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Fri, 18 Sep 2020 06:44:23 +0000 Received: from BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::8d64:f85b:1cce:a1c0]) by BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::8d64:f85b:1cce:a1c0%5]) with mapi id 15.20.3370.019; Fri, 18 Sep 2020 06:44:22 +0000 Subject: Re: [PATCH v2] sched, mm: Optimize current_gfp_context() To: Waiman Long Cc: Andrew Morton , Peter Zijlstra , Ingo Molnar , Mathieu Desnoyers , Michel Lespinasse , linux-kernel@vger.kernel.org References: <20200618212936.9776-1-longman@redhat.com> <20200811152921.1d43e34ad7aff928c6c1267f@linux-foundation.org> From: "Xu, Yanfei" Message-ID: Date: Fri, 18 Sep 2020 14:44:15 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 In-Reply-To: <20200811152921.1d43e34ad7aff928c6c1267f@linux-foundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: HK2PR0302CA0004.apcprd03.prod.outlook.com (2603:1096:202::14) To BY5PR11MB4241.namprd11.prod.outlook.com (2603:10b6:a03:1ca::13) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [128.224.162.160] (60.247.85.82) by HK2PR0302CA0004.apcprd03.prod.outlook.com (2603:1096:202::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3412.8 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-OriginatorOrg: windriver.com X-MS-Exchange-CrossTenant-Network-Message-Id: 76833633-60f1-40f1-c58b-08d85b9e46da X-MS-Exchange-CrossTenant-AuthSource: BY5PR11MB4241.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Sep 2020 06:44:22.6706 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ddb2873-a1ad-4a18-ae4e-4644631433be X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: NDUw5unkCYi3xgk9HvpfMg0R96YthX7BF7Y9rxZRFtq25oRaNQmsTmCNJ/bfcAl5XHqx1JyrqBJINTu8V+C/9Q== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR11MB3815 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Waiman, On 8/12/20 6:29 AM, Andrew Morton wrote: > On Thu, 18 Jun 2020 17:29:36 -0400 Waiman Long wrote: > >> The current_gfp_context() converts a number of PF_MEMALLOC_* per-process >> flags into the corresponding GFP_* flags for memory allocation. In >> that function, current->flags is accessed 3 times. That may lead to >> duplicated access of the same memory location. >> I have a puzzle about this comment, what's the meaning about "That may lead to duplicated access of the same memory location". After using variable 'pflags', will it not duplicated access the same memory location? Looking forward to your reply :) Thanks, Yanfei >> This is not usually a problem with minimal debug config options on as the >> compiler can optimize away the duplicated memory accesses. With most >> of the debug config options on, however, that may not be the case. >> For example, the x86-64 object size of the __need_fs_reclaim() in a >> debug kernel that calls current_gfp_context() was 309 bytes. With this >> patch applied, the object size is reduced to 202 bytes. This is a saving >> of 107 bytes and will probably be slightly faster too. >> >> ... >> >> --- a/include/linux/sched/mm.h >> +++ b/include/linux/sched/mm.h >> @@ -181,18 +181,20 @@ static inline bool in_vfork(struct task_struct *tsk) >> */ >> static inline gfp_t current_gfp_context(gfp_t flags) >> { >> - if (unlikely(current->flags & >> + unsigned int pflags = READ_ONCE(current->flags); > > Why use READ_ONCE() here? > >> + if (unlikely(pflags & >> (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_NOCMA))) { >> /* >> * NOIO implies both NOIO and NOFS and it is a weaker context >> * so always make sure it makes precedence >> */ >> - if (current->flags & PF_MEMALLOC_NOIO) >> + if (pflags & PF_MEMALLOC_NOIO) >> flags &= ~(__GFP_IO | __GFP_FS); >> - else if (current->flags & PF_MEMALLOC_NOFS) >> + else if (pflags & PF_MEMALLOC_NOFS) >> flags &= ~__GFP_FS; >> #ifdef CONFIG_CMA >> - if (current->flags & PF_MEMALLOC_NOCMA) >> + if (pflags & PF_MEMALLOC_NOCMA) >> flags &= ~__GFP_MOVABLE; >> #endif >> } >