Date: Mon, 12 Dec 2011 07:55:38 -0800 From: mdf@FreeBSD.org To: Andriy Gapon <avg@freebsd.org> Cc: usb@freebsd.org, hackers@freebsd.org Subject: Re: kern_yield vs ukbd_yield Message-ID: <CAMBSHm_ChT5X1bhE3oLNiRJPf4LZ5OmmxthjTXxbE2kJNxOyTw@mail.gmail.com> In-Reply-To: <4EE5B56A.4000106@FreeBSD.org> References: <4EE51CB5.1060505@FreeBSD.org> <CAMBSHm-Gmyfm83wDnhKTWAM%2BM90SEdub9uNOexG7QvuGWvN3iQ@mail.gmail.com> <4EE5B56A.4000106@FreeBSD.org>
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On Mon, Dec 12, 2011 at 12:03 AM, Andriy Gapon <avg@freebsd.org> wrote: > on 11/12/2011 23:48 mdf@FreeBSD.org said the following: >> On Sun, Dec 11, 2011 at 1:12 PM, Andriy Gapon <avg@freebsd.org> wrote: >>> >>> Does the following change do what I think that it does? >>> Thank you! >>> >>> Author: Andriy Gapon <avg@icyb.net.ua> >>> Date: =A0 Thu Sep 1 16:50:13 2011 +0300 >>> >>> =A0 =A0ukbd: drop local duplicate of kern_yield and use that instead >>> >>> diff --git a/sys/dev/usb/input/ukbd.c b/sys/dev/usb/input/ukbd.c >>> index 086c178..8078cbb 100644 >>> --- a/sys/dev/usb/input/ukbd.c >>> +++ b/sys/dev/usb/input/ukbd.c >>> @@ -399,33 +399,6 @@ ukbd_put_key(struct ukbd_softc *sc, uint32_t key) >>> =A0} >>> >>> =A0static void >>> -ukbd_yield(void) >>> -{ >>> - =A0 =A0 =A0 struct thread *td =3D curthread; >>> - =A0 =A0 =A0 uint32_t old_prio; >>> - >>> - =A0 =A0 =A0 DROP_GIANT(); >>> - >>> - =A0 =A0 =A0 thread_lock(td); >>> - >>> - =A0 =A0 =A0 /* get current priority */ >>> - =A0 =A0 =A0 old_prio =3D td->td_base_pri; >>> - >>> - =A0 =A0 =A0 /* set new priority */ >>> - =A0 =A0 =A0 sched_prio(td, td->td_user_pri); >>> - >>> - =A0 =A0 =A0 /* cause a task switch */ >>> - =A0 =A0 =A0 mi_switch(SW_INVOL | SWT_RELINQUISH, NULL); >>> - >>> - =A0 =A0 =A0 /* restore priority */ >>> - =A0 =A0 =A0 sched_prio(td, old_prio); >>> - >>> - =A0 =A0 =A0 thread_unlock(td); >>> - >>> - =A0 =A0 =A0 PICKUP_GIANT(); >>> -} >>> - >>> -static void >>> =A0ukbd_do_poll(struct ukbd_softc *sc, uint8_t wait) >>> =A0{ >>> >>> @@ -439,7 +412,7 @@ ukbd_do_poll(struct ukbd_softc *sc, uint8_t wait) >>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0while (sc->sc_inputs =3D=3D 0) { >>> >>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0/* give USB threads a ch= ance to run */ >>> - =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ukbd_yield(); >>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 kern_yield(-1); >> >> Not quite. >> >> 1) -1 should be spelled PRI_UNCHANGED, except ukbd_yield() uses >> td_user_pri, but then puts it back again, so I think UNCHANGED is what >> is meant. >> 2) kern_yield() calls it a SW_VOL rather than SW_INVOL, which seems >> the desired behaviour here anyways, since this is an explicit (i.e. >> voluntary) yield. > > Thank you for the explanation. =A0So would you say that the patch is OK? As far as I know, yes. There may be a difference in behaviour, though, while yielding, if the priority of the thread remains high (as this change would make it) -- I'm not completely sure how the scheduler chooses threads, because I'm pretty sure I've seen it take threads with lower (higher numbered) priorities even when there's runnable threads with a higher (lower numbered) priority available. It has always seemed weird to me that the priorities in the kernel are strictly higher than user-space -- but only after a prio change like that done implicitly by many of the calls to sleep(9). So it may be that the better patch is to use PRI_USER, not PRI_UNCHANGED, which would revert any potentially changed thread prio (e.g. due to a sleep(9)) back to its user-space level, so that it contended as expected with other threads. hselasky@ or someone else familiar with the various usb threads would have to answer that.
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