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SubjectRe: [PATCH] per-interrupt stacks - try 2
Date
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> per-CPU per-IRQ i mean, of course. It's a basic performance issue, on SMP
> we do not want dirty IRQ stacks to bounce between CPUs ...

Do you have benchmarks or something to show that is this actually a
_significant_ problem?

After all, unless you bind the interrupts to particular IRQs, loads of data -
including the irq_desc[] table - are going to be bouncing too.

I'm not convinced that it's going to be worth doing stacks on a per-CPU basis,
and not a per-IRQ basis. The only way I can see that it could be a problem is
that if it will be permitted to enter handle_IRQ_event() on two or more
different CPUs simultaneously for the same IRQ, provided that do_IRQ() itself
switches the stack just around the call to handle_IRQ_event() [see attached].

I think, however, that per-CPU softirq stacks would be a good idea, as these
are used quite capaciously by TCP/IP.

David

asmlinkage unsigned int do_IRQ(struct pt_regs regs)
{
/*
* We ack quickly, we don't want the irq controller
* thinking we're snobs just because some other CPU has
* disabled global interrupts (we have already done the
* INT_ACK cycles, it's too late to try to pretend to the
* controller that we aren't taking the interrupt).
*
* 0 return value means that this irq is already being
* handled by some other CPU. (or is disabled)
*/
int irq = regs.orig_eax & 0xff; /* high bits used in ret_from_ code */
int cpu = smp_processor_id();
irq_desc_t *desc = irq_desc + irq;
struct thread_info *curctx;
struct irqaction *action;
union irq_ctx *irqctx;
unsigned int status;

irq_enter();
kstat.irqs[cpu][irq]++;
spin_lock(&desc->lock);
desc->handler->ack(irq);
/*
REPLAY is when Linux resends an IRQ that was dropped earlier
WAITING is used by probe to mark irqs that are being tested
*/
status = desc->status & ~(IRQ_REPLAY | IRQ_WAITING);
status |= IRQ_PENDING; /* we _want_ to handle it */

/*
* If the IRQ is disabled for whatever reason, we cannot
* use the action we have.
*/
action = NULL;
if (likely(!(status & (IRQ_DISABLED | IRQ_INPROGRESS)))) {
action = desc->action;
status &= ~IRQ_PENDING; /* we commit to handling */
status |= IRQ_INPROGRESS; /* we are handling it */
}
desc->status = status;

/*
* If there is no IRQ handler or it was disabled, exit early.
Since we set PENDING, if another processor is handling
a different instance of this same irq, the other processor
will take care of it.
*/
if (unlikely(!action))
goto out;

/*
* Edge triggered interrupts need to remember
* pending events.
* This applies to any hw interrupts that allow a second
* instance of the same irq to arrive while we are in do_IRQ
* or in the handler. But the code here only handles the _second_
* instance of the irq, not the third or fourth. So it is mostly
* useful for irq hardware that does not mask cleanly in an
* SMP environment.
*/
curctx = current_thread_info();
irqctx = desc->hardirq_stack;
irqctx->tinfo.task = curctx->task;

for (;;) {
u32 *isp;

spin_unlock(&desc->lock);

/* build the stack frame on the IRQ stack */
isp = (u32*) ((char*)irqctx + sizeof(*irqctx));
*--isp = (u32) action;
*--isp = (u32) &regs;
*--isp = (u32) irq;

asm volatile(
" movl %1,%%eax \n"
" xchgl %%ebx,%%esp \n"
" call *%%eax \n"
" xchgl %%ebx,%%esp \n"
:
: "b"(isp), "i"(handle_IRQ_event)
: "memory", "cc", "eax", "edx", "ecx"
);

spin_lock(&desc->lock);

if (likely(!(desc->status & IRQ_PENDING)))
break;
desc->status &= ~IRQ_PENDING;
}

irqctx->tinfo.task = NULL;
desc->status &= ~IRQ_INPROGRESS;

out:
/*
* The ->end() handler has to deal with interrupts which got
* disabled while the handler was running.
*/
desc->handler->end(irq);
spin_unlock(&desc->lock);

irq_exit();

return 1;
}
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