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Date: Thu, 25 Jun 2015 15:10:17 -0400
From: Vivek Goyal <vgoyal@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: axboe@...nel.dk, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, avanzini.arianna@...il.com,
kernel-team@...com
Subject: Re: [PATCH 5/7] blk-throttle: remove asynchrnous percpu stats
allocation mechanism
On Tue, Jun 23, 2015 at 10:44:11PM -0400, Tejun Heo wrote:
> Because percpu allocator couldn't do non-blocking allocations,
> blk-throttle was forced to implement an ad-hoc asynchronous allocation
> mechanism for its percpu stats for cases where blkg's (blkcg_gq's) are
> allocated from an IO path without sleepable context.
>
> Now that percpu allocator can handle gfp_mask and blkg_policy_data
> alloc / free are handled by policy methods, the ad-hoc asynchronous
> allocation mechanism can be replaced with direct allocation from
> tg_stats_alloc_fn(). Rit it out.
>
> This ensures that an active throtl_grp always has valid non-NULL
> ->stats_cpu. Remove checks on it.
This is a nice cleanup. Allocating stats memory with the help of
a worker thread was twisted.
Thanks
Vivek
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> ---
> block/blk-throttle.c | 112 ++++++++++++---------------------------------------
> 1 file changed, 25 insertions(+), 87 deletions(-)
>
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index f1dd691..3c86976 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -144,9 +144,6 @@ struct throtl_grp {
>
> /* Per cpu stats pointer */
> struct tg_stats_cpu __percpu *stats_cpu;
> -
> - /* List of tgs waiting for per cpu stats memory to be allocated */
> - struct list_head stats_alloc_node;
> };
>
> struct throtl_data
> @@ -168,13 +165,6 @@ struct throtl_data
> struct work_struct dispatch_work;
> };
>
> -/* list and work item to allocate percpu group stats */
> -static DEFINE_SPINLOCK(tg_stats_alloc_lock);
> -static LIST_HEAD(tg_stats_alloc_list);
> -
> -static void tg_stats_alloc_fn(struct work_struct *);
> -static DECLARE_DELAYED_WORK(tg_stats_alloc_work, tg_stats_alloc_fn);
> -
> static void throtl_pending_timer_fn(unsigned long arg);
>
> static inline struct throtl_grp *pd_to_tg(struct blkg_policy_data *pd)
> @@ -256,53 +246,6 @@ static struct throtl_data *sq_to_td(struct throtl_service_queue *sq)
> } \
> } while (0)
>
> -static void tg_stats_init(struct tg_stats_cpu *tg_stats)
> -{
> - blkg_rwstat_init(&tg_stats->service_bytes);
> - blkg_rwstat_init(&tg_stats->serviced);
> -}
> -
> -/*
> - * Worker for allocating per cpu stat for tgs. This is scheduled on the
> - * system_wq once there are some groups on the alloc_list waiting for
> - * allocation.
> - */
> -static void tg_stats_alloc_fn(struct work_struct *work)
> -{
> - static struct tg_stats_cpu *stats_cpu; /* this fn is non-reentrant */
> - struct delayed_work *dwork = to_delayed_work(work);
> - bool empty = false;
> -
> -alloc_stats:
> - if (!stats_cpu) {
> - int cpu;
> -
> - stats_cpu = alloc_percpu(struct tg_stats_cpu);
> - if (!stats_cpu) {
> - /* allocation failed, try again after some time */
> - schedule_delayed_work(dwork, msecs_to_jiffies(10));
> - return;
> - }
> - for_each_possible_cpu(cpu)
> - tg_stats_init(per_cpu_ptr(stats_cpu, cpu));
> - }
> -
> - spin_lock_irq(&tg_stats_alloc_lock);
> -
> - if (!list_empty(&tg_stats_alloc_list)) {
> - struct throtl_grp *tg = list_first_entry(&tg_stats_alloc_list,
> - struct throtl_grp,
> - stats_alloc_node);
> - swap(tg->stats_cpu, stats_cpu);
> - list_del_init(&tg->stats_alloc_node);
> - }
> -
