linux-kernel - Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per

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Message-ID: <18172465-37e3-48a9-9d67-0d44fdcb93bd@redhat.com>
Date: Tue, 28 Jan 2025 14:42:52 +0100
From: David Hildenbrand <david@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: kernel test robot <oliver.sang@...el.com>,
 Qi Zheng <zhengqi.arch@...edance.com>, oe-lkp@...ts.linux.dev,
 lkp@...el.com, linux-kernel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 David Rientjes <rientjes@...gle.com>, Hugh Dickins <hughd@...gle.com>,
 Jann Horn <jannh@...gle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, Mel Gorman <mgorman@...e.de>,
 Muchun Song <muchun.song@...ux.dev>, Peter Xu <peterx@...hat.com>,
 Will Deacon <will@...nel.org>, Zach O'Keefe <zokeefe@...gle.com>,
 Dan Carpenter <dan.carpenter@...aro.org>, Rik van Riel <riel@...riel.com>
Subject: Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per_sec
 63.0% regression

On 28.01.25 14:28, Peter Zijlstra wrote:
> On Tue, Jan 28, 2025 at 12:39:51PM +0100, David Hildenbrand wrote:
>> On 28.01.25 12:31, Peter Zijlstra wrote:
> 
>>>> I recall a recent series to select MMU_GATHER_RCU_TABLE_FREE on x86
>>>> unconditionally (@Peter, @Rik).
>>>
>>> Those changes should not have made it to Linus yet.
>>>
>>> /me updates git and checks...
>>>
>>> nope, nothing changed there ... yet
>>
>> Sorry, I wasn't quite clear. CONFIG_PT_RECLAIM made it upstream, which has
>> "select MMU_GATHER_RCU_TABLE_FREE" in kconfig.
>>
>> So I'm wondering if the degradation we see in this report is due to
>> MMU_GATHER_RCU_TABLE_FREE being selected by CONFIG_PT_RECLAIM, and we'd get
>> the same result (degradation) when unconditionally enabling
>> MMU_GATHER_RCU_TABLE_FREE.
> 
> Ah, yes, put a RHEL based config (as is the case here) should already
> have it selected due to PARAVIRT.

Ah, right. Most distros will just have it enabled either way.

But that would then mean that MMU_GATHER_RCU_TABLE_FREE is not the cause 
for the regression here, and something else is going wrong.

-- 
Cheers,

David / dhildenb