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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4282b89aa7bsm59626985e9.5.2024.08.01.06.44.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Aug 2024 06:44:13 -0700 (PDT) Message-ID: <5c0979a2-9a56-4284-82d2-42da62bda4a5@redhat.com> Date: Thu, 1 Aug 2024 15:44:12 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [linus:master] [mm] c0bff412e6: stress-ng.clone.ops_per_sec -2.9% regression To: Mateusz Guzik Cc: "Yin, Fengwei" , kernel test robot , Peter Xu , oe-lkp@lists.linux.dev, lkp@intel.com, linux-kernel@vger.kernel.org, Andrew Morton , Huacai Chen , Jason Gunthorpe , Matthew Wilcox , Nathan Chancellor , Ryan Roberts , WANG Xuerui , linux-mm@kvack.org, ying.huang@intel.com, feng.tang@intel.com 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> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 01.08.24 15:37, Mateusz Guzik wrote: > On Thu, Aug 1, 2024 at 3:34 PM David Hildenbrand wrote: >> >> On 01.08.24 15:30, Mateusz Guzik wrote: >>> On Thu, Aug 01, 2024 at 08:49:27AM +0200, David Hildenbrand wrote: >>>> Yes indeed. fork() can be extremely sensitive to each added instruction. >>>> >>>> I even pointed out to Peter why I didn't add the PageHuge check in there >>>> originally [1]. >>>> >>>> "Well, and I didn't want to have runtime-hugetlb checks in >>>> PageAnonExclusive code called on certainly-not-hugetlb code paths." >>>> >>>> >>>> We now have to do a page_folio(page) and then test for hugetlb. >>>> >>>> return folio_test_hugetlb(page_folio(page)); >>>> >>>> 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: >> >> 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. -- Cheers, David / dhildenb