From: Kent Overstreet Date: Wed, 20 Feb 2013 02:16:50 +0000 (+1100) Subject: generic-dynamic-per-cpu-refcounting-doc X-Git-Tag: next-20130220~1^2~50 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=009f82b3a62406013968aa3ff5ac85c40cb1fffc;p=karo-tx-linux.git generic-dynamic-per-cpu-refcounting-doc percpu-refcount: Documentation Signed-off-by: Kent Overstreet Signed-off-by: Andrew Morton --- diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h index 922860474f00..bed9a0d29f66 100644 --- a/include/linux/percpu-refcount.h +++ b/include/linux/percpu-refcount.h @@ -1,3 +1,71 @@ +/* + * Dynamic percpu refcounts: + * (C) 2012 Google, Inc. + * Author: Kent Overstreet + * + * This implements a refcount with similar semantics to atomic_t - atomic_inc(), + * atomic_dec_and_test() - but potentially percpu. + * + * There's one important difference between percpu refs and normal atomic_t + * refcounts; you have to keep track of your initial refcount, and then when you + * start shutting down you call percpu_ref_kill() _before_ dropping the initial + * refcount. + * + * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the + * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() + * puts the ref back in single atomic_t mode, collecting the per cpu refs and + * issuing the appropriate barriers, and then marks the ref as shutting down so + * that percpu_ref_put() will check for the ref hitting 0. After it returns, + * it's safe to drop the initial ref. + * + * BACKGROUND: + * + * Percpu refcounts are quite useful for performance, but if we blindly + * converted all refcounts to percpu counters we'd waste quite a bit of memory + * think about all the refcounts embedded in kobjects, files, etc. most of which + * aren't used much. + * + * These start out as simple atomic counters - a little bigger than a bare + * atomic_t, 16 bytes instead of 4 - but if we exceed some arbitrary number of + * gets in one second, we then switch to percpu counters. + * + * This heuristic isn't perfect because it'll fire if the refcount was only + * being used on one cpu; ideally we'd be able to count the number of cache + * misses on percpu_ref_get() or something similar, but that'd make the non + * percpu path significantly heavier/more complex. We can count the number of + * gets() without any extra atomic instructions, on arches that support + * atomic64_t - simply by changing the atomic_inc() to atomic_add_return(). + * + * USAGE: + * + * See fs/aio.c for some example usage; it's used there for struct kioctx, which + * is created when userspaces calls io_setup(), and destroyed when userspace + * calls io_destroy() or the process exits. + * + * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it + * calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove + * the kioctx from the proccess's list of kioctxs - after that, there can't be + * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop + * the initial ref with percpu_ref_put(). + * + * Code that does a two stage shutdown like this often needs some kind of + * explicit synchronization to ensure the initial refcount can only be dropped + * once - percpu_ref_kill() does this for you, it returns true once and false if + * someone else already called it. The aio code uses it this way, but it's not + * necessary if the code has some other mechanism to synchronize teardown. + * + * As mentioned previously, we decide when to convert a ref to percpu counters + * in percpu_ref_get(). However, since percpu_ref_get() will often be called + * with rcu_read_lock() held, it's not done there - percpu_ref_get() returns + * true if the ref should be converted to percpu counters. + * + * The caller should then call percpu_ref_alloc() after dropping + * rcu_read_lock(); if there is an uncommonly used codepath where it's + * inconvenient to call percpu_ref_alloc() after get(), it may be safely skipped + * and percpu_ref_get() will return true again the next time the counter wraps + * around. + */ + #ifndef _LINUX_PERCPU_REFCOUNT_H #define _LINUX_PERCPU_REFCOUNT_H @@ -16,11 +84,29 @@ int percpu_ref_put(struct percpu_ref *ref); int percpu_ref_kill(struct percpu_ref *ref); int percpu_ref_dead(struct percpu_ref *ref); +/** + * percpu_ref_get - increment a dynamic percpu refcount + * + * Increments @ref and possibly converts it to percpu counters. Must be called + * with rcu_read_lock() held, and may potentially drop/reacquire rcu_read_lock() + * to allocate percpu counters - if sleeping/allocation isn't safe for some + * other reason (e.g. a spinlock), see percpu_ref_get_noalloc(). + * + * Analagous