Andi Kleen wrote:
As far as I can tell, the advantage of percpu is that you can define
new "fields" anywhere in the code and independently from the rest of
the system.
- Independent maintenance as you noted
- Fast access and relatively compact code
- Avoids false sharing by keeping cache lines of different CPUs separate
- Doesn't waste a lot of memory in padding like NR_CPUs arrays usually
need to to avoid the previous point.
Any replacement that doesn't have these properties too will probably
be not useful.
Thank you for the details. I'll try to stick to per-cpu wherever
possible for now.
Anyway, what do you think about adding the above text to the code (percpu.h
maybe) as documentation? See the patch below. (Dunno if the Signed-off-by
line is appropriate as most of the text is yours.)
Signed-off-by: Julio M. Merino Vidal <[email protected]>
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 600e3d3..b8e8b8c 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -1,6 +1,21 @@
#ifndef __LINUX_PERCPU_H
#define __LINUX_PERCPU_H
+/*
+ * percpu provides a mechanism to define variables that are specific to
each
+ * CPU in the system.
+ *
+ * Each variable is defined as an independent array of NR_CPUS elements.
+ * This approach is used instead of a per-CPU structure because it has the
+ * following advantages:
+ * - Independent maintenance: a source file can define new per-CPU
+ * variables without distorting others.
+ * - Fast access and relatively compact code.
+ * - Avoids false sharing by keeping cache lines of different CPUs
separate.
+ * - Doesn't waste a lot of memory in padding like NR_CPUs arrays usually
+ * need to to avoid the previous point.
+ */
+
#include <linux/spinlock.h> /* For preempt_disable() */
#include <linux/slab.h> /* For kmalloc() */
#include <linux/smp.h>
Kind regards.
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