linux-kernel - Re: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled

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Message-ID: <1324029272.18942.73.camel@twins>
Date:	Fri, 16 Dec 2011 10:54:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	frank.rowand@...sony.com
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	catalin.marinas@....com, rostedt@...dmis.org
Subject: Re: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled

[ now without loosing CC's ]

On Thu, 2011-12-15 at 19:20 -0800, Frank Rowand wrote:
> ARMv6 and later have VIPT caches and the TLBs are tagged with an ASID
> (application specific ID). The number of ASIDs is limited to 256 and
> the allocation algorithm requires IPIs when all the ASIDs have been
> used.  The IPIs require interrupts enabled during context switch for
> deadlock avoidance.
> 
> The RT patch mm-protect-activate-switch-mm.patch disables irqs around
> activate_mm() and switch_mm(), which are the portion of the ARMv6
> context switch that require interrupts enabled.
> 
> The solution for the ARMv6 processors could be to _not_ disable irqs.
> A more conservative solution is to provide the same environment that
> the scheduler provides, that is preempt_disable().  This is more
> resilient for possible future changes to the ARM context switch code
> that is not aware of the RT patches.
> 
> This patch will conflict slightly with Catalin's patch set to remove
> __ARCH_WANT_INTERRUPTS_ON_CTXSW, when that is accepted:
> 
>    http://lkml.indiana.edu/hypermail/linux/kernel/1111.3/01893.html
> 
> When Catalin's patch set is accepted, this RT patch will need to reverse
> the change in patch 6 to arch/arm/include/asm/system.h:


We could just merge Catalin's stuff in -rt to give it a test ride and
see if anything horrible happens.. :-)
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