Commit 86302ddf authored by Heiko Carstens's avatar Heiko Carstens
Browse files

Merge branch 'preempt' 

Heiko Carstens says:

====================

The option to select PREEMPT_NONE will go away for all architectures which
support PREEMPT_LAZY [1]. Until now all distributions provide kernels built
with PREEMPT_NONE enabled for s390. In particular this means that all
preempt_disable() / preempt_enable() pairs are optimized away during
compile time.

With PREEMPT_LAZY this is not the case. Switching to PREEMPT_LAZY leads
to a kernel image size increase of ~218kb (defconfig, gcc15).

s390 provides optimized preempt primitives, however there is still room for
improvement. Since support for relocatable lowcore was added access to
preempt_count in lowcore requires an extra call of get_lowcore(), which
generates an extra instruction. Also all instructions have to use a base
register which is not zero to access preempt_count.

Address this by adding a couple of inline assemblies with alternatives.

This generates better code and reduces the size of a PREEMPT_LAZY built
kernel image by ~58kb.

[1] https://lore.kernel.org/all/20251219101502.GB1132199@noisy.programming.kicks-ass.net/



====================

Signed-off-by: default avatarHeiko Carstens <hca@linux.ibm.com>
parents 12ea976f 48b4790f

arch/s390/include/asm/asm.h

+1 −1
Original line number Diff line number Diff line
@@ -30,7 +30,7 @@ 
 */

#if defined(__GCC_ASM_FLAG_OUTPUTS__) && !(IS_ENABLED(CONFIG_CC_ASM_FLAG_OUTPUT_BROKEN))



#define __HAVE_ASM_FLAG_OUTPUTS__

#define __HAVE_ASM_FLAG_OUTPUTS__ 1



#define CC_IPM(sym)

#define CC_OUT(sym, var)	"=@cc" (var)

arch/s390/include/asm/preempt.h

+44 −3
Original line number Diff line number Diff line
@@ -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)
@@ -68,7 +84,17 @@ static __always_inline void __preempt_count_add(int val) 
	 */

	if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES)) {

		if (__builtin_constant_p(val) && (val >= -128) && (val <= 127)) {

			__atomic_add_const(val, &get_lowcore()->preempt_count);

			unsigned long lc_preempt;



			lc_preempt = offsetof(struct lowcore, preempt_count);

			asm_inline(

				ALTERNATIVE("asi	%[offzero](%%r0),%[val]\n",

					    "asi	%[offalt](%%r0),%[val]\n",

					    ALT_FEATURE(MFEATURE_LOWCORE))

				: "+m" (((struct lowcore *)0)->preempt_count)

				: [offzero] "i" (lc_preempt), [val] "i" (val),

				  [offalt] "i" (lc_preempt + LOWCORE_ALT_ADDRESS)

				: "cc");

			return;

		}

	}
@@ -87,7 +113,22 @@ static __always_inline void __preempt_count_sub(int val) 
 */

static __always_inline bool __preempt_count_dec_and_test(void)

{

#ifdef __HAVE_ASM_FLAG_OUTPUTS__

	unsigned long lc_preempt;

	int cc;



	lc_preempt = offsetof(struct lowcore, preempt_count);

	asm_inline(

		ALTERNATIVE("alsi	%[offzero](%%r0),%[val]\n",

			    "alsi	%[offalt](%%r0),%[val]\n",

			    ALT_FEATURE(MFEATURE_LOWCORE))

		: "=@cc" (cc), "+m" (((struct lowcore *)0)->preempt_count)

		: [offzero] "i" (lc_preempt), [val] "i" (-1),

		[offalt] "i" (lc_preempt + LOWCORE_ALT_ADDRESS));

	return (cc == 0) || (cc == 2);

#else

	return __atomic_add_const_and_test(-1, &get_lowcore()->preempt_count);

#endif

}



/*