On Wed, Dec 16, 2020 at 04:50:42PM -0300, Martin Pieuchot wrote:
> On 16/12/20(Wed) 12:50, Scott Cheloha wrote:
> > On Tue, Dec 15, 2020 at 01:47:24PM +0100, Mark Kettenis wrote:
> > > > Date: Tue, 15 Dec 2020 13:32:22 +0100
> > > > From: Claudio Jeker <cje...@diehard.n-r-g.com>
> > > >
> > > > On Fri, Dec 11, 2020 at 07:07:56PM -0600, Scott Cheloha wrote:
> > > > > Hi,
> > > > >
> > > > > I'd like to remove lbolt from the kernel. I think having it in the
> > > > > kernel complicates otherwise simple code.
> > > > >
> > > > > We can start with sdmmc(4).
> > > > >
> > > > > The goal in sdmmc_io_function_enable() is calling
> > > > > sdmmc_io_function_ready()
> > > > > up to six times and sleep 1 second between each attempt. Here's
> > > > > rewritten
> > > > > code that does with without lbolt.
> > > > >
> > > > > ok?
> > > > >
> > > > > Index: sdmmc_io.c
> > > > > ===================================================================
> > > > > RCS file: /cvs/src/sys/dev/sdmmc/sdmmc_io.c,v
> > > > > retrieving revision 1.41
> > > > > diff -u -p -r1.41 sdmmc_io.c
> > > > > --- sdmmc_io.c 31 Dec 2019 10:05:33 -0000 1.41
> > > > > +++ sdmmc_io.c 12 Dec 2020 01:04:59 -0000
> > > > > @@ -231,8 +231,8 @@ sdmmc_io_function_enable(struct sdmmc_fu
> > > > > {
> > > > > struct sdmmc_softc *sc = sf->sc;
> > > > > struct sdmmc_function *sf0 = sc->sc_fn0;
> > > > > + int chan, retry = 5;
> > > > > u_int8_t rv;
> > > > > - int retry = 5;
> > > > >
> > > > > rw_assert_wrlock(&sc->sc_lock);
> > > > >
> > > > > @@ -244,7 +244,7 @@ sdmmc_io_function_enable(struct sdmmc_fu
> > > > > sdmmc_io_write_1(sf0, SD_IO_CCCR_FN_ENABLE, rv);
> > > > >
> > > > > while (!sdmmc_io_function_ready(sf) && retry-- > 0)
> > > > > - tsleep_nsec(&lbolt, PPAUSE, "pause", INFSLP);
> > > > > + tsleep_nsec(&chan, PPAUSE, "pause", SEC_TO_NSEC(1));
> > > > > return (retry >= 0) ? 0 : ETIMEDOUT;
> > > > > }
> > > > >
> > > >
> > > > Why not use &retry as wait channel instead of adding a new variable
> > > > chan? Result is the same. Would it make sense to allow NULL as wait
> > > > channel to make the tsleep not wakeable. At least that could be used in
> > > > a
> > > > few places where timeouts are implemented with tsleep and would make the
> > > > intent more obvious.
> > >
> > > Or have an appropriately named global variable? Something like "int
> > > nowake"?
> >
> > Something like the attached patch?
> >
> > I think the idea of a "dead channel" communicates the intent. Nobody
> > broadcasts wakeups on the dead channel. If you don't want to receive
> > wakeup broadcasts you sleep on the dead channel. Hence, "deadchan".
>
> Why did we choose to use a variable over NULL? Any technical reason?
The sleep subsytem requires a non-NULL value for ident. Changing this
seems not trivial.
> I'm wondering it the locality of the variable might not matter in a
> distant future. Did you dig a bit deeper about the FreeBSD solution?
> Why did they choose a per-CPU value?
Currently all sleep channels are hashed into IIRC 128 buckets. If all
timeouts use the same sleep channel then this queue may get overcrowded.
I guess only instrumentation and measurements will tell us how bad the
sleep queue is hashed.
> > Index: kern/kern_synch.c
> > ===================================================================
> > RCS file: /cvs/src/sys/kern/kern_synch.c,v
> > retrieving revision 1.172
> > diff -u -p -r1.172 kern_synch.c
> > --- kern/kern_synch.c 7 Dec 2020 16:55:29 -0000 1.172
> > +++ kern/kern_synch.c 16 Dec 2020 18:50:12 -0000
> > @@ -87,6 +87,12 @@ sleep_queue_init(void)
> > TAILQ_INIT(&slpque[i]);
> > }
> >
> > +/*
> > + * Threads that do not want to receive wakeup(9) broadcasts should
> > + * sleep on deadchan.
> > + */
> > +static int __deadchan;
> > +int *deadchan = &__deadchan;
> >
> > /*
> > * During autoconfiguration or after a panic, a sleep will simply
> > Index: sys/systm.h
> > ===================================================================
> > RCS file: /cvs/src/sys/sys/systm.h,v
> > retrieving revision 1.148
> > diff -u -p -r1.148 systm.h
> > --- sys/systm.h 26 Aug 2020 03:29:07 -0000 1.148
> > +++ sys/systm.h 16 Dec 2020 18:50:12 -0000
> > @@ -107,6 +107,8 @@ extern struct vnode *rootvp; /* vnode eq
> > extern dev_t swapdev; /* swapping device */
> > extern struct vnode *swapdev_vp;/* vnode equivalent to above */
> >
> > +extern int *deadchan; /* dead wakeup(9) channel */
> > +
> > struct proc;
> > struct process;
> > #define curproc curcpu()->ci_curproc
> > Index: dev/sdmmc/sdmmc_io.c
> > ===================================================================
> > RCS file: /cvs/src/sys/dev/sdmmc/sdmmc_io.c,v
> > retrieving revision 1.41
> > diff -u -p -r1.41 sdmmc_io.c
> > --- dev/sdmmc/sdmmc_io.c 31 Dec 2019 10:05:33 -0000 1.41
> > +++ dev/sdmmc/sdmmc_io.c 16 Dec 2020 18:50:12 -0000
> > @@ -244,7 +244,7 @@ sdmmc_io_function_enable(struct sdmmc_fu
> > sdmmc_io_write_1(sf0, SD_IO_CCCR_FN_ENABLE, rv);
> >
> > while (!sdmmc_io_function_ready(sf) && retry-- > 0)
> > - tsleep_nsec(&lbolt, PPAUSE, "pause", INFSLP);
> > + tsleep_nsec(deadchan, PPAUSE, "pause", SEC_TO_NSEC(1));
> > return (retry >= 0) ? 0 : ETIMEDOUT;
> > }
> >
> >
--
:wq Claudio
