Messages in this thread | | | Date | Mon, 19 Dec 2011 10:02:17 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled |
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On Fri, Dec 16, 2011 at 11:23:30PM +0000, 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> > > > > 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.
Option 1 is the only one that would work with the current ARM switch_mm() implementation and I think it's the simplest (but I don't know the background to the IRQs being disabled by the RT patches for the switch_mm() call).
-- Catalin
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