linux-kernel - Re: [REPORT] Softlockups on PowerNV with upstream

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Message-ID: <675d6580-814f-4fae-9dc5-9470645adc07@redhat.com>
Date: Thu, 10 Apr 2025 19:44:54 +1000
From: Gavin Shan <gshan@...hat.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: Aditya Gupta <adityag@...ux.ibm.com>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>, Danilo Krummrich
 <dakr@...nel.org>, David Hildenbrand <david@...hat.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Sourabh Jain <sourabhjain@...ux.ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [REPORT] Softlockups on PowerNV with upstream

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