to atomic_inc(). + */ static inline void percpu_ref_get(struct percpu_ref *ref) { __percpu_ref_get(ref, true); } +/** + * percpu_ref_get_noalloc - increment a dynamic percpu refcount + * + * Increments @ref, to be used when it's not safe to allocate percpu counters. + * Must be called with rcu_read_lock() held. + * + * Analagous to atomic_inc(). + */ static inline void percpu_ref_get_noalloc(struct percpu_ref *ref) { __percpu_ref_get(ref, false); diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index d87ab9766edf..79c61580a211 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -5,6 +5,51 @@ #include #include +/* + * A percpu refcount can be in 4 different modes. The state is tracked in the + * low two bits of percpu_ref->pcpu_count: + * + * PCPU_REF_NONE - the initial state, no percpu counters allocated. + * + * PCPU_REF_PTR - using percpu counters for the refcount. + * + * PCPU_REF_DYING - we're shutting down so get()/put() should use the embedded + * atomic counter, but we're not finished updating the atomic counter from the + * percpu counters - this means that percpu_ref_put() can't check for the ref + * hitting 0 yet. + * + * PCPU_REF_DEAD - we've finished the teardown sequence, percpu_ref_put() should + * now check for the ref hitting 0. + * + * In PCPU_REF_NONE mode, we need to count the number of times percpu_ref_get() + * is called; this is done with the high bits of the raw atomic counter. We also + * track the time, in jiffies, when the get count last wrapped - this is done + * with the remaining bits of percpu_ref->percpu_count. + * + * So, when percpu_ref_get() is called it increments the get count and checks if + * it wrapped; if it did, it checks if the last time it wrapped was less than + * one second ago; if so, we want to allocate percpu counters. + * + * PCPU_COUNT_BITS determines the threshold where we convert to percpu: of the + * raw 64 bit counter, we use PCPU_COUNT_BITS for the refcount, and the + * remaining (high) bits to count the number of times percpu_ref_get() has been + * called. It's currently (completely arbitrarily) 16384 times in one second. + * + * Percpu mode (PCPU_REF_PTR): + * + * In percpu mode all we do on get and put is increment or decrement the cpu + * local counter, which is a 32 bit unsigned int. + * + * Note that all the gets() could be happening on one cpu, and all the puts() on + * another - the individual cpu counters can wrap (potentially many times). + * + * But this is fine because we don't need to check for the ref hitting 0 in + * percpu mode; before we set the state to PCPU_REF_DEAD we simply sum up all + * the percpu counters and add them to the atomic counter. Since addition and + * subtraction in modular arithmatic is still associative, the result will be + * correct. + */ + #define PCPU_COUNT_BITS 50 #define PCPU_COUNT_MASK ((1LL << PCPU_COUNT_BITS) - 1) @@ -18,6 +63,12 @@ #define REF_STATUS(count) (count & PCPU_STATUS_MASK) +/** + * percpu_ref_init - initialize a dynamic percpu refcount + * + * Initializes the refcount in single atomic counter mode with a refcount of 1; + * analagous to atomic_set(ref, 1). + */ void percpu_ref_init(struct percpu_ref *ref) { unsigned long now = jiffies; @@ -79,6 +130,13 @@ void __percpu_ref_get(struct percpu_ref *ref, bool alloc) } } +/** + * percpu_ref_put - decrement a dynamic percpu refcount + * + * Returns true if the result is 0, otherwise false; only checks for the ref + * hitting 0 after percpu_ref_kill() has been called. Analagous to + * atomic_dec_and_test(). + */ int percpu_ref_put(struct percpu_ref *ref) { unsigned long pcpu_count; @@ -112,6 +170,17 @@ int percpu_ref_put(struct percpu_ref *ref) return ret; } +/** + * percpu_ref_kill - prepare a dynamic percpu refcount for teardown + * + * Must be called before dropping the initial ref, so that percpu_ref_put() + * knows to check for the refcount hitting 0. If the refcount was in percpu + * mode, converts it back to single atomic counter mode. + * + * Returns true the first time called on @ref and false if @ref is already + * shutting down, so it may be used by the caller for synchronizing other parts + * of a two stage shutdown. + */ int percpu_ref_kill(struct percpu_ref *ref) { unsigned long old, new, status, pcpu_count; @@ -160,6 +229,11 @@ int percpu_ref_kill(struct percpu_ref *ref) return 1; } +/** + * percpu_ref_dead - check if a dynamic percpu refcount is shutting down + * + * Returns true if percpu_ref_kill() has been called on @ref, false otherwise. + */ int percpu_ref_dead(struct percpu_ref *ref) { unsigned status = REF_STATUS(ref->pcpu_count);