From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f198.google.com (mail-pg1-f198.google.com [209.85.215.198]) by kanga.kvack.org (Postfix) with ESMTP id 10E1F6B000E for ; Wed, 17 Oct 2018 11:07:08 -0400 (EDT) Received: by mail-pg1-f198.google.com with SMTP id z8-v6so20179704pgp.20 for ; Wed, 17 Oct 2018 08:07:08 -0700 (PDT) Received: from mga03.intel.com (mga03.intel.com. [134.134.136.65]) by mx.google.com with ESMTPS id b190-v6si15115919pfb.166.2018.10.17.08.07.06 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 17 Oct 2018 08:07:06 -0700 (PDT) Subject: Re: [mm PATCH v3 1/6] mm: Use mm_zero_struct_page from SPARC on all 64b architectures References: <20181015202456.2171.88406.stgit@localhost.localdomain> <20181015202656.2171.92963.stgit@localhost.localdomain> <20181017084744.GH18839@dhcp22.suse.cz> From: Alexander Duyck Message-ID: <9700b00f-a8a4-e318-f6a8-71fd1e7021b3@linux.intel.com> Date: Wed, 17 Oct 2018 08:07:06 -0700 MIME-Version: 1.0 In-Reply-To: <20181017084744.GH18839@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-mm@kvack.org, akpm@linux-foundation.org, pavel.tatashin@microsoft.com, dave.jiang@intel.com, linux-kernel@vger.kernel.org, willy@infradead.org, davem@davemloft.net, yi.z.zhang@linux.intel.com, khalid.aziz@oracle.com, rppt@linux.vnet.ibm.com, vbabka@suse.cz, sparclinux@vger.kernel.org, dan.j.williams@intel.com, ldufour@linux.vnet.ibm.com, mgorman@techsingularity.net, mingo@kernel.org, kirill.shutemov@linux.intel.com On 10/17/2018 1:47 AM, Michal Hocko wrote: > On Mon 15-10-18 13:26:56, Alexander Duyck wrote: >> This change makes it so that we use the same approach that was already in >> use on Sparc on all the archtectures that support a 64b long. >> >> This is mostly motivated by the fact that 8 to 10 store/move instructions >> are likely always going to be faster than having to call into a function >> that is not specialized for handling page init. >> >> An added advantage to doing it this way is that the compiler can get away >> with combining writes in the __init_single_page call. As a result the >> memset call will be reduced to only about 4 write operations, or at least >> that is what I am seeing with GCC 6.2 as the flags, LRU poitners, and >> count/mapcount seem to be cancelling out at least 4 of the 8 assignments on >> my system. >> >> One change I had to make to the function was to reduce the minimum page >> size to 56 to support some powerpc64 configurations. > > This really begs for numbers. I do not mind the change itself with some > minor comments below. > > [...] >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index bb0de406f8e7..ec6e57a0c14e 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -102,8 +102,42 @@ static inline void set_max_mapnr(unsigned long limit) { } >> * zeroing by defining this macro in . >> */ >> #ifndef mm_zero_struct_page > > Do we still need this ifdef? I guess we can wait for an arch which > doesn't like this change and then add the override. I would rather go > simple if possible. We probably don't, but as soon as I remove it somebody will probably complain somewhere. I guess I could drop it for now and see if anybody screams. Adding it back should be pretty straight forward since it would only be 2 lines. >> +#if BITS_PER_LONG == 64 >> +/* This function must be updated when the size of struct page grows above 80 >> + * or reduces below 64. The idea that compiler optimizes out switch() >> + * statement, and only leaves move/store instructions >> + */ >> +#define mm_zero_struct_page(pp) __mm_zero_struct_page(pp) >> +static inline void __mm_zero_struct_page(struct page *page) >> +{ >> + unsigned long *_pp = (void *)page; >> + >> + /* Check that struct page is either 56, 64, 72, or 80 bytes */ >> + BUILD_BUG_ON(sizeof(struct page) & 7); >> + BUILD_BUG_ON(sizeof(struct page) < 56); >> + BUILD_BUG_ON(sizeof(struct page) > 80); >> + >> + switch (sizeof(struct page)) { >> + case 80: >> + _pp[9] = 0; /* fallthrough */ >> + case 72: >> + _pp[8] = 0; /* fallthrough */ >> + default: >> + _pp[7] = 0; /* fallthrough */ >> + case 56: >> + _pp[6] = 0; >> + _pp[5] = 0; >> + _pp[4] = 0; >> + _pp[3] = 0; >> + _pp[2] = 0; >> + _pp[1] = 0; >> + _pp[0] = 0; >> + } > > This just hit my eyes. I have to confess I have never seen default: to > be not the last one in the switch. Can we have case 64 instead or does gcc > complain? I would be surprised with the set of BUILD_BUG_ONs. I can probably just replace the "default:" with "case 64:". I think I have seen other switch statements in the kernel without a default so odds are it should be okay.