[PATCH v2 3/9] s390/percpu: Infrastructure for more efficient this_cpu operations

Heiko Carstens
2026-03-19T12:05:19Z

With the intended removal of PREEMPT_NONE this_cpu operations based on
atomic instructions, guarded with preempt_disable()/preempt_enable() pairs
become more expensive: the preempt_disable() / preempt_enable() pairs are
not optimized away anymore during compile time.

In particular the conditional call to preempt_schedule_notrace() after
preempt_enable() adds additional code and register pressure.

E.g. this simple C code sequence

DEFINE_PER_CPU(long, foo);
long bar(long a) { return this_cpu_add_return(foo, a); }

generates this code:

11a976: eb af f0 68 00 24 stmg %r10,%r15,104(%r15)
11a97c: b9 04 00 ef lgr %r14,%r15
11a980: b9 04 00 b2 lgr %r11,%r2
11a984: e3 f0 ff c8 ff 71 lay %r15,-56(%r15)
11a98a: e3 e0 f0 98 00 24 stg %r14,152(%r15)
11a990: eb 01 03 a8 00 6a asi 936,1 <- __preempt_count_add(1)
11a996: c0 10 00 d2 ac b5 larl %r1,1b70300 <- address of percpu var
11a9a0: e3 10 23 b8 00 08 ag %r1,952 <- add percpu offset
11a9a6: eb ab 10 00 00 e8 laag %r10,%r11,0(%r1) <- atomic op
11a9ac: eb ff 03 a8 00 6e alsi 936,-1 <- __preempt_count_dec_and_test()
11a9b2: a7 54 00 05 jnhe 11a9bc <bar+0x4c>
11a9b6: c0 e5 00 76 d1 bd brasl %r14,ff4d30 <preempt_schedule_notrace>
11a9bc: b9 e8 b0 2a agrk %r2,%r10,%r11
11a9c0: eb af f0 a0 00 04 lmg %r10,%r15,160(%r15)
11a9c6 07 fe br %r14

Even though the above example is more or less the worst case, since the
branch to preempt_schedule_notrace() requires a stackframe, which
otherwise wouldn't be necessary, there is also the conditional jnhe branch
instruction.

Get rid of the conditional branch with the following code sequence:

11a8e6: c0 30 00 d0 c5 0d larl %r3,1b33300
11a8ec: b9 04 00 43 lgr %r4,%r3
11a8f0: eb 00 43 c0 00 52 mviy 960,4
11a8f6: e3 40 03 b8 00 08 ag %r4,952
11a8fc: eb 52 40 00 00 e8 laag %r5,%r2,0(%r4)
11a902: eb 00 03 c0 00 52 mviy 960,0
11a908: b9 08 00 25 agr %r2,%r5
11a90c 07 fe br %r14

The general idea is that this_cpu operations based on atomic instructions
are guarded with mvyi instructions:

- The first mvyi instruction writes the register number, which contains
the percpu address variable to lowcore. This also indicates that a
percpu code section is executed.

- The first instruction following the mvyi instruction must be the ag
instruction which adds the percpu offset to the percpu address register.

- Afterwards the atomic percpu operation follows.

- Then a second mvyi instruction writes a zero to lowcore, which indicates
the end of the percpu code section.

- In case of an interrupt/exception/nmi the register number which was
written to lowcore is copied to the exception frame (pt_regs), and a zero
is written to lowcore.

