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memcg: simplify move_account() check 

In memcg, for avoiding take-lock-irq-off at accessing page_cgroup, a
logic, flag + rcu_read_lock(), is used.  This works as following

     CPU-A                     CPU-B
                             rcu_read_lock()
    set flag
                             if(flag is set)
                                   take heavy lock
                             do job.
    synchronize_rcu()        rcu_read_unlock()
    take heavy lock.

In recent discussion, it's argued that using per-cpu value for this flag
just complicates the code because 'set flag' is very rare.

This patch changes 'flag' implementation from percpu to atomic_t.  This
will be much simpler.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Ying Han <yinghan@google.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
KAMEZAWA Hiroyuki 2012-03-21 16:34:23 -07:00 committed by Linus Torvalds
parent 9e3357907c
commit 619d094b58
1 changed files with 30 additions and 40 deletions
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70
mm/memcontrol.c
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@ -89,7 +89,6 @@ enum mem_cgroup_stat_index {
MEM_CGROUP_STAT_FILE_MAPPED, /* # of pages charged as file rss */
MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
MEM_CGROUP_STAT_DATA, /* end of data requires synchronization */
MEM_CGROUP_ON_MOVE, /* someone is moving account between groups */
MEM_CGROUP_STAT_NSTATS,
};
@ -297,6 +296,10 @@ struct mem_cgroup {
* mem_cgroup ? And what type of charges should we move ?
*/
unsigned long move_charge_at_immigrate;
/*
* set > 0 if pages under this cgroup are moving to other cgroup.
*/
atomic_t moving_account;
/*
* percpu counter.
*/
@ -1287,35 +1290,36 @@ int mem_cgroup_swappiness(struct mem_cgroup *memcg)
return memcg->swappiness;
}
/*
* memcg->moving_account is used for checking possibility that some thread is
* calling move_account(). When a thread on CPU-A starts moving pages under
* a memcg, other threads should check memcg->moving_account under
* rcu_read_lock(), like this:
*
* CPU-A CPU-B
* rcu_read_lock()
* memcg->moving_account+1 if (memcg->mocing_account)
* take heavy locks.
* synchronize_rcu() update something.
* rcu_read_unlock()
* start move here.
*/
static void mem_cgroup_start_move(struct mem_cgroup *memcg)
{
int cpu;
get_online_cpus();
spin_lock(&memcg->pcp_counter_lock);
for_each_online_cpu(cpu)
per_cpu(memcg->stat->count[MEM_CGROUP_ON_MOVE], cpu) += 1;
memcg->nocpu_base.count[MEM_CGROUP_ON_MOVE] += 1;
spin_unlock(&memcg->pcp_counter_lock);
put_online_cpus();
atomic_inc(&memcg->moving_account);
synchronize_rcu();
}
static void mem_cgroup_end_move(struct mem_cgroup *memcg)
{
int cpu;
if (!memcg)
return;
get_online_cpus();
spin_lock(&memcg->pcp_counter_lock);
for_each_online_cpu(cpu)
per_cpu(memcg->stat->count[MEM_CGROUP_ON_MOVE], cpu) -= 1;
memcg->nocpu_base.count[MEM_CGROUP_ON_MOVE] -= 1;
spin_unlock(&memcg->pcp_counter_lock);
put_online_cpus();
/*
* Now, mem_cgroup_clear_mc() may call this function with NULL.
* We check NULL in callee rather than caller.
*/
if (memcg)
atomic_dec(&memcg->moving_account);
}
/*
* 2 routines for checking "mem" is under move_account() or not.
*
@ -1331,7 +1335,7 @@ static void mem_cgroup_end_move(struct mem_cgroup *memcg)
static bool mem_cgroup_stealed(struct mem_cgroup *memcg)
{
VM_BUG_ON(!rcu_read_lock_held());
return this_cpu_read(memcg->stat->count[MEM_CGROUP_ON_MOVE]) > 0;
return atomic_read(&memcg->moving_account) > 0;
}
static bool mem_cgroup_under_move(struct mem_cgroup *memcg)
@ -1882,8 +1886,8 @@ bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
* by flags.
*
* Considering "move", this is an only case we see a race. To make the race
* small, we check MEM_CGROUP_ON_MOVE percpu value and detect there are
* possibility of race condition. If there is, we take a lock.
* small, we check mm->moving_account and detect there are possibility of race
* If there is, we take a lock.
*/
void mem_cgroup_update_page_stat(struct page *page,
@ -2100,17 +2104,6 @@ static void mem_cgroup_drain_pcp_counter(struct mem_cgroup *memcg, int cpu)
per_cpu(memcg->stat->events[i], cpu) = 0;
memcg->nocpu_base.events[i] += x;
}
/* need to clear ON_MOVE value, works as a kind of lock. */
per_cpu(memcg->stat->count[MEM_CGROUP_ON_MOVE], cpu) = 0;
spin_unlock(&memcg->pcp_counter_lock);
}
static void synchronize_mem_cgroup_on_move(struct mem_cgroup *memcg, int cpu)
{
int idx = MEM_CGROUP_ON_MOVE;
spin_lock(&memcg->pcp_counter_lock);
per_cpu(memcg->stat->count[idx], cpu) = memcg->nocpu_base.count[idx];
spin_unlock(&memcg->pcp_counter_lock);
}
@ -2122,11 +2115,8 @@ static int __cpuinit memcg_cpu_hotplug_callback(struct notifier_block *nb,
struct memcg_stock_pcp *stock;
struct mem_cgroup *iter;
if ((action == CPU_ONLINE)) {
for_each_mem_cgroup(iter)
synchronize_mem_cgroup_on_move(iter, cpu);
if (action == CPU_ONLINE)
return NOTIFY_OK;
}
if ((action != CPU_DEAD) || action != CPU_DEAD_FROZEN)
return NOTIFY_OK;