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

[Michael Ellerman <mpe@ellerman.id.au>]

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 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.

> 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.

cheers


> 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