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