Messages in this thread | | | Date | Tue, 20 Dec 2011 12:09:26 -0800 | From | Frank Rowand <> | Subject | Re: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled |
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On 12/20/11 03:54, Catalin Marinas wrote: > On Tuesday, 20 December 2011, Frank Rowand <frank.rowand@am.sony.com> wrote: >> On 12/19/11 02:02, Catalin Marinas wrote: >>> On Fri, Dec 16, 2011 at 11:23:30PM +0000, frank.rowand@am.sony.com<mailto:frank.rowand@am.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@am.sony.com<mailto:frank.rowand@am.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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