[RFC] Strange code in cpu_idle()

[RFC] Strange code in cpu_idle()

[Paul E. McKenney]

Hello!

Strange code in i386, ia64, and x86-64 cpu_idle():

	void cpu_idle (void)
	{
		/* endless idle loop with no priority at all */
		while (1) {
			while (!need_resched()) {
				void (*idle)(void);
				/*
				 * Mark this as an RCU critical section so that
				 * synchronize_kernel() in the unload path waits
				 * for our completion.
				 */
				rcu_read_lock();
				idle = pm_idle;
				if (!idle)
					idle = default_idle;
				idle();
				rcu_read_unlock();
			}
			schedule();
		}
	}

Unless idle_cpu() is busted, it seems like the above is, given the code in
rcu_check_callbacks():

	void rcu_check_callbacks(int cpu, int user)
	{
		if (user || 
		    (idle_cpu(cpu) && !in_softirq() && 
					hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
			rcu_qsctr_inc(cpu);
			rcu_bh_qsctr_inc(cpu);
		} else if (!in_softirq())
			rcu_bh_qsctr_inc(cpu);
		tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
	}

And idle_cpu() is pretty straightforward:

	int idle_cpu(int cpu)
	{
		return cpu_curr(cpu) == cpu_rq(cpu)->idle;
	}

So I would say that the rcu_read_lock() in cpu_idle() is having no
effect, because any timer interrupt from cpu_idle() will mark a
quiescent state notwithstanding.  What am I missing here?

If I am not missing anything, then the attached patch would be in
order here, though there might be some additional work required.
(Though I thought that the try_stop_module() stuff took care of
all of this these days...)

Note that we really, really do want the idle loop to be an extended
quiescent state, otherwise one gets indefinite grace periods and
runs out of memory...

						Thanx, Paul

diff -urpN -X ../dontdiff linux-2.5/arch/i386/kernel/process.c linux-2.5-idle_rcu/arch/i386/kernel/process.c
--- linux-2.5/arch/i386/kernel/process.c	Mon Nov 29 10:47:14 2004
+++ linux-2.5-idle_rcu/arch/i386/kernel/process.c	Sat Dec  4 14:53:37 2004
@@ -144,14 +144,12 @@ void cpu_idle (void)
 {
 	/* endless idle loop with no priority at all */
 	while (1) {
+		/*
+		 * Note that it is illegal to use RCU read-side
+		 * critical sections within the idle loop.
+		 */
 		while (!need_resched()) {
 			void (*idle)(void);
-			/*
-			 * Mark this as an RCU critical section so that
-			 * synchronize_kernel() in the unload path waits
-			 * for our completion.
-			 */
-			rcu_read_lock();
 			idle = pm_idle;
 
 			if (!idle)
@@ -159,7 +157,6 @@ void cpu_idle (void)
 
 			irq_stat[smp_processor_id()].idle_timestamp = jiffies;
 			idle();
-			rcu_read_unlock();
 		}
 		schedule();
 	}
diff -urpN -X ../dontdiff linux-2.5/arch/ia64/kernel/process.c linux-2.5-idle_rcu/arch/ia64/kernel/process.c
--- linux-2.5/arch/ia64/kernel/process.c	Mon Nov 29 10:47:18 2004
+++ linux-2.5-idle_rcu/arch/ia64/kernel/process.c	Sat Dec  4 14:54:30 2004
@@ -230,6 +230,10 @@ cpu_idle (void *unused)
 
