Re: [PATCH v2] cpufreq: powernv: fix stack bloat and NR_CPUS limitation

[John Hubbard <jhubbard@nvidia.com>]

On 10/30/19 7:39 PM, Michael Ellerman wrote:
> Hi John,
> 
> Sorry I didn't reply to this sooner, too many patches :/
> 
> John Hubbard <jhubbard@nvidia.com> writes:
>> The following build warning occurred on powerpc 64-bit builds:
>>
>> drivers/cpufreq/powernv-cpufreq.c: In function 'init_chip_info':
>> drivers/cpufreq/powernv-cpufreq.c:1070:1: warning: the frame size of 1040 bytes is larger than 1024 bytes [-Wframe-larger-than=]
> 
> Oddly I don't see that warning in my builds, eg with GCC9:
> 
>    https://travis-ci.org/linuxppc/linux/jobs/604870722

This is with a cross-compiler based on gcc 8.1.0, which I got from:
   https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/8.1.0/

I'll put that in the v3 commit description.

> 
>> This is due to putting 1024 bytes on the stack:
>>
>>      unsigned int chip[256];
>>
>> ...and while looking at this, it also has a bug: it fails with a stack
>> overrun, if CONFIG_NR_CPUS > 256.
> 
> It _probably_ doesn't, because it only increments the index when the
> chip_id of the CPU changes, ie. it doesn't create a chip for every CPU.
> But I agree it's flaky the way it's written.

I'll soften up the wording accordingly.

> 
>> Fix both problems by dynamically allocating based on CONFIG_NR_CPUS.
> 
> Shouldn't it use num_possible_cpus() ?
> 
> Given the for loop is over possible CPUs that seems like the upper
> bound. In practice it should be lower because some CPUs will share a
> chip.
> 

OK, I see, that's more consistent with the code, I'll change to that.


thanks,
-- 
John Hubbard
NVIDIA

> 
> 
>> Fixes: 053819e0bf840 ("cpufreq: powernv: Handle throttling due to Pmax capping at chip level")
>> Cc: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
>> Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
>> Cc: Viresh Kumar <viresh.kumar@linaro.org>
>> Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
>> Cc: linux-pm@vger.kernel.org
>> Cc: linuxppc-dev@lists.ozlabs.org
>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>> ---
>>
>> Changes since v1: includes Viresh's review commit fixes.
>>
>>   drivers/cpufreq/powernv-cpufreq.c | 17 +++++++++++++----
>>   1 file changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
>> index 6061850e59c9..5b2e968cb5ea 100644
>> --- a/drivers/cpufreq/powernv-cpufreq.c
>> +++ b/drivers/cpufreq/powernv-cpufreq.c
>> @@ -1041,9 +1041,14 @@ static struct cpufreq_driver powernv_cpufreq_driver = {
>>   
>>   static int init_chip_info(void)
>>   {
>> -	unsigned int chip[256];
>> +	unsigned int *chip;
>>   	unsigned int cpu, i;
>>   	unsigned int prev_chip_id = UINT_MAX;
>> +	int ret = 0;
>> +
>> +	chip = kcalloc(CONFIG_NR_CPUS, sizeof(*chip), GFP_KERNEL);
>> +	if (!chip)
>> +		return -ENOMEM;
>>   
>>   	for_each_possible_cpu(cpu) {
>>   		unsigned int id = cpu_to_chip_id(cpu);
>> @@ -1055,8 +1060,10 @@ static int init_chip_info(void)
>>   	}
>>   
>>   	chips = kcalloc(nr_chips, sizeof(struct chip), GFP_KERNEL);
>> -	if (!chips)
>> -		return -ENOMEM;
>> +	if (!chips) {
>> +		ret = -ENOMEM;
>> +		goto free_and_return;
>> +	}
>>   
>>   	for (i = 0; i < nr_chips; i++) {
>>   		chips[i].id = chip[i];
>> @@ -1066,7 +1073,9 @@ static int init_chip_info(void)
>>   			per_cpu(chip_info, cpu) =  &chips[i];
>>   	}
>>   
>> -	return 0;
>> +free_and_return:
>> +	kfree(chip);
>> +	return ret;
>>   }
>>   
>>   static inline void clean_chip_info(void)
>> -- 
>> 2.23.0