linux-kernel - Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get

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Date: Tue, 26 Mar 2024 17:51:21 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>, Mark Rutland
 <mark.rutland@....com>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64

On 26/03/2024 17:39, David Hildenbrand wrote:
> On 26.03.24 18:32, Ryan Roberts wrote:
>> On 26/03/2024 17:04, David Hildenbrand wrote:
>>>>>>>
>>>>>>> Likely, we just want to read "the real deal" on both sides of the pte_same()
>>>>>>> handling.
>>>>>>
>>>>>> Sorry I'm not sure I understand? You mean read the full pte including
>>>>>> access/dirty? That's the same as dropping the patch, right? Of course if
>>>>>> we do
>>>>>> that, we still have to keep pte_get_lockless() around for this case. In an
>>>>>> ideal
>>>>>> world we would convert everything over to ptep_get_lockless_norecency() and
>>>>>> delete ptep_get_lockless() to remove the ugliness from arm64.
>>>>>
>>>>> Yes, agreed. Patch #3 does not look too crazy and it wouldn't really affect
>>>>> any
>>>>> architecture.
>>>>>
>>>>> I do wonder if pte_same_norecency() should be defined per architecture and the
>>>>> default would be pte_same(). So we could avoid the mkold etc on all other
>>>>> architectures.
>>>>
>>>> Wouldn't that break it's semantics? The "norecency" of
>>>> ptep_get_lockless_norecency() means "recency information in the returned pte
>>>> may
>>>> be incorrect". But the "norecency" of pte_same_norecency() means "ignore the
>>>> access and dirty bits when you do the comparison".
>>>
>>> My idea was that ptep_get_lockless_norecency() would return the actual result on
>>> these architectures. So e.g., on x86, there would be no actual change in
>>> generated code.
>>
>> I think this is a bad plan... You'll end up with subtle differences between
>> architectures.
>>
>>>
>>> But yes, the documentation of these functions would have to be improved.
>>>
>>> Now I wonder if ptep_get_lockless_norecency() should actively clear
>>> dirty/accessed bits to more easily find any actual issues where the bits still
>>> matter ...
>>
>> I did a version that took that approach. Decided it was not as good as this way
>> though. Now for the life of me, I can't remember my reasoning.
> 
> Maybe because there are some code paths that check accessed/dirty without
> "correctness" implications? For example, if the PTE is already dirty, no need to
> set it dirty etc?

I think I decided I was penalizing the architectures that don't care because all
their ptep_get_norecency() and ptep_get_lockless_norecency() need to explicitly
clear access/dirty. And I would have needed ptep_get_norecency() from day 1 so
that I could feed its result into pte_same().