- On return to the previous context it is checked if a percpu code section
was executed (saved register number not zero), and if the process was
migrated to a different cpu. If the percpu offset was already added to
the percpu address register (instruction address does _not_ point to the
ag instruction) the content of the percpu address register is adjusted so
it points to percpu variable of the new cpu.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
arch/s390/include/asm/entry-percpu.h | 57 ++++++++++++++++++++++++++++
arch/s390/include/asm/lowcore.h | 3 +-
arch/s390/include/asm/percpu.h | 54 ++++++++++++++++++++++++++
arch/s390/include/asm/ptrace.h | 2 +
arch/s390/kernel/irq.c | 17 +++++++--
arch/s390/kernel/nmi.c | 3 ++
arch/s390/kernel/traps.c | 3 ++
7 files changed, 134 insertions(+), 5 deletions(-)
create mode 100644 arch/s390/include/asm/entry-percpu.h

diff --git a/arch/s390/include/asm/entry-percpu.h b/arch/s390/include/asm/entry-percpu.h
new file mode 100644
index 000000000000..7ccfefdd8162
--- /dev/null
+++ b/arch/s390/include/asm/entry-percpu.h
@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef ARCH_S390_ENTRY_PERCPU_H
+#define ARCH_S390_ENTRY_PERCPU_H
+
+#include <linux/percpu.h>
+#include <asm/lowcore.h>
+#include <asm/ptrace.h>
+#include <asm/asm-offsets.h>
+
+static __always_inline void percpu_entry(struct pt_regs *regs)
+{
+ struct lowcore *lc = get_lowcore();
+
+ if (user_mode(regs))
+ return;
+ regs->cpu = lc->cpu_nr;
+ regs->percpu_register = lc->percpu_register;
+ lc->percpu_register = 0;
+}
+
+static __always_inline void percpu_exit(struct pt_regs *regs)
+{
+ struct lowcore *lc = get_lowcore();
+ unsigned int insn, disp;
+ unsigned char reg;
+
+ if (user_mode(regs))
+ return;
+ reg = regs->percpu_register;
+ if (!reg)
+ return;
+ lc->percpu_register = reg;
+ if (regs->cpu == lc->cpu_nr)
+ return;
+ /*
+ * Within a percpu code section and process has been migrated to
+ * a different CPU. Check if the percpu base register needs to be
+ * updated. This is the case if the PSW does not point to the ADD
+ * instruction within the section
+ * - AG %rx,percpu_offset_in_lowcore(%r0,%r0)
+ * which adds the percpu offset to the percpu base register.
+ */
+ if ((*(u16 *)psw_bits(regs->psw).ia & 0xff0f) != 0xe300)
+ goto fixup;
+ disp = offsetof(struct lowcore, percpu_offset);
+ if (machine_has_relocated_lowcore())
+ disp += LOWCORE_ALT_ADDRESS;
+ insn = (disp & 0xff000) >> 4 | (disp & 0x00fff) << 16 | 0x8;
+ if (*(u32 *)(psw_bits(regs->psw).ia + 2) != insn)
+ return;
+fixup:
+ /* Fixup percpu base register */
+ regs->gprs[reg] -= __per_cpu_offset[regs->cpu];
+ regs->gprs[reg] += lc->percpu_offset;
+}
+
+#endif
diff --git a/arch/s390/include/asm/lowcore.h b/arch/s390/include/asm/lowcore.h
index 50ffe75adeb4..cd1ddfdb5d35 100644
--- a/arch/s390/include/asm/lowcore.h
+++ b/arch/s390/include/asm/lowcore.h
@@ -165,7 +165,8 @@ struct lowcore {
__u32 spinlock_index; /* 0x03b0 */
__u8 pad_0x03b4[0x03b8-0x03b4]; /* 0x03b4 */
__u64 percpu_offset; /* 0x03b8 */
- __u8 pad_0x03c0[0x0400-0x03c0]; /* 0x03c0 */
+ __u8 percpu_register; /* 0x03c0 */
+ __u8 pad_0x03c1[0x0400-0x03c1]; /* 0x03c1 */

__u32 return_lpswe; /* 0x0400 */
__u32 return_mcck_lpswe; /* 0x0404 */
diff --git a/arch/s390/include/asm/percpu.h b/arch/s390/include/asm/percpu.h
index b18a96f3a334..05eb91428b42 100644
--- a/arch/s390/include/asm/percpu.h
+++ b/arch/s390/include/asm/percpu.h
@@ -60,6 +60,60 @@
#define this_cpu_or_1(pcp, val) arch_this_cpu_to_op_simple(pcp, val, |)
#define this_cpu_or_2(pcp, val) arch_this_cpu_to_op_simple(pcp, val, |)

