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

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Message-ID: <e0c0dd89-ca82-4423-aae5-eb8ec60b31da@bytedance.com>
Date: Tue, 28 Jan 2025 23:59:07 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>
Cc: kernel test robot <oliver.sang@...el.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 2025/1/28 21:42, David Hildenbrand wrote:
> 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 

In addition, commit 718b13861d22 ("x86: mm: free page table pages by RCU 
instead of semi RCU")
also made a change, that is, when freeing a single page table pages, use
RCU to free the page table page instead of sending the IPI.

But in theory this should not cause performance regression, and I have
tested the performance of munmap with bpftrace and found no regression.

> for the regression here, and something else is going wrong.

It looks like it, I will investigate it carefully.

Thanks!

>