Re: [PATCH 1/1] sparc64: unify thread stack sizing and add explicit 32KB stack
From: Tony Rodriguez
Date: Thu Jun 18 2026 - 03:32:49 EST
On 6/17/26 10:53 PM, Andreas Larsson wrote:
On 2026-06-16 21:58, David Laight wrote:
On Tue, 16 Jun 2026 16:18:33 +0200No, the next step up is 32 KiB as the stack allocation is sized by
Andreas Larsson <andreas.larsson@xxxxxxxxxxx> wrote:
On 2026-05-19 09:57, Tony Rodriguez wrote:That whole logic is impenetrable.
This patch restructures the thread‑stack sizing logic into a singleYes
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)
Non‑SPARC64 with PAGE_SHIFT == 13:No, not to my understanding, see comments below.
Retains the existing 16KB stack behavior
Fallback:
Retains the existing 8KB stack behavior
Signed-off-by: Tony Rodriguez <unixpro1970@xxxxxxxxx>As far as I can see, given that this header is included by
---
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
#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...this code dead, where the else branch code already was dead (but then
+ #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
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.
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?
THREAD_SIZE_ORDER.
Cheers,
Andreas
After additional testing and debugging on a SPARC64 S7-2 system running kernel v7.1-mainline, I've made several important observations regarding the USB core stack overflow issue.
1. The Stack Overflow is Real and Consistent
My initial patch (increasing kernel stack to 32KB) appears to work with v7.1-mainline as well. However, the underlying problem remains: the USB core's stack usage consistently exceeds the default 16KB limit during hub enumeration.
2. The "Static Analysis vs. Runtime Reality" Contradiction
When I compile the kernel with -fstack-usage to generate .su files, the static analysis shows small stack frames for all USB core functions.
For example:
hub_event: 2457 bytes (static)
hub_activate: 1892 bytes (static)
usb_control_msg: 1248 bytes (static)
However, my runtime stack tracing shows a dramatically different picture:
STACKTRACE: hub_event():entry: 31856 bytes used
STACKTRACE: hub_activate():entry: 31680 bytes used
STACKTRACE: usb_control_msg():entry: 30768 bytes used
Please see:
https://github.com/unixpro1970/Sparc64-Kernel-Debugging-Dumps/blob/main/usbcore-stacktrace.txt
Perhaps the issue is the accumulation of register window spills across multiple nested function calls?
3. The 32KB Limit is Also at Risk
I've observed that stack usage can approach the 32K limit as well.
4. Testing:
TO DO: I will try adding a stack flush at the entry and exit of hub_event() . Hopefully it will prevent the accumulation of register windows. The theory is that flushing register windows between work items may prevent the stack growth from carrying over from one event to the next.
If the flush helps, I may also look into "stack_trace_flush" David Miller's stack_trace_flush() implementation. Unsure if stack_trace_flush is supported with v7.1.
Regards,
Tony