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Subject[PATCH 08/72] Blackfin: allow high priority domains to preempt schedule_tail()
Date
From: Philippe Gerum <rpm@xenomai.org>

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>
---
arch/blackfin/kernel/entry.S | 24 ++++++++++++++++++++++--
1 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/blackfin/kernel/entry.S b/arch/blackfin/kernel/entry.S
index a9cfba9..3f8769b 100644
--- a/arch/blackfin/kernel/entry.S
+++ b/arch/blackfin/kernel/entry.S
@@ -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;
--
1.6.4.2


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