Re: [REPORT] Softlockups on PowerNV with upstream

[Gavin Shan <gshan@redhat.com>]

On 4/10/25 6:23 PM, Oscar Salvador wrote:
> On Thu, Apr 10, 2025 at 03:35:19PM +1000, Gavin Shan wrote:
>> Thanks, Oscar. You're correct that the overhead is introduced by for_each_present_section_nr().
>> I already had the fix, working on IBM's Power9 machine, where the issue can be
>> reproduced. Please see the attached patch.
>>
>> I'm having most tests on ARM64 machine for the fix.
> 
> Looks good to me.
> But we need a comment explaining why block_id is set to ULONG_MAX
> at the beginning as this might not be obvious.
> 
> Also, do we need
>   if (block_id != ULONG_MAX && memory_block_id(nr) == block_id) ?
> 
> Cannot just be
> 
>   if (memory_block_id(nr) == block_id) ?
> 
> AFAICS, the first time we loop through 'memory_block_id(nr) == ULONG_MAX'
> will evaluate false and and we will set block_id afterwards.
> 
> Either way looks fine to me.
> Another way I guess would be:
> 

Yeah, we need to record the last handled block ID by @block_id. For the
first time to register the block memory device in the loop, @block_id needs
to be invalid (ULONG_MAX), bypassing the check of 'memory_block_id(nr) == block_id'.
I will post the fix for review after Aditya confirms it works for him, with extra
comment to explain why @block_id is initialized to ULONG_MAX.

Aditya, please have a try when you get a chance, thanks! I verified it on Power9
machine where the issue exists and on one of my ARM64 machine.

Thanks,
Gavin