diff options
| author | Joel Fernandes <joelagnelf@nvidia.com> | 2026-08-04 09:14:23 -0700 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2026-08-08 22:44:05 +0200 |
| commit | b54aa0edf0594a6e1138e626bfd8a352ed582ae9 (patch) | |
| tree | 28724fee91acfa132bd3bfe9b605bb3bc5b78b4e /include/linux | |
| parent | fcb8ada1287227a1392930ee52d7b6c6cab0f0b2 (diff) | |
preempt: Track NMI nesting to separate per-CPU counter
Move NMI nesting tracking from the preempt_count bits to a separate
per-CPU counter (nmi_nesting). This is to free up the NMI bits in the
preempt_count, allowing those bits to be repurposed for other uses.
Reduce NMI_BITS from 4 to 1, using it only to detect if we're in an NMI.
The per-CPU counter currently caps nesting at 15.
[boqun: Address Steven Rostedt's comment on the BUG_ON() condition]
[boqun: Use preempt_count_set() in __nmi_exit() to avoid underflow]
Suggested-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260121223933.1568682-3-lyude@redhat.com
Link: https://patch.msgid.link/20260804161447.84806-2-boqun@kernel.org
Diffstat (limited to 'include/linux')
| -rw-r--r-- | include/linux/hardirq.h | 17 | ||||
| -rw-r--r-- | include/linux/preempt.h | 9 |
2 files changed, 20 insertions, 6 deletions
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h index d57cab4d4c06..8d4895531a45 100644 --- a/include/linux/hardirq.h +++ b/include/linux/hardirq.h @@ -10,6 +10,8 @@ #include <linux/vtime.h> #include <asm/hardirq.h> +DECLARE_PER_CPU(unsigned int, nmi_nesting); + extern void synchronize_irq(unsigned int irq); extern bool synchronize_hardirq(unsigned int irq); @@ -102,14 +104,17 @@ void irq_exit_rcu(void); */ /* - * nmi_enter() can nest up to 15 times; see NMI_BITS. + * nmi_enter() can nest - nesting is tracked in a per-CPU counter. */ #define __nmi_enter() \ do { \ lockdep_off(); \ arch_nmi_enter(); \ - BUG_ON(in_nmi() == NMI_MASK); \ - __preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET); \ + /* Maximum NMI nesting is 15. */ \ + BUG_ON(__this_cpu_read(nmi_nesting) >= 15); \ + __this_cpu_inc(nmi_nesting); \ + __preempt_count_add(HARDIRQ_OFFSET); \ + preempt_count_set(preempt_count() | NMI_MASK); \ } while (0) #define nmi_enter() \ @@ -124,8 +129,12 @@ void irq_exit_rcu(void); #define __nmi_exit() \ do { \ + unsigned int nesting; \ BUG_ON(!in_nmi()); \ - __preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET); \ + __preempt_count_sub(HARDIRQ_OFFSET); \ + nesting = __this_cpu_dec_return(nmi_nesting); \ + if (!nesting) \ + preempt_count_set(preempt_count() & ~NMI_MASK); \ arch_nmi_exit(); \ lockdep_on(); \ } while (0) diff --git a/include/linux/preempt.h b/include/linux/preempt.h index d964f965c8ff..586f96688325 100644 --- a/include/linux/preempt.h +++ b/include/linux/preempt.h @@ -17,6 +17,8 @@ * * - bits 0-7 are the preemption count (max preemption depth: 256) * - bits 8-15 are the softirq count (max # of softirqs: 256) + * - bits 16-19 are the hardirq count (max # of hardirqs: 16) + * - bit 20 is the NMI flag (no nesting count, tracked separately) * * The hardirq count could in theory be the same as the number of * interrupts in the system, but we run all interrupt handlers with @@ -24,16 +26,19 @@ * there are a few palaeontologic drivers which reenable interrupts in * the handler, so we need more than one bit here. * + * NMI nesting depth is tracked in a separate per-CPU variable + * (nmi_nesting) to save bits in preempt_count. + * * PREEMPT_MASK: 0x000000ff * SOFTIRQ_MASK: 0x0000ff00 * HARDIRQ_MASK: 0x000f0000 - * NMI_MASK: 0x00f00000 + * NMI_MASK: 0x00100000 * PREEMPT_NEED_RESCHED: 0x80000000 */ #define PREEMPT_BITS 8 #define SOFTIRQ_BITS 8 #define HARDIRQ_BITS 4 -#define NMI_BITS 4 +#define NMI_BITS 1 #define PREEMPT_SHIFT 0 #define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS) |