Date: Tue, 29 Apr 2003 17:23:30 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: John Baldwin <jhb@FreeBSD.org> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/dev/fxp if_fxp.c if_fxpvar.h Message-ID: <16046.60754.563011.201060@grasshopper.cs.duke.edu> In-Reply-To: <XFMail.20030429165916.jhb@FreeBSD.org> References: <16046.55840.945040.149302@grasshopper.cs.duke.edu> <XFMail.20030429165916.jhb@FreeBSD.org>
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John Baldwin writes:
> test for the special case in foo_intr(). Hmmmm. You almost want a way
> to turn off the interrupt handler as the very first thing (maybe just mark
> it as dead if it is still running, but not block) while holding the driver
> lock, then turn off the ability to generate interrupts, then sit and wait
> for the interrupt handler to remove itself. Maybe:
>
>
> mtx_lock(&sc->sc_mtx);
> /* handler no longer executes */
> bus_disable_intr(inthand_cookie, &sc->sc_mtx);
> /* dink with device */
> mtx_unlock(&sc->sc_mtx);
> bus_teardown_intr(inthande_cookie);
>
> With bus_disable_intr() calling a function like this:
>
> void
> ithread_disable_handler(void *cookie, struct mtx *m)
> {
> struct intrhand *ih;
> struct ithread *it;
>
> ih = (struct intrhand *)cookie;
> it = ih->ih_it;
> mtx_lock(&it->it_lock);
> if (ithread_is_running) {
> it->it_need = 1;
> ih->ih_flags |= IH_DISABLING;
> ih->ih_mutex = m;
> mtx_unlock(&it->it_lock);
> msleep(ih, m, ...);
> } else {
> ih->ih_flags |= IH_DISABLED;
> mtx_unlock(&it->it_lock);
> }
> }
>
> and some changes to ithread_loop() along the lines of:
>
> CTR6(... /* existing */)
> if (ih->ih_flags & IH_DISABLED)
> continue;
> if (ih->ih_flags & IH_DISABLING) {
> struct mtx *m;
>
> m = ih->ih_mutex;
> ih->ih_flags |= IH_DISABLED;
> ih->ih_flags &= ~IH_DISABLING;
> mtx_unlock(&it->it_lock);
> mtx_lock(&m);
> wakeup(ih);
> mtx_unlock(&m);
> goto restart;
> }
> if (ih->ih_flags & IH_DEAD) {
> TAILQ_REMOVE(...);
> wakeup(ih);
> continue;
> }
>
> I would need to hold the ithread lock around the TAILQ_FOREACH()
> bit and drop it around the execution of the handlers, but I probably
> need that anyways. Of course, all this really does, is take your
> gone test and move it up in the stack. You are still checking a flag
> on every interrupt handler invocation.
Since you we already need to check for IH_DEAD, you could avoid extra
compares by doing
if (ih->ih_flags & (IH_DISABLED | IH_DISABLING | IH_DEAD)) {
/* sort out rare cases outside the critical path */
}
Now we're doing only one check in the common case on something that
you apparently need to check anyway. I'd be all for something like
this.
> > No, in theory increased performance should come from increased
> > parallelism with no increased overhead. Any increased overhead is a
> > bug. Linux 2.4 runs a thread safe kernel with less overhead than we
> > have in 4.x. Its possible.
>
> How are we going to achieve increased paralellism w/o increased overhead?
> Discounting redesigning algo's which would have been a win in the
> non-parallel kernel as well. A mutex is far more expensive than an spl.
> You have to protect against more things. Of course overhead is going to
> go up.
I have no idea really. All I know is that linux 2.4 is more parallel
than 5.x is today, yet it outperforms 4.x for things like system call
latency and networking packets/sec through a single interface.
Therefore it must be possible.
Drew
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