[PATCH] elevator: define ioc counting mechanism

None of the in-kernel primitives for handling "atomic" counting seem
to be a good fit. We need something that is essentially free for
incrementing/decrementing, while the read side may be more expensive
as we only ever need to do that when a device is removed from the
kernel.

Use a per-cpu variable for maintaining a per-cpu ioc count and define
a reading mechanism that just sums up the values.

Signed-off-by: Jens Axboe <axboe@suse.de>

diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index cc81645..9c5a04f 100644 (file)
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_ELEVATOR_H
 #define _LINUX_ELEVATOR_H
 
+#include <linux/percpu.h>
+
 typedef int (elevator_merge_fn) (request_queue_t *, struct request **,
                                 struct bio *);
 
@@ -178,4 +180,27 @@ enum {
        INIT_LIST_HEAD(&(rq)->donelist);        \
        } while (0)
 
+/*
+ * io context count accounting
+ */
+#define elv_ioc_count_mod(name, __val)                         \
+       do {                                                    \
+               preempt_disable();                              \
+               __get_cpu_var(name) += (__val);                 \
+               preempt_enable();                               \
+       } while (0)
+
+#define elv_ioc_count_inc(name)        elv_ioc_count_mod(name, 1)
+#define elv_ioc_count_dec(name)        elv_ioc_count_mod(name, -1)
+
+#define elv_ioc_count_read(name)                               \
+({                                                             \
+       unsigned long __val = 0;                                \
+       int __cpu;                                              \
+       smp_wmb();                                              \
+       for_each_possible_cpu(__cpu)                            \
+               __val += per_cpu(name, __cpu);                  \
+       __val;                                                  \
+})
+
 #endif