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blkcg: alloc per cpu stats from worker thread in a delayed manner 

Current per cpu stat allocation assumes GFP_KERNEL allocation flag. But in
IO path there are times when we want GFP_NOIO semantics. As there is no
way to pass the allocation flags to alloc_percpu(), this patch delays the
allocation of stats using a worker thread.

v2-> tejun suggested following changes. Changed the patch accordingly.
	- move alloc_node location in structure
	- reduce the size of names of some of the fields
	- Reduce the scope of locking of alloc_list_lock
	- Simplified stat_alloc_fn() by allocating stats for all
	  policies in one go and then assigning these to a group.

v3 -> Andrew suggested to put some comments in the code. Also raised
      concerns about trying to allocate infinitely in case of allocation
      failure. I have changed the logic to sleep for 10ms before retrying.
      That should take care of non-preemptible UP kernels.

v4 -> Tejun had more suggestions.
	- drop list_for_each_entry_all()
	- instead of msleep() use queue_delayed_work()
	- Some cleanups realted to more compact coding.

v5-> tejun suggested more cleanups leading to more compact code.

tj: - Relocated pcpu_stats into blkio_stat_alloc_fn().
    - Minor comment update.
    - This also fixes suspicious RCU usage warning caused by invoking
      cgroup_path() from blkg_alloc() without holding RCU read lock.
      Now that blkg_alloc() doesn't require sleepable context, RCU
      read lock from blkg_lookup_create() is maintained throughout
      blkg_alloc().

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Vivek Goyal 2012-03-08 10:53:56 -08:00 committed by Jens Axboe
parent 671058fb2a
commit 1cd9e039fc
2 changed files with 91 additions and 40 deletions
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129
block/blk-cgroup.c
View File

