Re: Interrupt starvation points

[Russell King <rmk+lkml@arm.linux.org.uk>]

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