Messages in this thread Patch in this message | | | Date | Sat, 9 Sep 2017 23:36:49 +0300 | From | Alexey Dobriyan <> | Subject | [PATCH 2/2] pid: delete struct pidmap::nr_free |
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There is a check in pid allocation code to skip a full page:
if (likely(atomic_read(&map->nr_free))) { ...
In practice it doesn't do anything. To skip a pidmap page one has to have 32K consecutive pids allocated at the same time which doesn't happen.
Currently the price is _every_ fork/exit on every system being slower than necessary.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> ---
include/linux/pid_namespace.h | 1 - kernel/pid.c | 28 ++++++++++------------------ kernel/pid_namespace.c | 6 ------ 3 files changed, 10 insertions(+), 25 deletions(-)
--- a/include/linux/pid_namespace.h +++ b/include/linux/pid_namespace.h @@ -11,7 +11,6 @@ #include <linux/ns_common.h> struct pidmap { - atomic_t nr_free; void *page; }; --- a/kernel/pid.c +++ b/kernel/pid.c @@ -68,9 +68,6 @@ static inline int mk_pid(struct pid_namespace *pid_ns, */ struct pid_namespace init_pid_ns = { .kref = KREF_INIT(2), - .pidmap = { - [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } - }, .last_pid = 0, .nr_hashed = PIDNS_HASH_ADDING, .level = 0, @@ -106,7 +103,6 @@ static void free_pidmap(struct upid *upid) int offset = nr & BITS_PER_PAGE_MASK; clear_bit(offset, map->page); - atomic_inc(&map->nr_free); } /* @@ -181,20 +177,17 @@ static int alloc_pidmap(struct pid_namespace *pid_ns) if (unlikely(!map->page)) return -ENOMEM; } - if (likely(atomic_read(&map->nr_free))) { - for ( ; ; ) { - if (!test_and_set_bit(offset, map->page)) { - atomic_dec(&map->nr_free); - set_last_pid(pid_ns, last, pid); - return pid; - } - offset = find_next_offset(map, offset); - if (offset >= BITS_PER_PAGE) - break; - pid = mk_pid(pid_ns, map, offset); - if (pid >= pid_max) - break; + for (;;) { + if (!test_and_set_bit(offset, map->page)) { + set_last_pid(pid_ns, last, pid); + return pid; } + offset = find_next_offset(map, offset); + if (offset >= BITS_PER_PAGE) + break; + pid = mk_pid(pid_ns, map, offset); + if (pid >= pid_max) + break; } if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) { ++map; @@ -591,7 +584,6 @@ void __init pidmap_init(void) init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL); /* Reserve PID 0. We never call free_pidmap(0) */ set_bit(0, init_pid_ns.pidmap[0].page); - atomic_dec(&init_pid_ns.pidmap[0].nr_free); init_pid_ns.pid_cachep = KMEM_CACHE(pid, SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT); --- a/kernel/pid_namespace.c +++ b/kernel/pid_namespace.c @@ -98,7 +98,6 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns struct pid_namespace *ns; unsigned int level = parent_pid_ns->level + 1; struct ucounts *ucounts; - int i; int err; err = -ENOSPC; @@ -135,11 +134,6 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns INIT_WORK(&ns->proc_work, proc_cleanup_work); set_bit(0, ns->pidmap[0].page); - atomic_set(&ns->pidmap[0].nr_free, BITS_PER_PAGE - 1); - - for (i = 1; i < PIDMAP_ENTRIES; i++) - atomic_set(&ns->pidmap[i].nr_free, BITS_PER_PAGE); - return ns; out_free_map:
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