Messages in this thread | | | Date | Sun, 23 May 2004 17:43:59 +0100 | From | Russell King <> | Subject | scheduler: IRQs disabled over context switches |
| |
Hi,
The 2.6.6 scheduler disables IRQs across context switches, which is bad news for IRQ latency on ARM - to the point where 16550A FIFO UARTs to overrun.
I'm considering defining prepare_arch_switch & co as follows on ARM, so that we release IRQs over the call to context_switch().
#define prepare_arch_switch(rq,next) \ do { \ spin_lock(&(next)->switch_lock); \ spin_unlock_irq(&(rq)->lock); \ } while (0) #define finish_arch_switch(rq,prev) \ spin_unlock(&(prev)->switch_lock) #define task_running(rq,p) \ ((rq)->curr == (p) || spin_is_locked(&(p)->switch_lock))
The question is... why are we keeping IRQs disabled over context_switch() in the first case? Looking at the code, the only thing which is touched outside of the two tasks is rq->prev_mm. Since runqueues are CPU- specific and we're holding at least one spinlock, I think the above is preempt safe and SMP safe.
However, I'd like to find out from someone who knows this code why IRQs are disabled by default here.
-- 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |