From: Marcelo Tosatti <mtosatti@redhat.com>
To: Fenghua Yu <fenghua.yu@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org
Subject: [PATCH v2] intelrdt: resctrl: recommend locking for resctrlfs
Date: Thu, 1 Dec 2016 12:55:44 -0200 [thread overview]
Message-ID: <20161201145541.GA19232@amt.cnet> (raw)
In-Reply-To: <20161130220530.GG35583@linux.intel.com>
There is a locking problem between different applications
reading/writing to resctrlfs directory at the same time (read the patch
below for details).
Suggest a standard locking scheme for applications to use.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
---
v2: Improve commentary about generality of locking problem ("Yu, Fenghua")
Proper patch -p level (Thomas Gleixner)
diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt
index d918d26..14b3214 100644
--- a/Documentation/x86/intel_rdt_ui.txt
+++ b/Documentation/x86/intel_rdt_ui.txt
@@ -212,3 +212,33 @@ 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 multiple files, must be atomic.
+
+As an example, the allocation of an exclusive reservation
+of L3 cache involves:
+
+ 1. read list of cbmmasks for 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 attempting to allocate space race with each other
+(if two processes execute the steps above in a interlocked fashion),
+they can end up using the same bits of CBMMASK, which renders the
+reservations non-exclusive but shared.
+
+To coordinate atomic operations on resctrl and avoid the problem
+above, the following locking procedure is recommended:
+
+A) open /var/lock/resctrl/fs.lock with O_CREAT|O_EXCL.
+B) if success, write pid of program accessing the directory
+ structure to this file.
+C) read/write the directory structure.
+D) remove file.
+
next prev parent reply other threads:[~2016-12-02 10:50 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-30 15:48 [PATCH] intelrdt: resctrl: recommend locking for resctrlfs Marcelo Tosatti
2016-11-30 21:10 ` Thomas Gleixner
2016-11-30 22:05 ` Fenghua Yu
2016-11-30 22:25 ` Marcelo Tosatti
2016-12-01 21:54 ` Fenghua Yu
2016-12-01 14:55 ` Marcelo Tosatti [this message]
2016-12-02 11:20 ` [PATCH v2] " Thomas Gleixner
2016-12-02 22:07 ` Marcelo Tosatti
2016-12-09 9:35 ` Thomas Gleixner
2016-12-14 17:08 ` [PATCH v3] " Marcelo Tosatti
2016-12-15 13:49 ` [tip:x86/cache] Documentation, x86, resctrl: Recommend " tip-bot for Marcelo Tosatti
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