+/*
+ * Macros to be used for percpu code section based on atomic instructions.
+ *
+ * Avoid the need to use preempt_disable() / preempt_disable() pairs and the
+ * conditional preempt_schedule_notrace() function calls which come with
+ * this. The idea is that this_cpu operations based on atomic instructions are
+ * guarded with mvyi instructions:
+ *
+ * - The first mvyi instruction writes the register number, which contains the
+ * percpu address variable to lowcore. This also indicates that a percpu
+ * code section is executed.
+ *
+ * - The first mvyi instruction following the mvyi instruction must be the ag
+ * instruction which adds the percpu offset to the percpu address register.
+ *
+ * - Afterwards the atomic percpu operation follows.
+ *
+ * - Then a second mvyi instruction writes a zero to lowcore, which indicates
+ * the end of the percpu code section.
+ *
+ * - In case of an interrupt/exception/nmi the register number which was
+ * written to lowcore is copied to the exception frame (pt_regs), and a zero
+ * is written to lowcore.
+ *
+ * - On return to the previous context it is checked if a percpu code section
+ * was executed (saved register number not zero), and if the process was
+ * migrated to a different cpu. If the percpu offset was already added to
+ * the percpu address register (instruction address does _not_ point to the
+ * ag instruction) the content of the percpu address register is adjusted so
+ * it points to percpu variable of the new cpu.
+ *
+ * Inline assemblies making use of this typically have a code sequence like:
+ *
+ * MVIY_PERCPU(...) <- start of percpu code section
+ * AG_ALT(...) <- add percpu offset; must be the second instruction
+ * atomic_op <- atomic op
+ * MVIY_ALT(...) <- end of percpu code section
+ */
+
+#define MVIY_PERCPU(disp, dispalt, base) \
+ ALTERNATIVE(" mviy " disp "(" base " ),0\n", \
+ " mviy " dispalt "(" base " ),0\n", \
+ ALT_PERCPU(0))
+
+#define MVIY_ALT(disp, dispalt, base) \
+ ALTERNATIVE(" mviy " disp "(" base " ),0\n", \
+ " mviy " dispalt "(" base " ),0\n", \
+ ALT_FEATURE(MFEATURE_LOWCORE))
+
+#define AG_ALT(disp, dispalt, reg) \
+ ALTERNATIVE(" ag " reg ", " disp "(%%r0)\n", \
+ " ag " reg ", " dispalt "(%%r0)\n", \
+ ALT_FEATURE(MFEATURE_LOWCORE))
+
#ifndef MARCH_HAS_Z196_FEATURES

#define this_cpu_add_4(pcp, val) arch_this_cpu_to_op_simple(pcp, val, +)
diff --git a/arch/s390/include/asm/ptrace.h b/arch/s390/include/asm/ptrace.h
index aaceb1d9110a..495e310c3d6d 100644
--- a/arch/s390/include/asm/ptrace.h
+++ b/arch/s390/include/asm/ptrace.h
@@ -134,6 +134,8 @@ struct pt_regs {
};
unsigned long flags;
unsigned long last_break;
+ unsigned int cpu;
+ unsigned char percpu_register;
};

/*
diff --git a/arch/s390/kernel/irq.c b/arch/s390/kernel/irq.c
index d10a17e6531d..43c92b8faade 100644
--- a/arch/s390/kernel/irq.c
+++ b/arch/s390/kernel/irq.c
@@ -33,6 +33,7 @@
#include <asm/softirq_stack.h>
#include <asm/vtime.h>
#include <asm/asm.h>
+#include <asm/entry-percpu.h>
#include "entry.h"

