diff options
| author | Boqun Feng <boqun@kernel.org> | 2026-08-04 09:14:32 -0700 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2026-08-10 10:50:19 +0200 |
| commit | 3b0e2a22d4086ed7c1294584c4417f7d19f1ac67 (patch) | |
| tree | e31c28486fac5900755c052db5ffd1fff5031906 /include/linux/preempt.h | |
| parent | 560fcaa92ef983315023eb4ebfc1ebae1132fb2a (diff) | |
preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS
With the changes that enable preempt count to track IRQ disabling
nesting, we don't have enough bits in 32-bit preempt count
implementation, as a result we move NMI nesting bits out of the 32-bit
preempt count. However on the architectures that can support 64-bit
preempt count implementation, we can keep the NMI nesting bits in the
32-bit preempt count and avoid maintaining NMI nesting bits outside of
the same cache line.
Therefore HAS_SEPARATE_PREEMPT_RESCHED_BITS is introduced to allow
architectures to select this. Note that under this Kconfig, preempt
count is maintained in a 64-bit word however preempt_count() still
remains as an int because all the effective bits still fit in
(previously we mask out NEED_RESCHED bit in preempt_count()). This
should make no functional changes for existing preempt_count() users.
Enable this for x86_64 along with the introduction of the Kconfig.
[boqun: Undo the __preempt_count_{add,sub}() optimization in 32-bit
preempt count since it may introduce {over,under}flow]
Originally-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260804161447.84806-11-boqun@kernel.org
Diffstat (limited to 'include/linux/preempt.h')
| -rw-r--r-- | include/linux/preempt.h | 23 |
1 files changed, 20 insertions, 3 deletions
diff --git a/include/linux/preempt.h b/include/linux/preempt.h index 33fc4c814a9f..8299657f0f86 100644 --- a/include/linux/preempt.h +++ b/include/linux/preempt.h @@ -34,14 +34,31 @@ * SOFTIRQ_MASK: 0x0000ff00 * HARDIRQ_DISABLE_MASK: 0x00ff0000 * HARDIRQ_MASK: 0x0f000000 + * + * When HAS_SEPARATE_PREEMPT_RESCHED_BITS=y, PREEMPT_NEED_RESCHED is put in a + * separate word and that allows 64bit load-store architectures to 'set' + * PREEMPT_NEED_RESCHED without messing up the otherwise symmetric + * modifications used on preempt_count and still load the whole thing + * (single-copy) atomically, without having to resort to full atomic + * operations. + * + * Because of the above, NMI_MASK bits are different depending on + * HAS_SEPARATE_PREEMPT_RESCHED_BITS: + * + * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=n: + * * NMI_MASK: 0x10000000 * PREEMPT_NEED_RESCHED: 0x80000000 + * + * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=y: + * NMI_MASK: 0xf0000000 + * (PREEMPT_NEED_RESCHED is in a different word) */ #define PREEMPT_BITS 8 #define SOFTIRQ_BITS 8 #define HARDIRQ_DISABLE_BITS 8 #define HARDIRQ_BITS 4 -#define NMI_BITS 1 +#define NMI_BITS (1 + 3*IS_ENABLED(CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS)) #define PREEMPT_SHIFT 0 #define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS) @@ -116,8 +133,8 @@ static __always_inline unsigned char interrupt_context_level(void) * preempt_count() is commonly implemented with READ_ONCE(). */ -#define nmi_count() (preempt_count() & NMI_MASK) -#define hardirq_count() (preempt_count() & HARDIRQ_MASK) +#define nmi_count() (preempt_count() & NMI_MASK) +#define hardirq_count() (preempt_count() & HARDIRQ_MASK) #ifdef CONFIG_PREEMPT_RT # define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK) # define irq_count() ((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count()) |