Re: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

[Saravana Kannan]

On 02/02/2012 04:36 PM, Russell King - ARM Linux wrote:
> On Thu, Feb 02, 2012 at 03:36:49PM -0800, Stephen Boyd wrote:
> > On 02/02/12 13:38, Nicolas Pitre wrote:
> > > On Thu, 2 Feb 2012, Russell King - ARM Linux wrote
> > > > On Thu, Feb 02, 2012 at 11:24:46AM -0800, Stephen Boyd wrote:
> > > > > Should we move get_thread_info into assembler.h? It seems odd
> > > > > to include entry-header.S but I saw that vfp was doing the same.
> > > > Probably yes, and probably also have preempt_disable and preempt_enable
> > > > assembler macros.  That's going to get rather icky if we have to
> > > > explicitly call the scheduler though (to solve (1)).
> > > What about a pair of helpers written in C instead?
> > >
> > > v7_flush_dcache_all() could be renamed, and a wrapper function called
> > > v7_flush_dcache_all() would call the preemption disable helper, call the
> > > former v7_flush_dcache_all code, then call the preemption enable helper.
> > >
> > > Then __v7_setup() could still call the core cache flush code without
> > > issues.
> >
> > I tried to put the preemption disable/enable right around the place
> > where it was needed. With this approach we would disable preemption
> > during the entire cache flush. I'm not sure if we want to make this
> > function worse for performance, do we? It certainly sounds easier than
> > writing all the preempt macros in assembly though.
>
> Err, why do you think it's a big task?
>
> preempt disable is a case of incrementing the thread preempt count, while
> preempt enable is a case of decrementing it, testing for zero, if zero,
> then checking whether TIF_NEED_RESCHED is set and calling a function.
>
> If that's too much, then the simple method in assembly to quickly disable
> preemption over a very few set of instructions is using mrs/msr and cpsid i.
> That'll be far cheaper than fiddling about with preempt counters or
> messing about with veneers in C code.

Russell,

I think you misunderstood Stephen's point about the performance. He 
isn't referring to the performance difference between a C call to preemt 
disable/enable vs. a few assembly level instructions.

I believe he is referring to the performance hit of having preemption 
disabled during the entirety of the cache flush operation vs. having 
preemption disabled only for the duration of writing to CSSELR and 
reading back CCSIDR.

I would think a cache flush is a fairly long operation and to have 
preemption disable across it doesn't sound appealing to me.

Thoughts?

Thanks,
Saravana

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