Subject: Re: [PATCH] cpufreq:powernv: Fix init_chip_info initialization in numa=off

From: Gautham R Shenoy <[email-masked]>
Date: Thu, 8 Jul 2021 14:38:08 +0530

Hello Pratik,

On Tue, Jun 15, 2021 at 10:39:49AM +0530, Pratik R. Sampat wrote:
> In the numa=off kernel command-line configuration init_chip_info() loops
> around the number of chips and attempts to copy the cpumask of that node
> which is NULL for all iterations after the first chip.

Thanks for taking a look into this. Indeed there is an issue here
because the code here assumes that node_mask as a proxy for the
chip_mask. This assumption breaks when run with numa=off, since there will only be a
single node, but multiple chips.

> 
> Hence adding a check to bail out after the first initialization if there
> is only one node.
> 
> Fixes: 053819e0bf84 ("cpufreq: powernv: Handle throttling due to Pmax capping at chip level")
> Signed-off-by: Pratik R. Sampat <[email-masked]>
> Reported-by: Shirisha Ganta <[email-masked]>
> ---
>  drivers/cpufreq/powernv-cpufreq.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
> index e439b43c19eb..663f9c4b5e3a 100644
> --- a/drivers/cpufreq/powernv-cpufreq.c
> +++ b/drivers/cpufreq/powernv-cpufreq.c
> @@ -1078,6 +1078,8 @@ static int init_chip_info(void)
>  		INIT_WORK(&chips[i].throttle, powernv_cpufreq_work_fn);
>  		for_each_cpu(cpu, &chips[i].mask)
>  			per_cpu(chip_info, cpu) =  &chips[i];
> +		if (num_possible_nodes() == 1)
> +			break;

With this we will only initialize the chip[0].throttle work function,
while for the rest of the chips chip[i].throttle will be
uninitialized. While we may be running in the numa=off mode, the fact
remains that those other chips do exist and they may experiencing
throttling, during which they will try to schedule work for chip[i] in
order to take corrective action, which will fail.

Hence a more correct approach may be to maintain a chip[i] mask
independent of the node mask.

>  	}
>  free_and_return:
> -- 
> 2.30.2