58,6 +58,7 @@
void clear_selinfo_list(struct thread *td);
void selrecord(struct thread *selector, struct selinfo *sip);
void selwakeup(struct selinfo *sip);
+void selwakeuppri(struct selinfo *sip, int pri);
#endif
#endif /* !_SYS_SELINFO_H_ */
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