 	/* endless idle loop with no priority at all */
 	while (1) {
+		/*
+		 * Note that it is illegal to use RCU read-side
+		 * critical sections within the idle loop.
+		 */
 #ifdef CONFIG_SMP
 		if (!need_resched())
 			min_xtp();
@@ -239,17 +243,10 @@ cpu_idle (void *unused)
 
 			if (mark_idle)
 				(*mark_idle)(1);
-			/*
-			 * Mark this as an RCU critical section so that
-			 * synchronize_kernel() in the unload path waits
-			 * for our completion.
-			 */
-			rcu_read_lock();
 			idle = pm_idle;
 			if (!idle)
 				idle = default_idle;
 			(*idle)();
-			rcu_read_unlock();
 		}
 
 		if (mark_idle)
diff -urpN -X ../dontdiff linux-2.5/arch/x86_64/kernel/process.c linux-2.5-idle_rcu/arch/x86_64/kernel/process.c
--- linux-2.5/arch/x86_64/kernel/process.c	Mon Nov 29 10:48:05 2004
+++ linux-2.5-idle_rcu/arch/x86_64/kernel/process.c	Sat Dec  4 14:55:13 2004
@@ -133,19 +133,16 @@ void cpu_idle (void)
 {
 	/* endless idle loop with no priority at all */
 	while (1) {
+		/*
+		 * Note that it is illegal to use RCU read-side
+		 * critical sections within the idle loop.
+		 */
 		while (!need_resched()) {
 			void (*idle)(void);
-			/*
-			 * Mark this as an RCU critical section so that
-			 * synchronize_kernel() in the unload path waits
-			 * for our completion.
-			 */
-			rcu_read_lock();
 			idle = pm_idle;
 			if (!idle)
 				idle = default_idle;
 			idle();
-			rcu_read_unlock();
 		}
 		schedule();
 	}

Re: [RFC] Strange code in cpu_idle()

[Michael Buesch]

[-- Attachment #1: Type: text/plain, Size: 8352 bytes --]

Hi Paul,

Well, your mail is _very_ interresting for me.

I get this oops for weeks with several kernel versions now:

Unable to handle kernel paging request at virtual address 00099108
 printing eip:
b01010c0
*pde = 00000000
Oops: 0000 [#1]
SMP 
Modules linked in: nfs lockd sunrpc nvidia ipv6 ohci_hcd tuner tvaudio msp3400 bttv video_buf firmware_class btcx_risc ehci_hcd uhci_hcd usbcore intel_agp agpgart evdev
CPU:    0
EIP:    0060:[<b01010c0>]    Tainted: P      VLI
EFLAGS: 00010286   (2.6.10-rc2-ck2-nozeroram-findvmastat) 
EIP is at cpu_idle+0x31/0x3f
eax: 00000001   ebx: 00099100   ecx: 00000000   edx: 0000001d
esi: 00000000   edi: b03dff9c   ebp: b03dffe4   esp: b03dffe0
ds: 007b   es: 007b   ss: 0068
Process swapper (pid: 0, threadinfo=b03de000 task=b034db40)
Stack: 00020800 b03dfff8 b03e0898 000000bd b03e0340 b040cb80 0044f007 b0100211 
Call Trace:
 [<b0103c00>] show_stack+0x7a/0x90
 [<b0103d81>] show_registers+0x152/0x1ca
 [<b0103f86>] die+0xf4/0x178
 [<b0114556>] do_page_fault+0x42a/0x645
 [<b01038a7>] error_code+0x2b/0x30
 [<b03e0898>] start_kernel+0x13a/0x151
 [<b0100211>] 0xb0100211
Code: e0 ff ff 21 e3 eb 24 8b 0d 84 c6 40 b0 b8 26 10 10 b0 8b 15 c0 eb 34 b0 85 c9 0f 44 c8 8b 43 10 c1 e0 07 89 90 84 52 41 b0 ff d1 <8b> 43 08 a8 08 74 d5 e8 d8 7f 1f 00 eb f2 55 89 e5 56 53 fb ba 
 <0>Kernel panic - not syncing: Attempted to kill the idle task!


I changed cpu_idle to look like this:

void cpu_idle (void)
{
 while (1) {
  while (!need_resched()) {
   irq_stat[smp_processor_id()].idle_timestamp = jiffies;
   default_idle();
  }
  schedule();
 }
}

I did not have an oops with the changed cpu_idle() until today.
But maybe that is just luck. The oops is not reproducible.
There were days when I got 3 oopses, but there were also weeks
when the machine was rock-stable.


