[PATCH] increase PREEMPT_BITS to 12 to avoid overflow when starting KVM
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From: Rik van Riel on 30 Mar 2010 13:40 Increase the PREEMPT_BITS to 12, to deal with a larger number of locks that can be taken in mm_take_all_locks or other places where many instances of the same type of lock can be taken. The overflow of PREEMPT_BITS should be harmless, since it simply increments the counter into the SOFTIRQ_BITS, and the counter will be decremented again later. However, the overflow does lead to backtraces with CONFIG_PREEMPT_DEBUG enabled. Signed-off-by: Rik van Riel <riel(a)redhat.com> Reported-by: Kent Overstreet <kent.overstreet(a)gmail.com> --- Kent, does this patch fix the issue you saw? Peter, I know you do not like this approach. However, I could not think of a way around mm_take_all_locks. We need to take those locks and want to track that fact for lock debugging... diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h index d5b3876..e74108f 100644 --- a/include/linux/hardirq.h +++ b/include/linux/hardirq.h @@ -14,8 +14,8 @@ * We put the hardirq and softirq counter into the preemption * counter. The bitmask has the following meaning: * - * - bits 0-7 are the preemption count (max preemption depth: 256) - * - bits 8-15 are the softirq count (max # of softirqs: 256) + * - bits 0-12 are the preemption count (max preemption depth: 4096) + * - bits 13-19 are the softirq count (max # of softirqs: 256) * * The hardirq count can in theory reach the same as NR_IRQS. * In reality, the number of nested IRQS is limited to the stack @@ -24,16 +24,16 @@ * hardirq nesting. An arch may choose to give less than 10 bits. * m68k expects it to be 8. * - * - bits 16-25 are the hardirq count (max # of nested hardirqs: 1024) - * - bit 26 is the NMI_MASK - * - bit 28 is the PREEMPT_ACTIVE flag + * - bits 26-29 are the hardirq count (max # of nested hardirqs: 1024) + * - bit 30 is the NMI_MASK + * - bit 31 is the PREEMPT_ACTIVE flag * - * PREEMPT_MASK: 0x000000ff - * SOFTIRQ_MASK: 0x0000ff00 - * HARDIRQ_MASK: 0x03ff0000 - * NMI_MASK: 0x04000000 + * PREEMPT_MASK: 0x00000fff + * SOFTIRQ_MASK: 0x000ff000 + * HARDIRQ_MASK: 0x3ff00000 + * NMI_MASK: 0x40000000 */ -#define PREEMPT_BITS 8 +#define PREEMPT_BITS 12 #define SOFTIRQ_BITS 8 #define NMI_BITS 1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |