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Mon, 12 Aug 2024 04:43:19 +0000 Received: from CO1PR11MB4820.namprd11.prod.outlook.com ([fe80::47aa:294c:21c9:a6b8]) by CO1PR11MB4820.namprd11.prod.outlook.com ([fe80::47aa:294c:21c9:a6b8%4]) with mapi id 15.20.7828.029; Mon, 12 Aug 2024 04:43:19 +0000 Message-ID: Date: Mon, 12 Aug 2024 12:43:08 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [linus:master] [mm] c0bff412e6: stress-ng.clone.ops_per_sec -2.9% regression To: David Hildenbrand , Mateusz Guzik CC: kernel test robot , Peter Xu , , , , Andrew Morton , Huacai Chen , Jason Gunthorpe , Matthew Wilcox , Nathan Chancellor , Ryan Roberts , WANG Xuerui , , , References: <202407301049.5051dc19-oliver.sang@intel.com> <193e302c-4401-4756-a552-9f1e07ecedcf@redhat.com> <439265d8-e71e-41db-8a46-55366fdd334e@intel.com> <90477952-fde2-41d7-8ff4-2102c45e341d@redhat.com> <6uxnuf2gysgabyai2r77xrqegb7t7cc2dlzjz6upwsgwrnfk3x@cjj6on3wqm4x> <5a67c103-1d9d-440d-8bed-bbfa7d3ecf71@redhat.com> <5c0979a2-9a56-4284-82d2-42da62bda4a5@redhat.com> Content-Language: en-US From: Yin Fengwei In-Reply-To: <5c0979a2-9a56-4284-82d2-42da62bda4a5@redhat.com> Content-Type: text/plain; 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>>>>> >>>>> Nowadays, folio_test_hugetlb() will be faster than at c0bff412e6 >>>>> times, so >>>>> maybe at least part of the overhead is gone. >>>>> >>>> >>>> I'll note page_folio expands to a call to _compound_head. >>>> >>>> While _compound_head is declared as an inline, it ends up being big >>>> enough that the compiler decides to emit a real function instead and >>>> real func calls are not particularly cheap. >>>> >>>> I had a brief look with a profiler myself and for single-threaded usage >>>> the func is quite high up there, while it manages to get out with the >>>> first branch -- that is to say there is definitely performance lost for >>>> having a func call instead of an inlined branch. >>>> >>>> The routine is deinlined because of a call to page_fixed_fake_head, >>>> which itself is annotated with always_inline. >>>> >>>> This is of course patchable with minor shoveling. >>>> >>>> I did not go for it because stress-ng results were too unstable for me >>>> to confidently state win/loss. >>>> >>>> But should you want to whack the regression, this is what I would look >>>> into. >>>> >>> >>> This might improve it, at least for small folios I guess: Do you want us to test this change? Or you have further optimization ongoing? Thanks. Regards Yin, Fengwei >>> >>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h >>> index 5769fe6e4950..7796ae116018 100644 >>> --- a/include/linux/page-flags.h >>> +++ b/include/linux/page-flags.h >>> @@ -1086,7 +1086,7 @@ PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc) >>>     */ >>>    static inline bool PageHuge(const struct page *page) >>>    { >>> -       return folio_test_hugetlb(page_folio(page)); >>> +       return PageCompound(page) && >>> folio_test_hugetlb(page_folio(page)); >>>    } >>> >>>    /* >>> >>> >>> We would avoid the function call for small folios. >>> >> >> why not massage _compound_head back to an inlineable form instead? for >> all i know you may even register a small win in total > > Agreed, likely it will increase code size a bit which is why the > compiler decides to not inline. We could force it with __always_inline. > > Finding ways to shrink page_fixed_fake_head() might be even better. >