Messages in this thread |  | | | Subject | Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM | | From | Peter Zijlstra <> | | Date | Sat, 28 May 2011 15:13:01 +0200 |
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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!).
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