From: Heiko Carstens <hca@linux.ibm.com>
Date: Tue, 13 Jan 2026 19:43:57 +0000 (+0100)
Subject: s390/preempt: Optimize preempt_count()
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6cce3609a1e0dedeef9b4bfdc87d0d4692f691d7;p=thirdparty%2Fkernel%2Flinux.git

s390/preempt: Optimize preempt_count()

Provide an inline assembly using alternatives to avoid the need of
a base register when reading preempt_count() from lowcore. Use the
LLGT instruction, which reads only the least significant 31 bits of
preempt_count. This masks out the encoded PREEMPT_NEED_RESCHED bit.

Generated code is changed from

000000000046e5d0 <vfree>:
  46e5d0:       c0 04 00 00 00 00       jgnop   46e5d0 <vfree>
  46e5d6:       a7 39 00 00             lghi    %r3,0
  46e5da:       58 10 33 a8             l       %r1,936(%r3)
  46e5de:       c0 1b 00 ff ff 00       nilf    %r1,16776960
  46e5e4:       a7 74 00 11             jne     46e606 <vfree+0x36>

to something like this:

000000000046e5d0 <vfree>:
  46e5d0:       c0 04 00 00 00 00       jgnop   46e5d0 <vfree>
  46e5d6:       e3 10 03 a8 00 17       llgt    %r1,936
  46e5dc:       ec 41 28 b7 00 55       risbgz  %r4,%r1,40,55
  46e5e2:       a7 74 00 0f             jne     46e600 <vfree+0x30>

Overall savings are only 82 bytes according to bloat-o-meter. This
is because of different inlining decisions, and there aren't many
preempt_count() users in the kernel.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---

diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
index 6ccd033acfe52..6d6a28bee4e70 100644
--- a/arch/s390/include/asm/preempt.h
+++ b/arch/s390/include/asm/preempt.h
@@ -8,7 +8,10 @@
 #include <asm/cmpxchg.h>
 #include <asm/march.h>
 
-/* We use the MSB mostly because its available */
+/*
+ * Use MSB so it is possible to read preempt_count with LLGT which
+ * reads the least significant 31 bits with a single instruction.
+ */
 #define PREEMPT_NEED_RESCHED	0x80000000
 
 /*
@@ -23,7 +26,20 @@
  */
 static __always_inline int preempt_count(void)
 {
-	return READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED;
+	unsigned long lc_preempt, count;
+
+	BUILD_BUG_ON(sizeof_field(struct lowcore, preempt_count) != sizeof(int));
+	lc_preempt = offsetof(struct lowcore, preempt_count);
+	/* READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED */
+	asm_inline(
+		ALTERNATIVE("llgt	%[count],%[offzero](%%r0)\n",
+			    "llgt	%[count],%[offalt](%%r0)\n",
+			    ALT_FEATURE(MFEATURE_LOWCORE))
+		: [count] "=d" (count)
+		: [offzero] "i" (lc_preempt),
+		  [offalt] "i" (lc_preempt + LOWCORE_ALT_ADDRESS),
+		  "m" (((struct lowcore *)0)->preempt_count));
+	return count;
 }
 
 static __always_inline void preempt_count_set(int pc)