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

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Date:	Tue, 20 Dec 2011 12:09:26 -0800
From:	Frank Rowand <frank.rowand@...sony.com>
To:	Catalin Marinas <catalin.marinas@....com>
CC:	"Rowand, Frank" <Frank_Rowand@...yusa.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"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 12/20/11 03:54, Catalin Marinas wrote:
> On Tuesday, 20 December 2011, Frank Rowand <frank.rowand@...sony.com> wrote:
>> On 12/19/11 02:02, Catalin Marinas wrote:
>>> On Fri, Dec 16, 2011 at 11:23:30PM +0000, frank.rowand@...sony.com<mailto:frank.rowand@...sony.com> wrote:
>>>> On 12/16/11 03:01, Catalin Marinas wrote:
>>>>> On Fri, Dec 16, 2011 at 03:20:45AM +0000, 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:
>>>>>>
>>>>>>    -#ifndef CONFIG_CPU_HAS_ASID
>>>>>>    -#define __ARCH_WANT_INTERRUPTS_ON_CTXSW
>>>>>>    -#endif
>>>>>>
>>>>>> Signed-off-by: Frank Rowand <frank.rowand@...sony.com<mailto:frank.rowand@...sony.com>>
>>>>>
>>>>> The whole point of my patches was to no longer define
>>>>> __ARCH_WANT_INTERRUPTS_ON_CTXSW on ARM, so bringing it back in is not
>>>>> feasible.
>>>>
>>>> Looking over Catalin's patches again, it looks like my hacky RT patch
>>>> will no longer be needed after Catalin's patch set is in place.  The
>>>> problem my patch deals with is that with the RT patches applied, use_mm()
>>>> calls switch_mm() with IRQs disabled.  The current ARM switch_mm() can
>>>> not be called with IRQs disabled.  But with Catalin's patch 4
>>>> (http://lkml.indiana.edu/hypermail/linux/kernel/1111.3/01898.html)
>>>> applied, switch_mm() can be called with IRQs enabled, because
>>>> switch_mm() no longer calls check_context() which calls __new_context()
>>>> which calls smp_call_function() which requires IRQs to be enabled....
>>>
>>> I don't think much has changed with my patches. The switch_mm() itself
>>> can be called with IRQs disabled but it wouldn't even do the pgd switch
>>> unless it is followed by a finish_arch_post_lock_switch() call (hook
>>> introduced by my patch, but only available in sched.c).
>>>
>>> I think you need a solution for the RT series without considering my
>>> context switch changes. As I understand, the RT code currently calls
>>> switch_mm() with interrupts disabled which is not supported on ARM. So
>>> we have two solutions:
>>>
>>> 1. Change the RT patches to call switch_mm() with interrupts enabled
>>>    (and I can modify the ARM code to cope with this scenario and do the
>>>    pgd switch in one go).
>>> 2. Call switch_mm() with interrupts disabled but invoke an arch hook
>>>    once the interrupts have been enabled to complete the pgd switch.
>>
>> I think I'm in agreement with you.
>>
>> Solution 1 works for the RT patch set with the current mainline (and my
>> short term modification to the RT patch set that calls switch_mm() with
>> interrupts enabled from use_mm()).  I don't think there is any need to
>> modify the ARM code for this to work.  I'm assuming that when you say
>> "do the pgd switch" that you are talking about the
>> "cpu_switch_mm(next->pgd, next)" that is currently in switch_mm().
> 
> Yes.
> 
>> Solution 2 will work after version 2 of your patches in "Remove the
>> __ARCH_WANT_INTERRUPTS_ON_CTXSW definition" is applied.  In this
>> case my short term modification to the RT patch set for solution 1
>> would be removed, and instead the RT patch set would call
>> finish_arch_post_lock_switch() after re-enabling IRQs in use_mm().
> 
> Isn't solution 1 enough with both current ARM code and the latest context switch patches?

Yes, solution 1 would also work with the latest context switch patches
_if_ the RT patch called switch_mm() from use_mm() with interrupts
enabled.  But the RT patch wants interrupts disabled for all other
architectures, so the RT patch would have to leave interrupts enabled
for ARM.

-Frank

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