The above oops shows a crash when we try to access the
thread_info->flags field. This is done by need_resched()
to check the TIF_NEED_RESCHED flag.
I don't know how to interpret the oops correctly to find
the source of the crash.

b010108f <cpu_idle>:
b010108f: 55                    push   %ebp
b0101090: 89 e5                 mov    %esp,%ebp
b0101092: 53                    push   %ebx
b0101093: bb 00 e0 ff ff        mov    $0xffffe000,%ebx
b0101098: 21 e3                 and    %esp,%ebx
b010109a: eb 24                 jmp    b01010c0 <cpu_idle+0x31>
b010109c: 8b 0d 84 c6 40 b0     mov    0xb040c684,%ecx
b01010a2: b8 26 10 10 b0        mov    $0xb0101026,%eax
b01010a7: 8b 15 c0 eb 34 b0     mov    0xb034ebc0,%edx
b01010ad: 85 c9                 test   %ecx,%ecx
b01010af: 0f 44 c8              cmove  %eax,%ecx
b01010b2: 8b 43 10              mov    0x10(%ebx),%eax
b01010b5: c1 e0 07              shl    $0x7,%eax
b01010b8: 89 90 84 52 41 b0     mov    %edx,0xb0415284(%eax)
b01010be: ff d1                 call   *%ecx
b01010c0: 8b 43 08              mov    0x8(%ebx),%eax
^^^^^^^^^^^^^^^^^^^^^ OOPS here. This is clearly the need_resched() check.
b01010c3: a8 08                 test   $0x8,%al
b01010c5: 74 d5                 je     b010109c <cpu_idle+0xd>
b01010c7: e8 d8 7f 1f 00        call   b02f90a4 <schedule>
b01010cc: eb f2                 jmp    b01010c0 <cpu_idle+0x31>


