Re: top stack (l)users for 2.5.69

Timothy Miller
2003-05-08T18:49:03Z

Jonathan Lundell wrote:
>
>
> In particular, the interrupt stack is the kernel stack of the current
> task. This is (in part) what leads to stack overflows. If the current
> task is running in the kernel, using a significant hunk of its stack, an
> interrupt is limited to the balance of that stack. And if that interrupt
> triggers a soft irq that runs, say, a network stack, and that softirq
> handler in turn gets interrupted, we've got, effectively, three
> processes sharing the stack. And of course hard interrupts can be
> nested, so it's pretty damn difficult to specify a safe upper limit for
> stack usage.

This is the sort of things that would severely limit our ability to
shrink the kernel stack. While it's perhaps feasible to shrink kernel
stack usage for typical syscalls, exactly the situation you describe is
unpredictable and very difficult to avoid.

My suggestion would be that if we do manage to get typical stack usage
down to the point where we can go to a 4K stack, then interrupt handlers
would have to be rewritten to recognize whether or not the interrupt
arrived on a user process kernel stack and then move the context over to
the "interrupt stack". The overhead would be low enough that it's worth
doing so that we could reduce process kernel stack size. Whenever an
interrupt service routine is itself interrupted, the interrupt stack
check code would realize that it is already using the interrupt stack
and not move the context. Here, then, we would need only one single
interrupt stack which we would size for worst case; so if we made it 8
or 12K, that's 8 or 12K once for each CPU which is allowed to receive
interrupts, not once per process.

You like? :)