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SubjectRe: [patch] softirq performance fixes, cleanups, 2.4.10.

On Fri, 28 Sep 2001, Oleg Nesterov wrote:

> I am trying to understand the basics of softirq handling.
>
> It seems to me that ksoftirqd()'s loop can be cleanuped a bit with
> following (untested) patch on top of 2.4.10-softirq-A7.

thanks - the patch looks mostly correct.

> It also removes the 'mask' variable in do_softirq().

(right, applied.)

> - schedule();
> - __set_current_state(TASK_INTERRUPTIBLE);

we need a single schedule to migrate the CPU to the right CPU. Btw, it's
not a big offense to run on the wrong CPU: do_softirq() will run
correctly. But there wont be a ksoftirqd exeuting on the 'right' CPU (for
this short amount of time until it schedules once), and we might miss a
single instance of softirq processing. So it's cleaner to do a schedule()
that guarantees that we migrate to the target CPU.

> + __set_current_state(TASK_RUNNING);
> + }

well, this basically reverts it to the original code and removes the
optimization - but i dont have any strong feelings in either way. I think
Andrea is right and it's more correct to set it TASK_RUNNING, for
task-statistics reasons. Reverted it to the old code.

and in fact while doing that i found another bug in the old code:
TASK_INTERRUPTIBLE was set in a non-IRQ-atomic way, so theoretically it
could be possible that the compiler reorders the setting of
TASK_INTERRUPTIBLE and the test for softirq_pending(), and if an IRQ gets
inbetween the we could miss processing softirq for 1/HZ again. Added an
optimization barrier(). (SMP-consistency is not needed, since we are
per-CPU.)

Ingo

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