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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429c74ffb1esm92390395e9.5.2024.08.12.01.12.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 12 Aug 2024 01:12:50 -0700 (PDT) Message-ID: <66c4fcc5-47f6-438c-a73a-3af6e19c3200@redhat.com> Date: Mon, 12 Aug 2024 10:12:49 +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 , Yin Fengwei Cc: 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> <5c0979a2-9a56-4284-82d2-42da62bda4a5@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 12.08.24 06:49, Mateusz Guzik wrote: > On Mon, Aug 12, 2024 at 12:43:08PM +0800, Yin Fengwei wrote: >> Hi David, >> >> On 8/1/24 09:44, David Hildenbrand wrote: >>> 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: >> Do you want us to test this change? Or you have further optimization >> ongoing? Thanks. > > I verified the thing below boots, I have no idea about performance. If > it helps it can be massaged later from style perspective. As quite a lot of setups already run with the vmemmap optimization enabled, I wonder how effective this would be (might need more fine tuning, did not look at the generated code): diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 085dd8dcbea2..7ddcdbd712ec 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -233,7 +233,7 @@ static __always_inline int page_is_fake_head(const struct page *page) return page_fixed_fake_head(page) != page; } -static inline unsigned long _compound_head(const struct page *page) +static __always_inline unsigned long _compound_head(const struct page *page) { unsigned long head = READ_ONCE(page->compound_head); -- Cheers, David / dhildenb