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

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Message-ID: <20111216111319.GB6342@arm.com>
Date:	Fri, 16 Dec 2011 11:13:19 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"frank.rowand@...sony.com" <frank.rowand@...sony.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>
Subject: Re: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled

On Fri, Dec 16, 2011 at 09:54:32AM +0000, Peter Zijlstra wrote:
> 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.. :-)

Russell agreed for me to push this to -next (in case -rt uses that, not
sure) to get a bit more exposure. Otherwise testing the patches in -rt
would really help spotting bugs.

But we need to sort out the dangling switch_mm() calls (without a
corresponding post-switch hook call) that I mentioned in my reply to
Frank.

-- 
Catalin
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