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Blackfin: allow high priority domains to preempt schedule_tail()

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ret_from_fork is always entered with hw interrupts off, which prevents
real-time domains to preempt the Linux kernel during part of the
initial context switch to the new task, which could in turn raise the
worst-case latency figures.

To avoid this, stall the root domain stage in the interrupt pipeline
to keep the scheduling tail code free from Linux-handled IRQs, then
enable hardware interrupts again.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
This commit is contained in:
Philippe Gerum 2009-06-22 18:23:32 +02:00 committed by Mike Frysinger
parent bc569f1a77
commit 6b8019c85e
1 changed files with 22 additions and 2 deletions
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24
arch/blackfin/kernel/entry.S
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@ -43,8 +43,28 @@
ENTRY(_ret_from_fork)
#ifdef CONFIG_IPIPE
[--sp] = reti; /* IRQs on. */
SP += 4;
/*
* Hw IRQs are off on entry, and we don't want the scheduling tail
* code to starve high priority domains from interrupts while it
* runs. Therefore we first stall the root stage to have the
* virtual interrupt state reflect IMASK.
*/
p0.l = ___ipipe_root_status;
p0.h = ___ipipe_root_status;
r4 = [p0];
bitset(r4, 0);
[p0] = r4;
/*
* Then we may enable hw IRQs, allowing preemption from high
* priority domains. schedule_tail() will do local_irq_enable()
* since Blackfin does not define __ARCH_WANT_UNLOCKED_CTXSW, so
* there is no need to unstall the root domain by ourselves
* afterwards.
*/
p0.l = _bfin_irq_flags;
p0.h = _bfin_irq_flags;
r4 = [p0];
sti r4;
#endif /* CONFIG_IPIPE */
SP += -12;
call _schedule_tail;