Re: [PATCH 1/1] sparc64: unify thread stack sizing and add explicit 32KB stack

[Andreas Larsson]

On 2026-06-16 21:58, David Laight wrote:
> On Tue, 16 Jun 2026 16:18:33 +0200
> Andreas Larsson <andreas.larsson@xxxxxxxxxxx> wrote:
> 
>> On 2026-05-19 09:57, Tony Rodriguez wrote:
>>> This patch restructures the thread‑stack sizing logic into a single
>>> if / elif / else chain and introduces an explicit 32KB kernel stack
>>> for SPARC64. The previous implementation relied on nested conditionals
>>> and PAGE_SHIFT‑dependent behavior, which produced 8KB or 16KB stacks
>>> depending on configuration. SPARC64 requires a larger,
>>> architecture‑specific stack due to its trapframe size, register‑window
>>> behavior, and deeper call paths.
>>>
>>> A reproducible failure case occurs when usbcore is enabled: USB hub
>>> enumeration (usb_new_device(), hub_port_connect(), PM/QoS helpers)
>>> allocates large on‑stack structures and recurses through several
>>> layers of device‑model code. Combined with SPARC64’s trapframe and
>>> register‑window overhead, this reliably exhausts a 16KB stack and
>>> results in early‑boot panics.  A 32KB stack eliminates these failures.
>>>
>>> The new logic is:
>>>     SPARC64:
>>>         THREAD_SIZE = 4 * PAGE_SIZE (32KB)
>>>         THREAD_SHIFT = PAGE_SHIFT + 2 (log₂(32KB))
>>>         THREAD_SIZE_ORDER = 2 (4 contiguous pages)  
>>
>> Yes
>>
>>>     Non‑SPARC64 with PAGE_SHIFT == 13:
>>>         Retains the existing 16KB stack behavior
>>>     Fallback:
>>>         Retains the existing 8KB stack behavior  
>>
>> No, not to my understanding, see comments below.
>>
>>>
>>> Signed-off-by: Tony Rodriguez <unixpro1970@xxxxxxxxx>
>>> ---
>>>  arch/sparc/include/asm/thread_info_64.h | 28 ++++++++++++-------------
>>>  1 file changed, 14 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/arch/sparc/include/asm/thread_info_64.h b/arch/sparc/include/asm/thread_info_64.h
>>> index c8a73dff27f8..6b12a2b66385 100644
>>> --- a/arch/sparc/include/asm/thread_info_64.h
>>> +++ b/arch/sparc/include/asm/thread_info_64.h
>>> @@ -99,13 +99,20 @@ struct thread_info {
>>>  #define FAULT_CODE_BLKCOMMIT	0x10	/* Use blk-commit ASI in copy_page */
>>>  #define	FAULT_CODE_BAD_RA	0x20	/* Bad RA for sun4v		   */
>>>
>>> -#if PAGE_SHIFT == 13
>>> -#define THREAD_SIZE (2*PAGE_SIZE)
>>> -#define THREAD_SHIFT (PAGE_SHIFT + 1)
>>> -#else /* PAGE_SHIFT == 13 */
>>> -#define THREAD_SIZE PAGE_SIZE
>>> -#define THREAD_SHIFT PAGE_SHIFT
>>> -#endif /* PAGE_SHIFT == 13 */
>>> +/* thread information allocation */
>>> +#ifdef CONFIG_SPARC64
>>> +	#define THREAD_SIZE (4 * PAGE_SIZE)
>>> +	#define THREAD_SHIFT (PAGE_SHIFT + 2)
>>> +	#define THREAD_SIZE_ORDER 2  
>>
>> As far as I can see, given that this header is included by
>>
>> #if defined(__sparc__) && defined(__arch64__)
>> #include <asm/thread_info_64.h>
>> #else
>> #include <asm/thread_info_32.h>
>> #endif
>>
>> the code above is the only code that will ever be compiled, while leaving...
>>
>>> +#elif PAGE_SHIFT == 13
>>> +	#define THREAD_SIZE (2 * PAGE_SIZE)
>>> +	#define THREAD_SHIFT (PAGE_SHIFT + 1)
>>> +	#define THREAD_SIZE_ORDER 1
>>> +#else
>>> +	#define THREAD_SIZE PAGE_SIZE
>>> +	#define THREAD_SHIFT PAGE_SHIFT
>>> +	#define THREAD_SIZE_ORDER 0
>>> +#endif  
>>
>> ...this code dead, where the else branch code already was dead (but then
>> in two separate else braches).
>>
>> I'd rather see the else branch here and the else branch below cleaned up
>> by a separate patch with a fixup tag for commit 15b9350a177b ("sparc64:
>> Only support 4MB huge pages and 8KB base pages.") that as far as I can
>> see should have removed the else branch. The else branches was to use
>> only one page when the page size was _larger_ than 8 KiB when that was
>> an option.
> 
> That whole logic is impenetrable.
> Why not set the 'desired thread size' in kB, then work out how many
> pages that ends up being based on the page size, and finally get the actual
> stack size.
> I'm not sure, but with vmalloc()ed stacks and 8k pages can't you have 24kB?

No, the next step up is 32 KiB as the stack allocation is sized by
THREAD_SIZE_ORDER.

Cheers,
Andreas