DEFINE_PER_CPU_SHARED_ALIGNED(struct irq_stat, irq_stat);
@@ -142,10 +143,13 @@ static int irq_pending(struct pt_regs *regs)

void noinstr do_io_irq(struct pt_regs *regs)
{
- irqentry_state_t state = irqentry_enter(regs);
- struct pt_regs *old_regs = set_irq_regs(regs);
+ struct pt_regs *old_regs;
+ irqentry_state_t state;
bool from_idle;

+ percpu_entry(regs);
+ state = irqentry_enter(regs);
+ old_regs = set_irq_regs(regs);
from_idle = test_and_clear_cpu_flag(CIF_ENABLED_WAIT);
if (from_idle)
update_timer_idle();
@@ -177,14 +181,18 @@ void noinstr do_io_irq(struct pt_regs *regs)

if (from_idle)
regs->psw.mask &= ~(PSW_MASK_EXT | PSW_MASK_IO | PSW_MASK_WAIT);
+ percpu_exit(regs);
}

void noinstr do_ext_irq(struct pt_regs *regs)
{
- irqentry_state_t state = irqentry_enter(regs);
- struct pt_regs *old_regs = set_irq_regs(regs);
+ struct pt_regs *old_regs;
+ irqentry_state_t state;
bool from_idle;

+ percpu_entry(regs);
+ state = irqentry_enter(regs);
+ old_regs = set_irq_regs(regs);
from_idle = test_and_clear_cpu_flag(CIF_ENABLED_WAIT);
if (from_idle)
update_timer_idle();
@@ -212,6 +220,7 @@ void noinstr do_ext_irq(struct pt_regs *regs)

if (from_idle)
regs->psw.mask &= ~(PSW_MASK_EXT | PSW_MASK_IO | PSW_MASK_WAIT);
+ percpu_exit(regs);
}

static void show_msi_interrupt(struct seq_file *p, int irq)
diff --git a/arch/s390/kernel/nmi.c b/arch/s390/kernel/nmi.c
index a55abbf65333..20fd319a3a8e 100644
--- a/arch/s390/kernel/nmi.c
+++ b/arch/s390/kernel/nmi.c
@@ -22,6 +22,7 @@
#include <linux/module.h>
#include <linux/sched/signal.h>
#include <linux/kvm_host.h>
+#include <asm/entry-percpu.h>
#include <asm/lowcore.h>
#include <asm/ctlreg.h>
#include <asm/fpu.h>
@@ -374,6 +375,7 @@ void notrace s390_do_machine_check(struct pt_regs *regs)
unsigned long mcck_dam_code;
int mcck_pending = 0;

+ percpu_entry(regs);
irq_state = irqentry_nmi_enter(regs);

if (user_mode(regs))
@@ -496,6 +498,7 @@ void notrace s390_do_machine_check(struct pt_regs *regs)
schedule_mcck_handler();

irqentry_nmi_exit(regs, irq_state);
+ percpu_exit(regs);
}
NOKPROBE_SYMBOL(s390_do_machine_check);

diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
index 1b5c6fc431cc..095937c2ca72 100644
--- a/arch/s390/kernel/traps.c
+++ b/arch/s390/kernel/traps.c
@@ -24,6 +24,7 @@
#include <linux/entry-common.h>
#include <linux/kmsan.h>
#include <linux/bug.h>
+#include <asm/entry-percpu.h>
#include <asm/asm-extable.h>
#include <asm/irqflags.h>
#include <asm/ptrace.h>
@@ -349,6 +350,7 @@ void noinstr __do_pgm_check(struct pt_regs *regs)
current->thread.gmap_int_code = regs->int_code & 0xffff;
return;
}
+ percpu_entry(regs);
state = irqentry_enter(regs);
if (user_mode(regs)) {
update_timer_sys();
@@ -386,6 +388,7 @@ void noinstr __do_pgm_check(struct pt_regs *regs)
out:
local_irq_disable();
irqentry_exit(regs, state);
+ percpu_exit(regs);
}

/*
--
2.51.0