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s390/preempt: Optimize preempt_count()

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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>
This commit is contained in:
Heiko Carstens 2026-01-13 20:43:57 +01:00
parent 12ea976f95
commit 6cce3609a1
1 changed files with 18 additions and 2 deletions
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20
arch/s390/include/asm/preempt.h
View File

@ -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)