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

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Message-ID: <8bd9e136-8575-4c40-bae2-9b015d823916@redhat.com>
Date: Tue, 26 Mar 2024 18:39:23 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....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 26.03.24 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?

-- 
Cheers,

David / dhildenb