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Message-ID: <a210104f-a3af-4554-b734-097cfa77a470@samsung.com>
Date: Thu, 14 Mar 2024 13:28:40 +0100
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Subject: Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase
 supported CPUs to 512
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To: Catalin Marinas <catalin.marinas@arm.com>, "Russell King (Oracle)"
	<linux@armlinux.org.uk>
Cc: Sudeep Holla <sudeep.holla@arm.com>, "Christoph Lameter (Ampere)"
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	<rafael@kernel.org>, Viresh Kumar <vireshk@kernel.org>, Will Deacon
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	vanshikonda@os.amperecomputing.com, yang@os.amperecomputing.com, Nishanth
	Menon <nm@ti.com>, Stephen Boyd <sboyd@kernel.org>
From: Marek Szyprowski <m.szyprowski@samsung.com>
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Dear All,

On 14.03.2024 09:39, Catalin Marinas wrote:
> On Wed, Mar 13, 2024 at 05:13:33PM +0000, Russell King wrote:
>> So, I wonder whether what you're seeing is a latent bug which is
>> being tickled by the presence of the CPU masks being off-stack
>> changing the kernel timing.
>>
>> I would suggest the printk debug approach may help here to see when
>> the OPPs are begun to be parsed, when they're created etc and their
>> timing relationship to being used. Given the suspicion, it's possible
>> that the mere addition of printk() may "fix" the problem, which again
>> would be another semi-useful data point.
> It might be an init order problem. Passing "initcall_debug" on the
> cmdline might help a bit.
>
> It would also be useful in dev_pm_opp_set_config(), in the WARN_ON
> block, to print opp_table->opp_list.next to get an idea whether it looks
> like a valid pointer or memory corruption.

I've finally found some time to do the step-by-step printk-based 
debugging of this issue and finally found what's broken!

Here is the fix:

diff --git a/drivers/cpufreq/cpufreq-dt.c b/drivers/cpufreq/cpufreq-dt.c
index 8bd6e5e8f121..2d83bbc65dd0 100644
--- a/drivers/cpufreq/cpufreq-dt.c
+++ b/drivers/cpufreq/cpufreq-dt.c
@@ -208,7 +208,7 @@ static int dt_cpufreq_early_init(struct device *dev, 
int cpu)
         if (!priv)
                 return -ENOMEM;

-       if (!alloc_cpumask_var(&priv->cpus, GFP_KERNEL))
+       if (!zalloc_cpumask_var(&priv->cpus, GFP_KERNEL))
                 return -ENOMEM;

         cpumask_set_cpu(cpu, priv->cpus);


It is really surprising that this didn't blow up for anyone else so 
far... This means that the $subject patch is fine.

I will send a proper patch fixing this issue in a few minutes.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland