Re: [PATCH] drivers/base/memory: Avoid overhead from for_each_present_section_nr()

[Oscar Salvador]

On Thu, Apr 10, 2025 at 10:51:10PM +1000, Gavin Shan wrote:
> for_each_present_section_nr() was introduced to add_boot_memory_block()
> by commit 61659efdb35c ("drivers/base/memory: improve add_boot_memory_block()").
> It causes unnecessary overhead when the present sections are really
> sparse. next_present_section_nr() called by the macro to find the next
> present section, which is far away from the spanning sections in the
> specified block. Too much time consumed by next_present_section_nr()
> in this case, which can lead to softlockup as observed by Aditya Gupta
> on IBM Power10 machine.
> 
>   watchdog: BUG: soft lockup - CPU#248 stuck for 22s! [swapper/248:1]
>   Modules linked in:
>   CPU: 248 UID: 0 PID: 1 Comm: swapper/248 Not tainted 6.15.0-rc1-next-20250408 #1 VOLUNTARY
>   Hardware name: 9105-22A POWER10 (raw) 0x800200 opal:v7.1-107-gfda75d121942 PowerNV
>   NIP:  c00000000209218c LR: c000000002092204 CTR: 0000000000000000
>   REGS: c00040000418fa30 TRAP: 0900   Not tainted  (6.15.0-rc1-next-20250408)
>   MSR:  9000000002009033 <SF,HV,VEC,EE,ME,IR,DR,RI,LE>  CR: 28000428  XER: 00000000
>   CFAR: 0000000000000000 IRQMASK: 0
>   GPR00: c000000002092204 c00040000418fcd0 c000000001b08100 0000000000000040
>   GPR04: 0000000000013e00 c000c03ffebabb00 0000000000c03fff c000400fff587f80
>   GPR08: 0000000000000000 00000000001196f7 0000000000000000 0000000028000428
>   GPR12: 0000000000000000 c000000002e80000 c00000000001007c 0000000000000000
>   GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>   GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>   GPR24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>   GPR28: c000000002df7f70 0000000000013dc0 c0000000011dd898 0000000008000000
>   NIP [c00000000209218c] memory_dev_init+0x114/0x1e0
>   LR [c000000002092204] memory_dev_init+0x18c/0x1e0
>   Call Trace:
>   [c00040000418fcd0] [c000000002092204] memory_dev_init+0x18c/0x1e0 (unreliable)
>   [c00040000418fd50] [c000000002091348] driver_init+0x78/0xa4
>   [c00040000418fd70] [c0000000020063ac] kernel_init_freeable+0x22c/0x370
>   [c00040000418fde0] [c0000000000100a8] kernel_init+0x34/0x25c
>   [c00040000418fe50] [c00000000000cd94] ret_from_kernel_user_thread+0x14/0x1c
> 
> Avoid the overhead by folding for_each_present_section_nr() to the outer
> loop. add_boot_memory_block() is dropped after that.
> 
> Fixes: 61659efdb35c ("drivers/base/memory: improve add_boot_memory_block()")
> Closes: https://lore.kernel.org/linux-mm/20250409180344.477916-1-adityag@xxxxxxxxxxxxx
> Reported-by: Aditya Gupta <adityag@xxxxxxxxxxxxx>
> Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>

Acked-by: Oscar Salvador <osalvador@xxxxxxx>


-- 
Oscar Salvador
SUSE Labs