[BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

[michal.simek@petalogix.com (Michal Simek)]

Peter Zijlstra wrote:
> On Fri, 2011-05-27 at 21:52 +0100, Russell King - ARM Linux wrote:
>> On Fri, May 27, 2011 at 02:06:29PM +0200, Ingo Molnar wrote:
>>> The expectations are to have irqs off (we are holding the runqueue 
>>> lock if !__ARCH_WANT_INTERRUPTS_ON_CTXSW), so that's not workable i 
>>> suspect.
>> Just a thought, but we _might_ be able to avoid a lot of this hastle if
>> we had a new arch hook in finish_task_switch(), after finish_lock_switch()
>> returns but before the old MM is dropped.
> 
> I'd be more than willing to provide this.
> 
>> For the new ASID-based switch_mm(), we currently do this:
>>
>> 1. check ASID validity
>> 2. flush branch predictor
>> 3. set reserved ASID value
>> 4. set new page tables
>> 5. set new ASID value
>>
>> This will be shortly changed to:
>>
>> 1. check ASID validity
>> 2. flush branch predictor
>> 3. set swapper_pg_dir tables
>> 4. set new ASID value
>> 5. set new page tables
>>
>> We could change switch_mm() to only do:
>>
>> 1. flush branch predictor
>> 2. set swapper_pg_dir tables
>> 3. check ASID validity
>> 4. set new ASID value
>>
>> At this point, we have no user mappings, and so nothing will be using the
>> ASID at this point.  Then in a new post-finish_lock_switch() arch hook:
>>
>> 5. check whether we need to do flushing as a result of ASID change
>> 6. set new page tables
>>
>> I think this may simplify the ASID code.  It needs prototyping out,
>> reviewing and testing, but I think it may work.
>>
>> And I think it may also be workable with the CPUs which need to flush
>> the caches on context switches - we can postpone their page table
>> switch to this new arch hook too, which will mean we wouldn't require
>> __ARCH_WANT_INTERRUPTS_ON_CTXSW on ARM at all.
>>
>> Any thoughts (if you've followed what I'm going on about) ?
> 
> Yeah, definitely worth a try, you mentioned on IRC the problem of
> detecting if switch_mm() happened in the new arch hook. Since
> switch_mm() gets a @next pointer we can set a TIF flag there and have
> the new arch hook test for that and conditionally perform the required
> work.
> 
> Now, supposing we can get ARM to not rely on
> __ARCH_WANT_INTERRUPTS_ON_CTXSW anymore, there's only microblaze left,
> Michal, would a similar scheme work for you? If so we can fully
> deprecate and remove this exception from the scheduler (yay!).

Hi,

please correct me if I am wrong but this is workaround just for ARM.
I am not aware that we need to do anything with caches. I enabled that options
after our discussion (http://lkml.org/lkml/2009/12/3/204) because of problems 
with lockdep. I will look if I can remove that option but it will be necessary 
to do some changes in code. switch_to should be called with irq OFF right?

Michal


Michal






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Michal Simek, Ing. (M.Eng)
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