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

From: Saravana Kannan
Date: Mon Feb 06 2012 - 22:34:07 EST

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

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.


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.



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