[patch] Remove tq_scheduler

[patch] Remove tq_scheduler

[Andrew Morton]

This patch removes tq_scheduler from the kernel.  All uses of
tq_scheduler are migrated over to use schedule_task().

Notes:

- In two places: drivers/block/paride/pseudo.h and
  drivers/net/wan/sdlamain.c we are re-adding tasks to tq_scheduler
  within the callback.  That means that these functions are being
  called _every_ time the machine calls schedule().

  It may be a limited case for paride, but sdlamain.c appears to be
  doing this wicked thing all the time.

  Now, if you do this with the current schedule_task() your machine
  will hang up.  So schedule_task() has been changed to support this
  practice.  If we see that the task queue has waiters on it we still
  call schedule(), but we do it in state TASK_RUNNING.

- The patch adds to context.c a new API function
  run_schedule_tasks() which immediately runs the queued tasks.  Must
  only be called from process context.  serial.c needs this in its
  shutdown routines.

- If anyone sleeps in a callback, then all other users of
  schedule_task() also sleep.  But there's nothing new here.  Kinda
  makes one wonder why schedule_task exists.  But what-hey, it's neat.

- Note the careful massaging of module reference counts.

  Yes my friends, much usage of task queues in modules is racy wrt
  module removal.  This patch fixes some of them.

  The approach taken here is to increment the module refcount when we
  enqueue a task and to decrement it in the handler:

	mainline()
	{
		...
		MOD_INC_USE_COUNT;
		if (schedule_task(some_wq) == 0)
			MOD_DEC_USE_COUNT;
	}

	handler()
	{
		...
		MOD_DEC_USE_COUNT;
		/* Wheee!  Tiny race here */
		return;
	}

  Note that queue_task and schedule_task have been enhanced to return
  a success indicator.  If this is non-zero you know that your task
  will be run.  If it returns zero then your tq_struct was already
  queued and you lose.

  The patch against test11-pre7 (1043 lines) is at

	http://www.uow.edu.au/~andrewm/linux/tq_scheduler.patch

  It affects the following files:

include/linux/tqueue.h
include/linux/sched.h
include/linux/compatmac.h
arch/ppc/8xx_io/uart.c
arch/ppc/8xx_io/fec.c
arch/ppc/8260_io/uart.c
drivers/net/wan/sdlamain.c
drivers/block/paride/pseudo.h
drivers/char/tty_io.c
drivers/char/esp.c
drivers/char/istallion.c
drivers/char/riscom8.c
drivers/char/serial.c
drivers/char/README.epca
drivers/char/specialix.c
drivers/char/epca.c
drivers/char/isicom.c
drivers/char/moxa.c
drivers/char/mxser.c
drivers/char/stallion.c
drivers/scsi/megaraid.c
drivers/scsi/qla1280.c
drivers/isdn/avmb1/b1capi.c
drivers/isdn/avmb1/kcapi.c
drivers/sbus/char/sab82532.c
drivers/sbus/char/aurora.c
drivers/ide/ide.c
drivers/usb/serial/keyspan_pda.c
drivers/usb/serial/digi_acceleport.c
drivers/i2o/i2o_lan.c
fs/smbfs/sock.c
kernel/sched.c
kernel/ksyms.c
kernel/timer.c
kernel/context.c
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Re: [patch] Remove tq_scheduler

[David Woodhouse]

On Sat, 18 Nov 2000, Andrew Morton wrote:

>
> This patch removes tq_scheduler from the kernel.  All uses of
> tq_scheduler are migrated over to use schedule_task().
>
> Notes:
> - If anyone sleeps in a callback, then all other users of
>   schedule_task() also sleep.  But there's nothing new here.  Kinda
>   makes one wonder why schedule_task exists.  But what-hey, it's neat.

Because you're only supposed to use it if the task that is scheduled:
	A) Doesn't care about a reasonable delay
	B) Doesn't sleep for an unreasonable amount of time.

As long as there's some value of 'reasonable' to match the set of tasks
which you are using schedule_task() for at any given moment, you should be
fine.

If it's really necessary in 2.5, we can consider using multiple queues to
get round this problem - either a task per subsystem or a pool of worker
threads. Hopefully it won't be necessary though. We'll see.

> - Note the careful massaging of module reference counts.
>
>   Yes my friends, much usage of task queues in modules is racy wrt
>   module removal.  This patch fixes some of them.

Cool. I was going to look into that. I had figured we should fix it
completely or not at all, though, which is why I didn't do the trick with
use counts. I probably should have done, though.

While you're in maintenance mode, do you feel like fixing up stuff to use
up_and_exit() for killing kernel threads? I started on net/sunrpc/sched.c
but it made my head hurt so I gave up and started hacking PCMCIA
instead :)

Also, drivers/usb/hub.c can probably use schedule_task() now instead of
its own kernel thread.

-- 
dwmw2


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Re: [patch] Remove tq_scheduler

[Alan Cox]

>   The patch against test11-pre7 (1043 lines) is at
> 
> 	http://www.uow.edu.au/~andrewm/linux/tq_scheduler.patch
> 
>   It affects the following files:

Andrew, can you put your patches on a properly configured site.  The site
admins have all ICMP packets blocked on www.uow.edu.au so you are a PMTU 
blackhole and unreachable via my tunnel.

Alan

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Re: [patch] Remove tq_scheduler

[bert hubert]

On Mon, Nov 20, 2000 at 05:07:14PM +0000, Alan Cox wrote:
> >   The patch against test11-pre7 (1043 lines) is at
> > 
> > 	http://www.uow.edu.au/~andrewm/linux/tq_scheduler.patch
> > 
> >   It affects the following files:
> 
> Andrew, can you put your patches on a properly configured site.  The site
> admins have all ICMP packets blocked on www.uow.edu.au so you are a PMTU 
> blackhole and unreachable via my tunnel.

Or configure your host to do MSS Clamping. I have a similar situation over
here and no problems.

Regards,

bert hubert

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