Re: [PATCH v2] perf: Use pre-empt safe cpu_get/put insted of smp_processor_id

[Roger Quadros <rogerq@ti.com>]

On 09/10/2012 08:53 PM, Stephen Boyd wrote:
> On 09/10/12 04:30, Roger Quadros wrote:
>> gets rid of below messages with CONFIG_DEBUG_PREEMPT enabled
>>
>> [   28.832916] debug_smp_processor_id: 18 callbacks suppressed
>> [   28.832946] BUG: using smp_processor_id() in preemptible [00000000] code: modprobe/1763
>> [   28.841491] caller is pwrdm_set_next_pwrst+0x54/0x120
>>
>> changes in v2:
>> - rebased on 3.6-rc5
>> - use put_cpu() immediately after get_cpu() in omap3_pm_idle()
>>
>> Signed-off-by: Roger Quadros <rogerq@ti.com>
>> ---
>>  arch/arm/mach-omap2/clock.c       |    9 ++++++---
>>  arch/arm/mach-omap2/pm34xx.c      |   12 ++++++++----
>>  arch/arm/mach-omap2/powerdomain.c |    6 ++++--
>>  3 files changed, 18 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/arm/mach-omap2/clock.c b/arch/arm/mach-omap2/clock.c
>> index ea3f565..06747b6 100644
>> --- a/arch/arm/mach-omap2/clock.c
>> +++ b/arch/arm/mach-omap2/clock.c
>> @@ -285,7 +285,8 @@ void omap2_clk_disable(struct clk *clk)
>>  	pr_debug("clock: %s: disabling in hardware\n", clk->name);
>>  
>>  	if (clk->ops && clk->ops->disable) {
>> -		trace_clock_disable(clk->name, 0, smp_processor_id());
>> +		trace_clock_disable(clk->name, 0, get_cpu());
>> +		put_cpu();
> 
> Why are you doing this? Why not just use raw_smp_processor_id()? Do you
> really care about the CPU number? get_cpu() and put_cpu() are about
> non-preemptible sections where you want to ensure the CPU the code is
> operating on is actually on that CPU.

You are right. raw_smp_processor_id() makes perfect sense here.

> 
> How about just put 0 all the time because the CPU number is already part
> of the trace event?
> 

I'm not sure about this. I can see that removing the cpu_id field from
trace_cpu_idle() was discussed here [1] and it was decided to keep it.
There was no conclusion about removing it from
trace_clock_enable/disable though.

For now, I will resend the patch with raw_smp_processor_id().

regards,
-roger

[1] https://lkml.org/lkml/2011/8/19/316