Re: [PATCH] 2.5.17 fix for running a SMP kernel on a UP box

[James Cleverdon <jamesclv@us.ibm.com>]

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I was looking at this with Jack Vogel and I can't figure out how it goes 
wrong, either.  However, the code in move() that uses the cpu number is a bit 
strange.  Entering loops in their middles is generally considered bug-prone 
by programming style books.  What about eliminating the goto by using 
something like the attached patch?

On Tuesday 21 May 2002 02:52 pm, Greg KH wrote:
> I can't seem to run a SMP 2.5.17 kernel on a UP machine, it locks up
> during the boot process.  In talking to Jack Vogel, he suggested I make
> the following patch, which seems to solve the problem for me.  In
> looking at the code, I have no idea of why this seems to work, so there
> probably is a better fix out there.
> 
> Any suggestions?
> 
> thanks,
> 
> greg k-h
> 
> 
> 
> diff -Nru a/arch/i386/kernel/io_apic.c b/arch/i386/kernel/io_apic.c
> --- a/arch/i386/kernel/io_apic.c	Tue May 21 14:47:06 2002
> +++ b/arch/i386/kernel/io_apic.c	Tue May 21 14:47:06 2002
> @@ -205,7 +205,7 @@
>  } ____cacheline_aligned irq_balance_t;
>  
>  static irq_balance_t irq_balance[NR_IRQS] __cacheline_aligned
> -			= { [ 0 ... NR_IRQS-1 ] = { 1, 0 } };
> +			= { [ 0 ... NR_IRQS-1 ] = { 0, 0 } };
>  
>  extern unsigned long irq_affinity [NR_IRQS];
>  
> -


-- 
James Cleverdon
IBM xSeries Linux Solutions
{jamesclv(Unix, preferred), cleverdj(Notes)} at us dot ibm dot com

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*** linux/arch/i386/kernel/io_apic.c.df	Mon May 20 22:07:36 2002
--- linux/arch/i386/kernel/io_apic.c	Tue May 21 18:41:54 2002
***************
*** 203,213 ****
  	unsigned int cpu;
  	unsigned long timestamp;
  } ____cacheline_aligned irq_balance_t;
  
  static irq_balance_t irq_balance[NR_IRQS] __cacheline_aligned
! 			= { [ 0 ... NR_IRQS-1 ] = { 1, 0 } };
  
  extern unsigned long irq_affinity [NR_IRQS];
  
  #endif
  
--- 203,213 ----
  	unsigned int cpu;
  	unsigned long timestamp;
  } ____cacheline_aligned irq_balance_t;
  
  static irq_balance_t irq_balance[NR_IRQS] __cacheline_aligned
! 			= { [ 0 ... NR_IRQS-1 ] = { 0, 0 } };
  
  extern unsigned long irq_affinity [NR_IRQS];
  
  #endif
  
***************
*** 220,248 ****
  static unsigned long move(int curr_cpu, unsigned long allowed_mask, unsigned long now, int direction)
  {
  	int search_idle = 1;
  	int cpu = curr_cpu;
  
! 	goto inside;
! 
! 	do {
! 		if (unlikely(cpu == curr_cpu))
! 			search_idle = 0;
! inside:
! 		if (direction == 1) {
! 			cpu++;
! 			if (cpu >= smp_num_cpus)
  				cpu = 0;
  		} else {
! 			cpu--;
! 			if (cpu == -1)
! 				cpu = smp_num_cpus-1;
  		}
! 	} while (!IRQ_ALLOWED(cpu,allowed_mask) ||
! 			(search_idle && !IDLE_ENOUGH(cpu,now)));
! 
! 	return cpu;
  }
  
  static inline void balance_irq(int irq)
  {
  #if CONFIG_SMP
--- 220,242 ----
  static unsigned long move(int curr_cpu, unsigned long allowed_mask, unsigned long now, int direction)
  {
  	int search_idle = 1;
  	int cpu = curr_cpu;
  
! 	for (;;) {
! 		if (direction) {
! 			if (++cpu >= smp_num_cpus)
  				cpu = 0;
  		} else {
! 			if (--cpu < 0)
! 				cpu = smp_num_cpus - 1;
  		}
! 		if (IRQ_ALLOWED(cpu, allowed_mask) && (!search_idle || IDLE_ENOUGH(cpu, now)))
! 			return cpu;
! 		if (unlikely(cpu == curr_cpu))
! 			search_idle = 0;
! 	}
  }
  
  static inline void balance_irq(int irq)
  {
  #if CONFIG_SMP