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Commit 6b8019c8 authored by Philippe Gerum's avatar Philippe Gerum Committed by Mike Frysinger
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Blackfin: allow high priority domains to preempt schedule_tail() 



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: default avatarPhilippe Gerum <rpm@xenomai.org>
Signed-off-by: default avatarMike Frysinger <vapier@gentoo.org>
parent bc569f1a

arch/blackfin/kernel/entry.S

+22 −2
Original line number Diff line number Diff line
@@ -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;