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

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Message-ID: <Z_d_8fyQzGuwzbIv@localhost.localdomain>
Date: Thu, 10 Apr 2025 10:23:13 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Gavin Shan <gshan@...hat.com>
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 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:


-- 
Oscar Salvador
SUSE Labs