From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758079AbcLONtX (ORCPT ); Thu, 15 Dec 2016 08:49:23 -0500 Received: from terminus.zytor.com ([198.137.202.10]:47342 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752360AbcLONtW (ORCPT ); Thu, 15 Dec 2016 08:49:22 -0500 Date: Thu, 15 Dec 2016 05:49:08 -0800 From: tip-bot for Marcelo Tosatti Message-ID: Cc: fenghua.yu@intel.com, linux-kernel@vger.kernel.org, mtosatti@redhat.com, tglx@linutronix.de, hpa@zytor.com, mingo@kernel.org Reply-To: mtosatti@redhat.com, linux-kernel@vger.kernel.org, fenghua.yu@intel.com, tglx@linutronix.de, hpa@zytor.com, mingo@kernel.org In-Reply-To: <20161214170835.GA16924@amt.cnet> References: <20161214170835.GA16924@amt.cnet> To: linux-tip-commits@vger.kernel.org Subject: [tip:x86/cache] Documentation, x86, resctrl: Recommend locking for resctrlfs Git-Commit-ID: 3c2a769de7955ff81818b49d388dd771bf6ae29d X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 3c2a769de7955ff81818b49d388dd771bf6ae29d Gitweb: http://git.kernel.org/tip/3c2a769de7955ff81818b49d388dd771bf6ae29d Author: Marcelo Tosatti AuthorDate: Wed, 14 Dec 2016 15:08:37 -0200 Committer: Thomas Gleixner CommitDate: Thu, 15 Dec 2016 14:44:27 +0100 Documentation, x86, resctrl: Recommend locking for resctrlfs Concurrent write or read/write access from applications to the resctrlfs directory can result in incorrect readouts or setups. Recommend a standard locking scheme for applications to use. Signed-off-by: Marcelo Tosatti Cc: Fenghua Yu Link: http://lkml.kernel.org/r/20161214170835.GA16924@amt.cnet Signed-off-by: Thomas Gleixner --- Documentation/x86/intel_rdt_ui.txt | 114 +++++++++++++++++++++++++++++++++++++ 1 file changed, 114 insertions(+) diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt index d918d26..51cf6fa 100644 --- a/Documentation/x86/intel_rdt_ui.txt +++ b/Documentation/x86/intel_rdt_ui.txt @@ -212,3 +212,117 @@ Finally we move core 4-7 over to the new group and make sure that the kernel and the tasks running there get 50% of the cache. # echo C0 > p0/cpus + +4) Locking between applications + +Certain operations on the resctrl filesystem, composed of read/writes +to/from multiple files, must be atomic. + +As an example, the allocation of an exclusive reservation of L3 cache +involves: + + 1. Read the cbmmasks from each directory + 2. Find a contiguous set of bits in the global CBM bitmask that is clear + in any of the directory cbmmasks + 3. Create a new directory + 4. Set the bits found in step 2 to the new directory "schemata" file + +If two applications attempt to allocate space concurrently then they can +end up allocating the same bits so the reservations are shared instead of +exclusive. + +To coordinate atomic operations on the resctrlfs and to avoid the problem +above, the following locking procedure is recommended: + +Locking is based on flock, which is available in libc and also as a shell +script command + +Write lock: + + A) Take flock(LOCK_EX) on /sys/fs/resctrl + B) Read/write the directory structure. + C) funlock + +Read lock: + + A) Take flock(LOCK_SH) on /sys/fs/resctrl + B) If success read the directory structure. + C) funlock + +Example with bash: + +# Atomically read directory structure +$ flock -s /sys/fs/resctrl/ find /sys/fs/resctrl + +# Read directory contents and create new subdirectory + +$ cat create-dir.sh +find /sys/fs/resctrl/ > output.txt +mask = function-of(output.txt) +mkdir /sys/fs/resctrl/newres/ +echo mask > /sys/fs/resctrl/newres/schemata + +$ flock /sys/fs/resctrl/ ./create-dir.sh + +Example with C: + +/* + * Example code do take advisory locks + * before accessing resctrl filesystem + */ +#include +#include + +void resctrl_take_shared_lock(int fd) +{ + int ret; + + /* take shared lock on resctrl filesystem */ + ret = flock(fd, LOCK_SH); + if (ret) { + perror("flock"); + exit(-1); + } +} + +void resctrl_take_exclusive_lock(int fd) +{ + int ret; + + /* release lock on resctrl filesystem */ + ret = flock(fd, LOCK_EX); + if (ret) { + perror("flock"); + exit(-1); + } +} + +void resctrl_release_lock(int fd) +{ + int ret; + + /* take shared lock on resctrl filesystem */ + ret = flock(fd, LOCK_UN); + if (ret) { + perror("flock"); + exit(-1); + } +} + +void main(void) +{ + int fd, ret; + + fd = open("/sys/fs/resctrl", O_DIRECTORY); + if (fd == -1) { + perror("open"); + exit(-1); + } + resctrl_take_shared_lock(fd); + /* code to read directory contents */ + resctrl_release_lock(fd); + + resctrl_take_exclusive_lock(fd); + /* code to read and write directory contents */ + resctrl_release_lock(fd); +}