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SubjectRe: [patch] 2.4.4 alpha semaphores optimization
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.

#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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