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

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Date: Tue, 26 Mar 2024 17:32:35 +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 17:04, 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.

> 
>>
>> I think you could only do the optimization you describe if you required that
>> pte_same_norecency() would only be given values returned by
>> ptep_get_lockless_norecency() (or ptep_get_norecency()). Even then, its not
>> quite the same; if a page is accessed between gets one will return true and the
>> other false.
> 
> Right.
>