On Fri, Feb 11, 2005 at 09:05:21AM -0800, Daniel Walker wrote:
> The other patch enabled interrupt before calling up on
> kernel_sem ..This one could use some thinking over. I did this cause
> up() is very expensive on ARM , and combined with the looping above
> interrupts can stay off for a long time ..
Please substantiate your claim that up() is very expensive on ARM.
I disagree:
#define __up_op(ptr,wake) \
({ \
__asm__ __volatile__( \
"@ up_op\n" \
" mrs ip, cpsr\n" \
" orr lr, ip, #128\n" \
" msr cpsr_c, lr\n" \
" ldr lr, [%0]\n" \
" adds lr, lr, %1\n" \
" str lr, [%0]\n" \
" msr cpsr_c, ip\n" \
" movle ip, %0\n" \
" blle " #wake \
: \
: "r" (ptr), "I" (1) \
: "ip", "lr", "cc", "memory"); \
})
static inline void up(struct semaphore * sem)
{
__up_op(sem, __up_wakeup);
}
Looks like 9 instructions for the uncontended case to me. If you're
worried about 9 instructions being expensive, please work on GCC to
improve its optimisation capabilities first. There's room there for
improvement across the whole kernel than just the above 9 instructions.
Plus, after you've read the above code, wouldn't you think that adding
the "enable interrupts + disable interrupts" around an up() operation
(which itself immediately disables interrupts again) is just adding
extra instructions to the kernel, which corresponds directly to lower
performance?
--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 PCMCIA - http://pcmcia.arm.linux.org.uk/
2.6 Serial core
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