> - empty = list_empty(&tg_stats_alloc_list);
> - spin_unlock_irq(&tg_stats_alloc_lock);
> - if (!empty)
> - goto alloc_stats;
> -}
> -
> static void throtl_qnode_init(struct throtl_qnode *qn, struct throtl_grp *tg)
> {
> INIT_LIST_HEAD(&qn->node);
> @@ -405,7 +348,27 @@ static void throtl_service_queue_exit(struct throtl_service_queue *sq)
>
> static struct blkg_policy_data *throtl_pd_alloc(gfp_t gfp, int node)
> {
> - return kzalloc_node(sizeof(struct throtl_grp), gfp, node);
> + struct throtl_grp *tg;
> + int cpu;
> +
> + tg = kzalloc_node(sizeof(*tg), gfp, node);
> + if (!tg)
> + return NULL;
> +
> + tg->stats_cpu = alloc_percpu_gfp(struct tg_stats_cpu, gfp);
> + if (!tg->stats_cpu) {
> + kfree(tg);
> + return NULL;
> + }
> +
> + for_each_possible_cpu(cpu) {
> + struct tg_stats_cpu *stats_cpu = per_cpu_ptr(tg->stats_cpu, cpu);
> +
> + blkg_rwstat_init(&stats_cpu->service_bytes);
> + blkg_rwstat_init(&stats_cpu->serviced);
> + }
> +
> + return &tg->pd;
> }
>
> static void throtl_pd_init(struct blkcg_gq *blkg)
> @@ -413,7 +376,6 @@ static void throtl_pd_init(struct blkcg_gq *blkg)
> struct throtl_grp *tg = blkg_to_tg(blkg);
> struct throtl_data *td = blkg->q->td;
> struct throtl_service_queue *parent_sq;
> - unsigned long flags;
> int rw;
>
> /*
> @@ -448,16 +410,6 @@ static void throtl_pd_init(struct blkcg_gq *blkg)
> tg->bps[WRITE] = -1;
> tg->iops[READ] = -1;
> tg->iops[WRITE] = -1;
> -
> - /*
> - * Ugh... We need to perform per-cpu allocation for tg->stats_cpu
> - * but percpu allocator can't be called from IO path. Queue tg on
> - * tg_stats_alloc_list and allocate from work item.
> - */
> - spin_lock_irqsave(&tg_stats_alloc_lock, flags);
> - list_add(&tg->stats_alloc_node, &tg_stats_alloc_list);
> - schedule_delayed_work(&tg_stats_alloc_work, 0);
> - spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);
> }
>
> /*
> @@ -487,20 +439,16 @@ static void throtl_pd_online(struct blkcg_gq *blkg)
> static void throtl_pd_exit(struct blkcg_gq *blkg)
> {
> struct throtl_grp *tg = blkg_to_tg(blkg);
> - unsigned long flags;
> -
> - spin_lock_irqsave(&tg_stats_alloc_lock, flags);
> - list_del_init(&tg->stats_alloc_node);
> - spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);
> -
> - free_percpu(tg->stats_cpu);
>
> throtl_service_queue_exit(&tg->service_queue);
> }
>
> static void throtl_pd_free(struct blkg_policy_data *pd)
> {
> - kfree(pd);
> + struct throtl_grp *tg = pd_to_tg(pd);
> +
> + free_percpu(tg->stats_cpu);
> + kfree(tg);
> }
>
> static void throtl_pd_reset_stats(struct blkcg_gq *blkg)
> @@ -508,9 +456,6 @@ static void throtl_pd_reset_stats(struct blkcg_gq *blkg)
> struct throtl_grp *tg = blkg_to_tg(blkg);
> int cpu;
>
> - if (tg->stats_cpu == NULL)
> - return;
> -
> for_each_possible_cpu(cpu) {
> struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);
>
> @@ -973,10 +918,6 @@ static void throtl_update_dispatch_stats(struct blkcg_gq *blkg, u64 bytes,
> struct tg_stats_cpu *stats_cpu;
> unsigned long flags;
>
> - /* If per cpu stats are not allocated yet, don't do any accounting. */
> - if (tg->stats_cpu == NULL)
> - return;
> -
> /*
> * Disabling interrupts to provide mutual exclusion between two
> * writes on same cpu. It probably is not needed for 64bit. Not
> @@ -1302,9 +1243,6 @@ static u64 tg_prfill_cpu_rwstat(struct seq_file *sf,
> struct blkg_rwstat rwstat = { }, tmp;
> int i, cpu;
>
> - if (tg->stats_cpu == NULL)
> - return 0;
> -
> for_each_possible_cpu(cpu) {
> struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);
>
> --
> 2.4.3
--
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