Re: [patch] 2.4.4 alpha semaphores optimization

[Ivan Kokshaysky]

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On Fri, May 04, 2001 at 04:33:59PM +0200, Andrea Arcangeli wrote:
> the 2^16 limit is not a per-user limit it is a global one so the max
> user process ulimit is irrelevant.
> 
> Only the number of pid and the max number of tasks supported by the
> architecture is a relevant limit for this.

Thanks for the correction. I thought about a case where one user could
exhaust 2^16 limit.

> > b. "long" count would cost extra 8 bytes in the struct rw_semaphore;
> 
> correct but that's the "feature" to be able to support 2^32 concurrent
> sleepers at not relevant runtime cost 8).

But I can't imagine how this "feature" could be useful in a real life :-)

> > c. I can use existing atomic routines which deal with ints.
> 
> I was thinking at a dedicated routine that implements the slow path by
> hand as well like x86 just do. Then using ldq instead of ldl isn't
> really a big deal programmer wise.

You meant "the fast path", I guess? Then it's true. However with those
atomic functions code is much more readable.

Anyway, I've attached asm-alpha/rwsem_xchgadd.h for your implementation.
However I got processes in D state early on boot with it -- maybe
I've made a typo somewhere...

Ivan.

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#ifndef _ALPHA_RWSEM_XCHGADD_H
#define _ALPHA_RWSEM_XCHGADD_H

#include <asm/types.h>	/* BITS_PER_LONG */

static inline void __down_read(struct rw_semaphore *sem)
{
	long count, temp;
	__asm__ __volatile__(
	"1:	ldq_l	%0,%1\n"
	"	addq	%0,1,%2\n"
	"	stq_c	%2,%1\n"
	"	beq	%2,2f\n"
#ifdef CONFIG_SMP
	"	mb\n"
#endif
	".subsection 2\n"
	"2:	br	1b\n"
	".previous"
	:"=&r" (count), "=m" (sem->count), "=&r" (temp)
	:"m" (sem->count) : "memory");

	if (count < 0)
		rwsem_down_failed(sem, RWSEM_READ_BLOCKING_BIAS);
}

static inline void __down_write(struct rw_semaphore *sem)
{
	long granted, temp = RWSEM_WRITE_BIAS + RWSEM_READ_BIAS;
	__asm__ __volatile__(
	"1:	ldq_l	%0,%1\n"
	"	addq	%0,%2,%2\n"
	"	stq_c	%2,%1\n"
	"	beq	%2,2f\n"
#ifdef CONFIG_SMP
	"	mb\n"
#endif
	"	cmpeq	%0,0,%0\n"
	".subsection 2\n"
	"2:	br	1b\n"
	".previous"
	:"=&r" (granted), "=m" (sem->count), "=&r" (temp)
	:"2" (temp),"m" (sem->count) : "memory");

	if (!granted)
		rwsem_down_failed(sem, RWSEM_WRITE_BLOCKING_BIAS);
}

static inline void __up_read(struct rw_semaphore *sem)
{
	long oldcount, temp;
	__asm__ __volatile__(
#ifdef CONFIG_SMP
	"	mb\n"
#endif
	"1:	ldq_l	%0,%1\n"
	"	subq	%0,1,%2\n"
	"	stq_c	%2,%1\n"
	"	beq	%2,2f\n"
	"	subl	%0,1,%2\n"
	".subsection 2\n"
	"2:	br	1b\n"
	".previous"
	:"=&r" (oldcount), "=m" (sem->count), "=&r" (temp)
	:"m" (sem->count) : "memory");

	if (oldcount < 0 && temp == 0)
			rwsem_wake(sem);
}

static inline void __up_write(struct rw_semaphore *sem)
{
	long count, temp = RWSEM_READ_BIAS + RWSEM_WRITE_BIAS;
	__asm__ __volatile__(
#ifdef CONFIG_SMP
	"	mb\n"
#endif
	"1:	ldq_l	%0,%1\n"
	"	subq	%0,%2,%2\n"
	"	mov	%2,%0\n"
	"	stq_c	%2,%1\n"
	"	beq	%2,2f\n"
	".subsection 2\n"
	"2:	br	1b\n"
	".previous"
	:"=&r" (count), "=m" (sem->count), "=&r" (temp)
	:"2" (temp), "m" (sem->count) : "memory");

	if (count < 0)
		rwsem_wake(sem);
}

static inline long rwsem_xchgadd(long value, long * count)
{
	long ret, temp;
	__asm__ __volatile__(
	"1:	ldq_l	%0,%1\n"
	"	addq	%0,%3,%2\n"
	"	stq_c	%2,%1\n"
	"	beq	%2,2f\n"
	".subsection 2\n"
	"2:	br	1b\n"
	".previous"
	:"=&r" (ret), "=m" (count), "=&r" (temp)
	:"Ir" (value), "m" (count) : "memory");

	return ret;
}

#endif

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