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Date:   Tue, 6 Jun 2023 11:11:09 +0800
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Subject: Re: [PATCH 00/14] Reduce preallocations for maple tree
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To:     Hugh Dickins <hughd@google.com>
CC:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Peng Zhang <zhangpeng.00@bytedance.com>,
        <maple-tree@lists.infradead.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, "Liu, Yujie" <yujie.liu@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
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From:   "Yin, Fengwei" <fengwei.yin@intel.com>
In-Reply-To: <ff754a68-4f94-e818-a31f-c8a1fd11b4b5@google.com>
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On 6/6/2023 11:08 AM, Hugh Dickins wrote:
> On Tue, 6 Jun 2023, Yin, Fengwei wrote:
>> On 6/6/2023 10:41 AM, Hugh Dickins wrote:
>>> On Mon, 5 Jun 2023, Liam R. Howlett wrote:
>>>>
>>>> You mean "mm: update validate_mm() to use vma iterator" here I guess.  I
>>>> have it as a different commit id in my branch.
>>>>
>>>> I 'restored' some of the checking because I was able to work around not
>>>> having the mt_dump() definition with the vma iterator.  I'm now
>>>> wondering how wide spread CONFIG_DEBUG_VM is used and if I should not
>>>> have added these extra checks.
>>>
>>> Most CONFIG_DEBUG_VM checks are quite cheap, mostly VM_BUG_ONs for
>> Indeed. I had CONFIG_DEBUG_VM enabled and didn't see surprise perf report.
>>
>>
>>> easily checked conditions.  If validate_mm() is still the kind of thing
>>> it used to be, checking through every vma on every mmap operation, please
>>> don't bring that into CONFIG_DEBUG_VM - it distorts performance too much,
>>> so always used to be under a separate CONFIG_DEBUG_VM_RB instead.
>> So does this mean CONFIG_DEBUG_VM is allowed to be enabled for performance
>> testing? Thanks.
> 
> I was going to say:
> No, I did not mean that: I just meant that even developers not doing
> strict performance testing still like to keep a rough eye on performance
> changes; and historically CONFIG_DEBUG_VM has not distorted very much.
> 
> But then I wonder about certain distros which (wrongly or rightly) turn
> CONFIG_DEBUG_VM on: I expect they do performance testing on their kernels.
Fair enough. Thanks for explanation.

Regards
Yin, Fengwei

> 
> Hugh