@ -30,6 +30,13 @@ static LIST_HEAD(blkio_list);
static DEFINE_MUTEX(all_q_mutex);
static LIST_HEAD(all_q_list);
/* List of groups pending per cpu stats allocation */
static DEFINE_SPINLOCK(alloc_list_lock);
static LIST_HEAD(alloc_list);
static void blkio_stat_alloc_fn(struct work_struct *);
static DECLARE_DELAYED_WORK(blkio_stat_alloc_work, blkio_stat_alloc_fn);
struct blkio_cgroup blkio_root_cgroup = { .weight = 2*BLKIO_WEIGHT_DEFAULT };
EXPORT_SYMBOL_GPL(blkio_root_cgroup);
@ -391,6 +398,10 @@ void blkiocg_update_dispatch_stats(struct blkio_group *blkg,
struct blkio_group_stats_cpu *stats_cpu;
unsigned long flags;
/* If per cpu stats are not allocated yet, don't do any accounting. */
if (pd->stats_cpu == NULL)
return;
/*
* Disabling interrupts to provide mutual exclusion between two
* writes on same cpu. It probably is not needed for 64bit. Not
@ -443,6 +454,10 @@ void blkiocg_update_io_merged_stats(struct blkio_group *blkg,
struct blkio_group_stats_cpu *stats_cpu;
unsigned long flags;
/* If per cpu stats are not allocated yet, don't do any accounting. */
if (pd->stats_cpu == NULL)
return;
/*
* Disabling interrupts to provide mutual exclusion between two
* writes on same cpu. It probably is not needed for 64bit. Not
@ -460,6 +475,60 @@ void blkiocg_update_io_merged_stats(struct blkio_group *blkg,
}
EXPORT_SYMBOL_GPL(blkiocg_update_io_merged_stats);
/*
* Worker for allocating per cpu stat for blk groups. This is scheduled on
* the system_nrt_wq once there are some groups on the alloc_list waiting
* for allocation.
*/
static void blkio_stat_alloc_fn(struct work_struct *work)
{
static void *pcpu_stats[BLKIO_NR_POLICIES];
struct delayed_work *dwork = to_delayed_work(work);
struct blkio_group *blkg;
int i;
bool empty = false;
alloc_stats:
for (i = 0; i < BLKIO_NR_POLICIES; i++) {
if (pcpu_stats[i] != NULL)
continue;
pcpu_stats[i] = alloc_percpu(struct blkio_group_stats_cpu);
/* Allocation failed. Try again after some time. */
if (pcpu_stats[i] == NULL) {
queue_delayed_work(system_nrt_wq, dwork,
msecs_to_jiffies(10));
return;
}
}
spin_lock_irq(&blkio_list_lock);
spin_lock(&alloc_list_lock);
/* cgroup got deleted or queue exited. */
if (!list_empty(&alloc_list)) {
blkg = list_first_entry(&alloc_list, struct blkio_group,
alloc_node);
for (i = 0; i < BLKIO_NR_POLICIES; i++) {
struct blkg_policy_data *pd = blkg->pd[i];
if (blkio_policy[i] && pd && !pd->stats_cpu)
swap(pd->stats_cpu, pcpu_stats[i]);
}
list_del_init(&blkg->alloc_node);
}
empty = list_empty(&alloc_list);
spin_unlock(&alloc_list_lock);
spin_unlock_irq(&blkio_list_lock);
if (!empty)
goto alloc_stats;
}
/**
* blkg_free - free a blkg
* @blkg: blkg to free
@ -491,9 +560,6 @@ static void blkg_free(struct blkio_group *blkg)
* @q: request_queue the new blkg is associated with
*
* Allocate a new blkg assocating @blkcg and @q.
*
* FIXME: Should be called with queue locked but currently isn't due to
* percpu stat breakage.
*/
static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
struct request_queue *q)
@ -509,6 +575,7 @@ static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
spin_lock_init(&blkg->stats_lock);
blkg->q = q;
INIT_LIST_HEAD(&blkg->q_node);
INIT_LIST_HEAD(&blkg->alloc_node);
blkg->blkcg = blkcg;
blkg->refcnt = 1;
cgroup_path(blkcg->css.cgroup, blkg->path, sizeof(blkg->path));
@ -530,13 +597,6 @@ static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
blkg->pd[i] = pd;
pd->blkg = blkg;
/* broken, read comment in the callsite */
pd->stats_cpu = alloc_percpu(struct blkio_group_stats_cpu);
if (!pd->stats_cpu) {
blkg_free(blkg);
return NULL;
}
}
/* invoke per-policy init */
@ -556,7 +616,7 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
bool for_root)
__releases(q->queue_lock) __acquires(q->queue_lock)
{
struct blkio_group *blkg, *new_blkg;
struct blkio_group *blkg;
WARN_ON_ONCE(!rcu_read_lock_held());
lockdep_assert_held(q->queue_lock);
@ -580,48 +640,27 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
/*
* Allocate and initialize.
*
* FIXME: The following is broken. Percpu memory allocation
* requires %GFP_KERNEL context and can't be performed from IO
* path. Allocation here should inherently be atomic and the
* following lock dancing can be removed once the broken percpu
* allocation is fixed.
*/
spin_unlock_irq(q->queue_lock);
rcu_read_unlock();
new_blkg = blkg_alloc(blkcg, q);
rcu_read_lock();
spin_lock_irq(q->queue_lock);
/* did bypass get turned on inbetween? */
if (unlikely(blk_queue_bypass(q)) && !for_root) {
blkg = ERR_PTR(blk_queue_dead(q) ? -EINVAL : -EBUSY);
goto out;
}
/* did someone beat us to it? */
blkg = blkg_lookup(blkcg, q);
if (unlikely(blkg))
goto out;
blkg = blkg_alloc(blkcg, q);
/* did alloc fail? */
if (unlikely(!new_blkg)) {
if (unlikely(!blkg)) {
blkg = ERR_PTR(-ENOMEM);
goto out;
}
/* insert */
spin_lock(&blkcg->lock);
swap(blkg, new_blkg);
hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
list_add(&blkg->q_node, &q->blkg_list);
spin_unlock(&blkcg->lock);
spin_lock(&alloc_list_lock);
list_add(&blkg->alloc_node, &alloc_list);
/* Queue per cpu stat allocation from worker thread. */
queue_delayed_work(system_nrt_wq, &blkio_stat_alloc_work, 0);
spin_unlock(&alloc_list_lock);
out:
blkg_free(new_blkg);
return blkg;
}
EXPORT_SYMBOL_GPL(blkg_lookup_create);
@ -654,6 +693,10 @@ static void blkg_destroy(struct blkio_group *blkg)
list_del_init(&blkg->q_node);
hlist_del_init_rcu(&blkg->blkcg_node);
spin_lock(&alloc_list_lock);
list_del_init(&blkg->alloc_node);
spin_unlock(&alloc_list_lock);
/*
* Put the reference taken at the time of creation so that when all
* queues are gone, group can be destroyed.
@ -752,6 +795,9 @@ static void blkio_reset_stats_cpu(struct blkio_group *blkg, int plid)
struct blkg_policy_data *pd = blkg->pd[plid];
struct blkio_group_stats_cpu *stats_cpu;
int i, j, k;
if (pd->stats_cpu == NULL)
return;
/*
* Note: On 64 bit arch this should not be an issue. This has the
* possibility of returning some inconsistent value on 32bit arch
@ -883,6 +929,9 @@ static uint64_t blkio_read_stat_cpu(struct blkio_group *blkg, int plid,
struct blkio_group_stats_cpu *stats_cpu;
u64 val = 0, tval;
if (pd->stats_cpu == NULL)
return val;
for_each_possible_cpu(cpu) {
unsigned int start;
stats_cpu = per_cpu_ptr(pd->stats_cpu, cpu);

2
block/blk-cgroup.h
View File

@ -190,6 +190,8 @@ struct blkio_group {
spinlock_t stats_lock;
struct blkg_policy_data *pd[BLKIO_NR_POLICIES];
/* List of blkg waiting for per cpu stats memory to be allocated */
struct list_head alloc_node;
struct rcu_head rcu_head;
};