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   Arnd Bergmann (arnd@society.oftrolls.com)'s status on Saturday, 04-Jan-2025 19:40:53 JST Arnd Bergmann

   I had a feeling that kernel compilation got slower recently and tried to find the slowest file across randconfig builds. It turned out to be arch/x86/xen/setup.c, which takes 15 seconds to preprocess on a reasonably fast Apple M1 Ultra.

   This all comes from one line "extra_pages = min3(EXTRA_MEM_RATIO * min(max_pfn, PFN_DOWN(MAXMEM)), extra_pages, max_pages - max_pfn);" that expands to 47MB of preprocessor output after commits 80fcac55385c..867046cc70277.

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     Lorenzo Stoakes (ljs@social.kernel.org)'s status on Saturday, 04-Jan-2025 19:40:46 JST Lorenzo Stoakes

     in reply to

     • Lorenzo Stoakes
     • Omar Sandoval
     @osandov @arnd For those following, posted https://lore.kernel.org/linux-mm/c83c17bb-be75-4c67-979d-54eee38774c6@lucifer.local/T/#u summarising situation + showing some results on my local box (ignore cringe hostname of said box)
     Haelwenn /элвэн/ :triskell: likes this.
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     Lorenzo Stoakes (ljs@social.kernel.org)'s status on Saturday, 04-Jan-2025 19:40:47 JST Lorenzo Stoakes

     in reply to

     • Omar Sandoval
     @osandov @arnd Regardless, this is an unacceptable build regression.
     
     Chatted to @arnd on IRC who suggests he is tied up with stuff to dig into this more so I'll have a wee look :)
     
     EDIT: Think that'll be more likely a report to list + leave others to solve as this seems quite involved and I have a ton of things to do :>)
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     Omar Sandoval (osandov@fosstodon.org)'s status on Saturday, 04-Jan-2025 19:40:49 JST Omar Sandoval

     in reply to

     • Lorenzo Stoakes

     @ljs @arnd eh I don't think it's fair to call this pedantry. This looks like a legitimate quality of life improvement for a common annoyance with min/max, but given the consequences, it's fair to say that it's not worth it.

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     Lorenzo Stoakes (ljs@social.kernel.org)'s status on Saturday, 04-Jan-2025 19:40:50 JST Lorenzo Stoakes

     in reply to
     @arnd we going to revert that series then?
     
     I feel there is pedantry at play...
   • Embed this notice
     Mans R (mansr@society.oftrolls.com)'s status on Saturday, 04-Jan-2025 19:46:40 JST Mans R

     in reply to

     • KeyJ
     • Dirk Steins

     @dirksteins @arnd @KeyJ There's no reason to doubt the correctness, but as has been noted, it is somewhat slow.

     Now as shocking as this might be, it is quite tame compared to the boost C++ library.

     Haelwenn /элвэн/ :triskell: likes this.
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     Dirk Steins (dirksteins@social.cologne)'s status on Saturday, 04-Jan-2025 19:46:41 JST Dirk Steins

     in reply to

     • Mans R
     • KeyJ

     @mansr @arnd @KeyJ probably. But I still ask myself if a 47MB expression is useful and if the compiler is able to process this correctly (it probably is, but is it efficient?).

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     Mans R (mansr@society.oftrolls.com)'s status on Saturday, 04-Jan-2025 19:46:43 JST Mans R

     in reply to

     • KeyJ
     • Dirk Steins

     @dirksteins @arnd @KeyJ It's a huge constant expression that gets evaluated by the compiler. The final code is (probably) fine.

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     Dirk Steins (dirksteins@social.cologne)'s status on Saturday, 04-Jan-2025 19:46:44 JST Dirk Steins

     in reply to

     • KeyJ

     @arnd @KeyJ and this is effective and efficient code? Doesn’t look like it to me - but I’m not a kernel developer.

   • Embed this notice
     Arnd Bergmann (arnd@society.oftrolls.com)'s status on Saturday, 04-Jan-2025 19:46:47 JST Arnd Bergmann

     in reply to

     • KeyJ

     @KeyJ it nests min() multiple levels deep with the use of min3(), and each one expands its argument 20 times times now (up from 6 back in linux-6.6). This gets 8000 expansions for each of the arguments, plus a lot of extra bits with each expansion. PFN_DOWN(MAXMEM) contributes a bit to the initial size as well.

     See https://pastebin.com/MmfWH7TM for the first few pages of it.

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     KeyJ (keyj@mastodon.gamedev.place)'s status on Saturday, 04-Jan-2025 19:46:48 JST KeyJ

     in reply to

     @arnd But ... how? Even if it expanded to 47 kilobytes, that would be excessive, but 47 *megabytes*? 🤯