Quoting "Paul E. McKenney" <paulmck@us.ibm.com>:
> Hello!
> 
> Strange code in i386, ia64, and x86-64 cpu_idle():
> 
>  void cpu_idle (void)
>  {
>   /* endless idle loop with no priority at all */
>   while (1) {
>    while (!need_resched()) {
>     void (*idle)(void);
>     /*
>      * Mark this as an RCU critical section so that
>      * synchronize_kernel() in the unload path waits
>      * for our completion.
>      */
>     rcu_read_lock();
>     idle = pm_idle;
>     if (!idle)
>      idle = default_idle;
>     idle();
>     rcu_read_unlock();
>    }
>    schedule();
>   }
>  }
> 
> Unless idle_cpu() is busted, it seems like the above is, given the code in
> rcu_check_callbacks():
> 
>  void rcu_check_callbacks(int cpu, int user)
>  {
>   if (user || 
>       (idle_cpu(cpu) && !in_softirq() && 
>      hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
>    rcu_qsctr_inc(cpu);
>    rcu_bh_qsctr_inc(cpu);
>   } else if (!in_softirq())
>    rcu_bh_qsctr_inc(cpu);
>   tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
>  }
> 
> And idle_cpu() is pretty straightforward:
> 
> 	int idle_cpu(int cpu)
> 	{
> 		return cpu_curr(cpu) == cpu_rq(cpu)->idle;
> 	}
> 
> So I would say that the rcu_read_lock() in cpu_idle() is having no
> effect, because any timer interrupt from cpu_idle() will mark a
> quiescent state notwithstanding.  What am I missing here?
> 
> If I am not missing anything, then the attached patch would be in
> order here, though there might be some additional work required.
> (Though I thought that the try_stop_module() stuff took care of
> all of this these days...)
> 
> Note that we really, really do want the idle loop to be an extended
> quiescent state, otherwise one gets indefinite grace periods and
> runs out of memory...
> 
> 						Thanx, Paul
> 
> diff -urpN -X ../dontdiff linux-2.5/arch/i386/kernel/process.c linux-2.5-idle_rcu/arch/i386/kernel/process.c
> --- linux-2.5/arch/i386/kernel/process.c	Mon Nov 29 10:47:14 2004
> +++ linux-2.5-idle_rcu/arch/i386/kernel/process.c	Sat Dec  4 14:53:37 2004
> @@ -144,14 +144,12 @@ void cpu_idle (void)
>  {
>  	/* endless idle loop with no priority at all */
>  	while (1) {
> +		/*
> +		 * Note that it is illegal to use RCU read-side
> +		 * critical sections within the idle loop.
> +		 */
>  		while (!need_resched()) {
>  			void (*idle)(void);
> -			/*
> -			 * Mark this as an RCU critical section so that
> -			 * synchronize_kernel() in the unload path waits
> -			 * for our completion.
> -			 */
> -			rcu_read_lock();
>  			idle = pm_idle;
>  
>  			if (!idle)
> @@ -159,7 +157,6 @@ void cpu_idle (void)
>  
>  			irq_stat[smp_processor_id()].idle_timestamp = jiffies;
>  			idle();
> -			rcu_read_unlock();
>  		}
>  		schedule();
>  	}
> diff -urpN -X ../dontdiff linux-2.5/arch/ia64/kernel/process.c linux-2.5-idle_rcu/arch/ia64/kernel/process.c
> --- linux-2.5/arch/ia64/kernel/process.c	Mon Nov 29 10:47:18 2004
> +++ linux-2.5-idle_rcu/arch/ia64/kernel/process.c	Sat Dec  4 14:54:30 2004
> @@ -230,6 +230,10 @@ cpu_idle (void *unused)
>  
>  	/* endless idle loop with no priority at all */
>  	while (1) {
> +		/*
> +		 * Note that it is illegal to use RCU read-side
> +		 * critical sections within the idle loop.
> +		 */
>  #ifdef CONFIG_SMP
>  		if (!need_resched())
>  			min_xtp();
> @@ -239,17 +243,10 @@ cpu_idle (void *unused)
>  
>  			if (mark_idle)
>  				(*mark_idle)(1);
> -			/*
> -			 * Mark this as an RCU critical section so that
> -			 * synchronize_kernel() in the unload path waits
> -			 * for our completion.
> -			 */
> -			rcu_read_lock();
>  			idle = pm_idle;
>  			if (!idle)
>  				idle = default_idle;
>  			(*idle)();
> -			rcu_read_unlock();
>  		}
>  
>  		if (mark_idle)
> diff -urpN -X ../dontdiff linux-2.5/arch/x86_64/kernel/process.c linux-2.5-idle_rcu/arch/x86_64/kernel/process.c
> --- linux-2.5/arch/x86_64/kernel/process.c	Mon Nov 29 10:48:05 2004
> +++ linux-2.5-idle_rcu/arch/x86_64/kernel/process.c	Sat Dec  4 14:55:13 2004
> @@ -133,19 +133,16 @@ void cpu_idle (void)
>  {
>  	/* endless idle loop with no priority at all */
>  	while (1) {
> +		/*
> +		 * Note that it is illegal to use RCU read-side
> +		 * critical sections within the idle loop.
> +		 */
>  		while (!need_resched()) {
>  			void (*idle)(void);
> -			/*
> -			 * Mark this as an RCU critical section so that
> -			 * synchronize_kernel() in the unload path waits
> -			 * for our completion.
> -			 */
> -			rcu_read_lock();
>  			idle = pm_idle;
>  			if (!idle)
>  				idle = default_idle;
>  			idle();
> -			rcu_read_unlock();
>  		}
>  		schedule();
>  	}
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 
> 

-- 
Regards Michael Buesch  [ http://www.tuxsoft.de.vu ]



[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]

Re: [RFC] Strange code in cpu_idle()

[Zwane Mwaikambo]

On Mon, 6 Dec 2004, Michael Buesch wrote:

> Hi Paul,
> 
> Well, your mail is _very_ interresting for me.
> 
> I get this oops for weeks with several kernel versions now:

There shouldn't be any oopses with the changes in question, the original 
bugfix was for crashes whilst unloading ACPI processor or APM module.

