linux_dsm_epyc7002/arch/sparc64
Thomas Gleixner b8f8c3cf0a nohz: prevent tick stop outside of the idle loop
Jack Ren and Eric Miao tracked down the following long standing
problem in the NOHZ code:

	scheduler switch to idle task
	enable interrupts

Window starts here

	----> interrupt happens (does not set NEED_RESCHED)
	      	irq_exit() stops the tick

	----> interrupt happens (does set NEED_RESCHED)

	return from schedule()
	
	cpu_idle(): preempt_disable();

Window ends here

The interrupts can happen at any point inside the race window. The
first interrupt stops the tick, the second one causes the scheduler to
rerun and switch away from idle again and we end up with the tick
disabled.

The fact that it needs two interrupts where the first one does not set
NEED_RESCHED and the second one does made the bug obscure and extremly
hard to reproduce and analyse. Kudos to Jack and Eric.

Solution: Limit the NOHZ functionality to the idle loop to make sure
that we can not run into such a situation ever again.

cpu_idle()
{
	preempt_disable();

	while(1) {
		 tick_nohz_stop_sched_tick(1); <- tell NOHZ code that we
		 			          are in the idle loop

		 while (!need_resched())
		       halt();

		 tick_nohz_restart_sched_tick(); <- disables NOHZ mode
		 preempt_enable_no_resched();
		 schedule();
		 preempt_disable();
	}
}

In hindsight we should have done this forever, but ... 

/me grabs a large brown paperbag.

Debugged-by: Jack Ren <jack.ren@marvell.com>, 
Debugged-by: eric miao <eric.y.miao@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2008-07-18 18:10:28 +02:00
..
boot sparc64: remove CVS keywords 2008-05-20 00:33:43 -07:00
kernel nohz: prevent tick stop outside of the idle loop 2008-07-18 18:10:28 +02:00
lib sparc64: remove CVS keywords 2008-05-20 00:33:43 -07:00
math-emu sparc64: remove CVS keywords 2008-05-20 00:33:43 -07:00
mm sparc64: Add global register dumping facility. 2008-05-20 00:33:45 -07:00
oprofile Combine instrumentation menus in kernel/Kconfig.instrumentation 2007-10-19 11:53:54 -07:00
prom sparc64: remove CVS keywords 2008-05-20 00:33:43 -07:00
defconfig sparc64: Update defconfig. 2008-05-16 13:36:27 -07:00
Kconfig sparc: Add kgdb support. 2008-04-29 02:38:50 -07:00
Kconfig.debug [SPARC64]: Remove DEBUG_BOOTMEM. 2008-02-13 01:00:26 -08:00
Makefile sparc64: remove CVS keywords 2008-05-20 00:33:43 -07:00