Re: [PATCH v4 2/2] arm64: Expand the stack trace feature to support IRQ stack

[Jungseok Lee]

On Oct 13, 2015, at 8:00 PM, James Morse wrote:
> Hi Jungseok,

Hi James,

> On 12/10/15 23:13, Jungseok Lee wrote:
>> On Oct 13, 2015, at 1:34 AM, James Morse wrote:
>>> Having two kmem_caches for 16K stacks on a 64K page system may be wasteful
>>> (especially for systems with few cpus)…
>> 
>> This would be a single concern. To address this issue, I drop the 'static'
>> keyword in thread_info_cache. Please refer to the below hunk.
> 
> Its only a problem on systems with 64K pages, which don't have a multiple
> of 4 cpus. I suspect if you turn on 64K pages, you have many cores with
> plenty of memory…

Yes, the problem 'two kmem_caches' comes from only 64K page system.

I don't get the statement 'which don't have a multiple of 4 cpus'.
Could you point out what I am missing?

Since I don't have platforms which have many cores and huge memory,
I cannot play with this series on them.

>>> The alternative is to defining CONFIG_ARCH_THREAD_INFO_ALLOCATOR and
>>> allocate all stack memory from arch code. (Largely copied code, prevents
>>> irq stacks being a different size, and nothing uses that define today!)
>>> 
>>> 
>>> Thoughts?
>> 
>> Almost same story I've been testing.
>> 
>> I'm aligned with yours Regarding CONFIG_ARCH_THREAD_INFO_ALLOCATOR.
>> 
>> Another approach I've tried is the following data structure, but it's not
>> a good fit for this case due to __per_cpu_offset which is page-size aligned,
>> not thread-size.
>> 
>> struct irq_stack {
>> 	char stack[THREAD_SIZE];
>> 	char *highest;
>> } __aligned(THREAD_SIZE);
>> 
>> DEFINE_PER_CPU(struct irq_stack, irq_stacks);
> 
> Yes, x86 does this - but it increases the Image size by 16K, as that space
> could have some initialisation values. This isn't a problem on x86 as
> no-one uses the uncompressed image.
> 
> I would avoid this approach due to the bloat!
> 
>> 
>> ----8<-----
>> diff --git a/arch/arm64/include/asm/irq.h b/arch/arm64/include/asm/irq.h
>> index 6ea82e8..d3619b3 100644
>> --- a/arch/arm64/include/asm/irq.h
>> +++ b/arch/arm64/include/asm/irq.h
>> @@ -1,7 +1,9 @@
>> #ifndef __ASM_IRQ_H
>> #define __ASM_IRQ_H
>> 
>> +#include <linux/gfp.h>
>> #include <linux/irqchip/arm-gic-acpi.h>
>> +#include <linux/slab.h>
>> 
>> #include <asm-generic/irq.h>
>> 
>> @@ -9,6 +11,21 @@ struct irq_stack {
>>        void *stack;
>> };
>> 
>> +#if THREAD_SIZE >= PAGE_SIZE
>> +static inline void *__alloc_irq_stack(void)
>> +{
>> +       return (void *)__get_free_pages(THREADINFO_GFP | __GFP_ZERO,
>> +                                       THREAD_SIZE_ORDER);
>> +}
>> +#else
>> +extern struct kmem_cache *thread_info_cache;
> 
> If this has been made a published symbol, it should go in a header file.

Sure.

>> +
>> +static inline void *__alloc_irq_stack(void)
>> +{
>> +       return kmem_cache_alloc(thread_info_cache, THREADINFO_GFP | __GFP_ZERO);
>> +}
>> +#endif
>> +
>> struct pt_regs;
>> 
>> extern void migrate_irqs(void);
>> diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
>> index a6bdf4d..4e13bdd 100644
>> --- a/arch/arm64/kernel/irq.c
>> +++ b/arch/arm64/kernel/irq.c
>> @@ -50,10 +50,13 @@ void __init set_handle_irq(void (*handle_irq)(struct pt_regs *))
>>        handle_arch_irq = handle_irq;
>> }
>> 
>> +static char boot_irq_stack[THREAD_SIZE] __aligned(THREAD_SIZE);
>> +
>> void __init init_IRQ(void)
>> {
>> -       if (alloc_irq_stack(smp_processor_id()))
>> -               panic("Failed to allocate IRQ stack for a boot cpu");
>> +       unsigned int cpu = smp_processor_id();
>> +
>> +       per_cpu(irq_stacks, cpu).stack = boot_irq_stack + THREAD_START_SP;
>> 
>>        irqchip_init();
>>        if (!handle_arch_irq)
>> @@ -128,7 +131,7 @@ int alloc_irq_stack(unsigned int cpu)
>>        if (per_cpu(irq_stacks, cpu).stack)
>>                return 0;
>> 
>> -       stack = (void *)__get_free_pages(THREADINFO_GFP, THREAD_SIZE_ORDER);
>> +       stack = __alloc_irq_stack();
>>        if (!stack)
>>                return -ENOMEM;
>> 
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 2845623..9c55f86 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -172,7 +172,7 @@ static inline void free_thread_info(struct thread_info *ti)
>>        free_kmem_pages((unsigned long)ti, THREAD_SIZE_ORDER);
>> }
>> # else
>> -static struct kmem_cache *thread_info_cache;
>> +struct kmem_cache *thread_info_cache;
>> 
>> static struct thread_info *alloc_thread_info_node(struct task_struct *tsk,
>>                                                  int node)
>> ----8<-----
> 
> 
> Looks good!

Thanks for reviewing the code!

Best Regards
Jungseok Lee--
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