> Unable to handle kernel paging request at virtual address 00099108
>  printing eip:
> b01010c0
> *pde = 00000000
> Oops: 0000 [#1]
> SMP 
> Modules linked in: nfs lockd sunrpc nvidia ipv6 ohci_hcd tuner tvaudio msp3400 bttv video_buf firmware_class btcx_risc ehci_hcd uhci_hcd usbcore intel_agp agpgart evdev
> CPU:    0
> EIP:    0060:[<b01010c0>]    Tainted: P      VLI
> EFLAGS: 00010286   (2.6.10-rc2-ck2-nozeroram-findvmastat)

It would be a lot easier to debug with a vanilla kernel and no nvidia 
loaded.

> The above oops shows a crash when we try to access the
> thread_info->flags field. This is done by need_resched()
> to check the TIF_NEED_RESCHED flag.
> I don't know how to interpret the oops correctly to find
> the source of the crash.

Looks like thread_info was. Wow i think you're in a different world of 
hurt. If you can reproduce with less patches and no nvidia please send 
over the bugreport.

Thanks,
	Zwane

Re: [RFC] Strange code in cpu_idle()

[Michael Buesch]

[-- Attachment #1: Type: text/plain, Size: 1654 bytes --]

Quoting Zwane Mwaikambo <zwane@arm.linux.org.uk>:
> On Mon, 6 Dec 2004, Michael Buesch wrote:
> 
> > Hi Paul,
> > 
> > Well, your mail is _very_ interresting for me.
> > 
> > I get this oops for weeks with several kernel versions now:
> 
> There shouldn't be any oopses with the changes in question, the original 
> bugfix was for crashes whilst unloading ACPI processor or APM module.
> 
> > Unable to handle kernel paging request at virtual address 00099108
> >  printing eip:
> > b01010c0
> > *pde = 00000000
> > Oops: 0000 [#1]
> > SMP 
> > Modules linked in: nfs lockd sunrpc nvidia ipv6 ohci_hcd tuner tvaudio msp3400 bttv video_buf firmware_class btcx_risc ehci_hcd uhci_hcd usbcore intel_agp agpgart evdev
> > CPU:    0
> > EIP:    0060:[<b01010c0>]    Tainted: P      VLI
> > EFLAGS: 00010286   (2.6.10-rc2-ck2-nozeroram-findvmastat)
> 
> It would be a lot easier to debug with a vanilla kernel and no nvidia 
> loaded.
> 
> > The above oops shows a crash when we try to access the
> > thread_info->flags field. This is done by need_resched()
> > to check the TIF_NEED_RESCHED flag.
> > I don't know how to interpret the oops correctly to find
> > the source of the crash.
> 
> Looks like thread_info was. Wow i think you're in a different world of 
> hurt. If you can reproduce with less patches and no nvidia please send 
> over the bugreport.

Yes. That's the problem.
It's impossible to reproduce and it's pure luck when it's triggered.
I'll look what I can do. nvidia already received this bugreport.

> Thanks,
>  Zwane
> 
> 
> 

-- 
Regards Michael Buesch  [ http://www.tuxsoft.de.vu ]



[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]

Re: [RFC] Strange code in cpu_idle()

[Zwane Mwaikambo]

Hello Paul,

On Sat, 4 Dec 2004, Paul E. McKenney wrote:

> Unless idle_cpu() is busted, it seems like the above is, given the code in
> rcu_check_callbacks():
> 
> 	void rcu_check_callbacks(int cpu, int user)
> 	{
> 		if (user || 
> 		    (idle_cpu(cpu) && !in_softirq() && 
> 					hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
> 			rcu_qsctr_inc(cpu);
> 			rcu_bh_qsctr_inc(cpu);
> 		} else if (!in_softirq())
> 			rcu_bh_qsctr_inc(cpu);
> 		tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
> 	}
> 
> And idle_cpu() is pretty straightforward:
> 
> 	int idle_cpu(int cpu)
> 	{
> 		return cpu_curr(cpu) == cpu_rq(cpu)->idle;
> 	}
> 
> So I would say that the rcu_read_lock() in cpu_idle() is having no
> effect, because any timer interrupt from cpu_idle() will mark a
> quiescent state notwithstanding.  What am I missing here?

What about the hardirq_count check since we're coming in from the timer 
interrupt?

> Note that we really, really do want the idle loop to be an extended
> quiescent state, otherwise one gets indefinite grace periods and
> runs out of memory...

I've (hopefully) covered this in another email.

Thanks,
	Zwane

Re: [RFC] Strange code in cpu_idle()

[Dipankar Sarma]

Hello Zwane,

On Mon, Dec 06, 2004 at 02:47:11AM -0700, Zwane Mwaikambo wrote:
> Hello Paul,
> 
> On Sat, 4 Dec 2004, Paul E. McKenney wrote:
> 
> > Unless idle_cpu() is busted, it seems like the above is, given the code in
> > rcu_check_callbacks():
> > 
> > 	void rcu_check_callbacks(int cpu, int user)
> > 	{
> > 		if (user || 
> > 		    (idle_cpu(cpu) && !in_softirq() && 
> > 					hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
> > 			rcu_qsctr_inc(cpu);
> > 			rcu_bh_qsctr_inc(cpu);
> > 		} else if (!in_softirq())
> > 			rcu_bh_qsctr_inc(cpu);
> > 		tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
> > 	}
> > 
> > So I would say that the rcu_read_lock() in cpu_idle() is having no
> > effect, because any timer interrupt from cpu_idle() will mark a
> > quiescent state notwithstanding.  What am I missing here?
> 
> What about the hardirq_count check since we're coming in from the timer 
> interrupt?

Look at the hardirq_count check closely - it only checks for reentrant
hardirqs. If the idle task gets interrupted by a timer interrupt,
the RCU quiscent state counter for the cpu will get incremented.
So, rcu_read_lock() in cpu_idle() is bogus.

Thanks
Dipankar

Re: [RFC] Strange code in cpu_idle()

[Zwane Mwaikambo]

On Mon, 6 Dec 2004, Dipankar Sarma wrote:

> > > So I would say that the rcu_read_lock() in cpu_idle() is having no
> > > effect, because any timer interrupt from cpu_idle() will mark a
> > > quiescent state notwithstanding.  What am I missing here?
> > 
> > What about the hardirq_count check since we're coming in from the timer 
> > interrupt?
> 
> Look at the hardirq_count check closely - it only checks for reentrant
> hardirqs. If the idle task gets interrupted by a timer interrupt,
> the RCU quiscent state counter for the cpu will get incremented.
> So, rcu_read_lock() in cpu_idle() is bogus.

Ah crafty, the only reason it 'works' right now then is because we exit 
the pm_idle callback shortly after processing the timer interrupt which 
marks the processor as quiescent.

Thanks for pointing that out,

	Zwane

RE: [RFC] Strange code in cpu_idle()

[Li, Shaohua]

>static void __exit apm_exit(void)
>{
>	int	error;
>
>	if (set_pm_idle) {
>		pm_idle = original_pm_idle;
>		/*
>		 * We are about to unload the current idle thread pm
callback
>		 * (pm_idle), Wait for all processors to update
cached/local
>		 * copies of pm_idle before proceeding.
>		 */
>		synchronize_kernel();
>	}
This patch is written by me. The detail information is in
http://bugzilla.kernel.org/show_bug.cgi?id=1716


>Unfortunately, the idle loop is a quiescent state, so it is
>possible for synchronize_kernel() to return before the idle threads
>have returned.  So I don't believe RCU is useful here.  One other
>approach would be to keep a cpu mask, in which apm_exit() sets all
>bits, and pm_idle() clears its CPU's bit only if it is set.
>Then apm_exit() could wait for all CPU's bits to clear.
This is my original idea. We think it's too complex, so we discard it.
Sorry for my bad.

Thanks,
Shaohua