From: Marcelo Tosatti <mtosatti@redhat.com>
To: "Yu, Fenghua" <fenghua.yu@intel.com>,
Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Subject: [PATCH] intelrdt: resctrl: recommend locking for resctrlfs
Date: Wed, 30 Nov 2016 13:48:10 -0200 [thread overview]
Message-ID: <20161130154809.GA27444@amt.cnet> (raw)
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>
--- Documentation/x86/intel_rdt_ui.txt.orig 2016-11-30 13:40:33.080233101 -0200
+++ Documentation/x86/intel_rdt_ui.txt 2016-11-30 13:45:01.253703259 -0200
@@ -212,3 +212,30 @@ Finally we move core 4-7 over to the new
kernel and the tasks running there get 50% of the cache.
# echo C0 > p0/cpus
+
+4) Locking between applications
+
+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 attempt 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 creation of reservations 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 reply other threads:[~2016-11-30 15:48 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-30 15:48 Marcelo Tosatti [this message]
2016-11-30 21:10 ` [PATCH] intelrdt: resctrl: recommend locking for resctrlfs 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 ` [PATCH v2] " Marcelo Tosatti
2016-12-02 11:20 ` 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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