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[91.12.105.184]) by smtp.gmail.com with ESMTPSA id b6sm1276048edd.18.2021.04.19.02.40.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Apr 2021 02:40:58 -0700 (PDT) Subject: Re: [PATCH] secretmem: optimize page_is_secretmem() From: David Hildenbrand To: Mike Rapoport Cc: Andrew Morton , Alexander Viro , Andy Lutomirski , Arnd Bergmann , Borislav Petkov , Catalin Marinas , Christopher Lameter , Dan Williams , Dave Hansen , Elena Reshetova , "H. Peter Anvin" , Ingo Molnar , James Bottomley , "Kirill A. Shutemov" , Matthew Wilcox , Matthew Garrett , Mark Rutland , Michal Hocko , Mike Rapoport , Michael Kerrisk , Palmer Dabbelt , Paul Walmsley , Peter Zijlstra , "Rafael J. Wysocki" , Rick Edgecombe , Roman Gushchin , Shakeel Butt , Shuah Khan , Thomas Gleixner , Tycho Andersen , Will Deacon , Yury Norov , linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org, x86@kernel.org, kernel test robot References: <20210419084218.7466-1-rppt@kernel.org> <3b30ac54-8a92-5f54-28f0-f110a40700c7@redhat.com> Organization: Red Hat Message-ID: <56d8b80c-ce2c-ed86-0eda-253768d8d463@redhat.com> Date: Mon, 19 Apr 2021 11:40:56 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On 19.04.21 11:38, David Hildenbrand wrote: > On 19.04.21 11:36, Mike Rapoport wrote: >> On Mon, Apr 19, 2021 at 11:15:02AM +0200, David Hildenbrand wrote: >>> On 19.04.21 10:42, Mike Rapoport wrote: >>>> From: Mike Rapoport >>>> >>>> Kernel test robot reported -4.2% regression of will-it-scale.per_thread_ops >>>> due to commit "mm: introduce memfd_secret system call to create "secret" >>>> memory areas". >>>> >>>> The perf profile of the test indicated that the regression is caused by >>>> page_is_secretmem() called from gup_pte_range() (inlined by gup_pgd_range): >>>> >>>> 27.76 +2.5 30.23 perf-profile.children.cycles-pp.gup_pgd_range >>>> 0.00 +3.2 3.19 ± 2% perf-profile.children.cycles-pp.page_mapping >>>> 0.00 +3.7 3.66 ± 2% perf-profile.children.cycles-pp.page_is_secretmem >>>> >>>> Further analysis showed that the slow down happens because neither >>>> page_is_secretmem() nor page_mapping() are not inline and moreover, >>>> multiple page flags checks in page_mapping() involve calling >>>> compound_head() several times for the same page. >>>> >>>> Make page_is_secretmem() inline and replace page_mapping() with page flag >>>> checks that do not imply page-to-head conversion. >>>> >>>> Reported-by: kernel test robot >>>> Signed-off-by: Mike Rapoport >>>> --- >>>> >>>> @Andrew, >>>> The patch is vs v5.12-rc7-mmots-2021-04-15-16-28, I'd appreciate if it would >>>> be added as a fixup to the memfd_secret series. >>>> >>>> include/linux/secretmem.h | 26 +++++++++++++++++++++++++- >>>> mm/secretmem.c | 12 +----------- >>>> 2 files changed, 26 insertions(+), 12 deletions(-) >>>> >>>> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h >>>> index 907a6734059c..b842b38cbeb1 100644 >>>> --- a/include/linux/secretmem.h >>>> +++ b/include/linux/secretmem.h >>>> @@ -4,8 +4,32 @@ >>>> #ifdef CONFIG_SECRETMEM >>>> +extern const struct address_space_operations secretmem_aops; >>>> + >>>> +static inline bool page_is_secretmem(struct page *page) >>>> +{ >>>> + struct address_space *mapping; >>>> + >>>> + /* >>>> + * Using page_mapping() is quite slow because of the actual call >>>> + * instruction and repeated compound_head(page) inside the >>>> + * page_mapping() function. >>>> + * We know that secretmem pages are not compound and LRU so we can >>>> + * save a couple of cycles here. >>>> + */ >>>> + if (PageCompound(page) || !PageLRU(page)) >>>> + return false; >>> >>> I'd assume secretmem pages are rare in basically every setup out there. So >>> maybe throwing in a couple of likely()/unlikely() might make sense. >> >> I'd say we could do unlikely(page_is_secretmem()) at call sites. Here I can >> hardly estimate which pages are going to be checked. >> >>>> + >>>> + mapping = (struct address_space *) >>>> + ((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS); >>>> + >>> >>> Not sure if open-coding page_mapping is really a good idea here -- or even >>> necessary after the fast path above is in place. Anyhow, just my 2 cents. >> >> Well, most if the -4.2% of the performance regression kbuild reported were >> due to repeated compount_head(page) in page_mapping(). So the whole point >> of this patch is to avoid calling page_mapping(). > > I would have thought the fast path "(PageCompound(page) || > !PageLRU(page))" would already avoid calling page_mapping() in many cases. (and I do wonder if a generic page_mapping() optimization would make sense instead) Willy can most probably give the best advise here :) -- Thanks, David / dhildenb