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Subject[PATCH 6/7] blk-mq: create hctx for each present CPU
Date
Currently we only create hctx for online CPUs, which can lead to a lot
of churn due to frequent soft offline / online operations. Instead
allocate one for each present CPU to avoid this and dramatically simplify
the code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
block/blk-mq.c | 120 +++++----------------------------------------
block/blk-mq.h | 5 --
include/linux/cpuhotplug.h | 1 -
3 files changed, 11 insertions(+), 115 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a69ad122ed66..01bbc30a2807 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -37,9 +37,6 @@
#include "blk-wbt.h"
#include "blk-mq-sched.h"

-static DEFINE_MUTEX(all_q_mutex);
-static LIST_HEAD(all_q_list);
-
static void blk_mq_poll_stats_start(struct request_queue *q);
static void blk_mq_poll_stats_fn(struct blk_stat_callback *cb);
static void __blk_mq_stop_hw_queues(struct request_queue *q, bool sync);
@@ -1985,8 +1982,8 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
INIT_LIST_HEAD(&__ctx->rq_list);
__ctx->queue = q;

- /* If the cpu isn't online, the cpu is mapped to first hctx */
- if (!cpu_online(i))
+ /* If the cpu isn't present, the cpu is mapped to first hctx */
+ if (!cpu_present(i))
continue;

hctx = blk_mq_map_queue(q, i);
@@ -2029,8 +2026,7 @@ static void blk_mq_free_map_and_requests(struct blk_mq_tag_set *set,
}
}

-static void blk_mq_map_swqueue(struct request_queue *q,
- const struct cpumask *online_mask)
+static void blk_mq_map_swqueue(struct request_queue *q)
{
unsigned int i, hctx_idx;
struct blk_mq_hw_ctx *hctx;
@@ -2048,13 +2044,11 @@ static void blk_mq_map_swqueue(struct request_queue *q,
}

/*
- * Map software to hardware queues
+ * Map software to hardware queues.
+ *
+ * If the cpu isn't present, the cpu is mapped to first hctx.
*/
- for_each_possible_cpu(i) {
- /* If the cpu isn't online, the cpu is mapped to first hctx */
- if (!cpumask_test_cpu(i, online_mask))
- continue;
-
+ for_each_present_cpu(i) {
hctx_idx = q->mq_map[i];
/* unmapped hw queue can be remapped after CPU topo changed */
if (!set->tags[hctx_idx] &&
@@ -2340,16 +2334,8 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
blk_queue_softirq_done(q, set->ops->complete);

blk_mq_init_cpu_queues(q, set->nr_hw_queues);
-
- get_online_cpus();
- mutex_lock(&all_q_mutex);
-
- list_add_tail(&q->all_q_node, &all_q_list);
blk_mq_add_queue_tag_set(set, q);
- blk_mq_map_swqueue(q, cpu_online_mask);
-
- mutex_unlock(&all_q_mutex);
- put_online_cpus();
+ blk_mq_map_swqueue(q);

if (!(set->flags & BLK_MQ_F_NO_SCHED)) {
int ret;
@@ -2375,18 +2361,12 @@ void blk_mq_free_queue(struct request_queue *q)
{
struct blk_mq_tag_set *set = q->tag_set;

- mutex_lock(&all_q_mutex);
- list_del_init(&q->all_q_node);
- mutex_unlock(&all_q_mutex);
-
blk_mq_del_queue_tag_set(q);
-
blk_mq_exit_hw_queues(q, set, set->nr_hw_queues);
}

/* Basically redo blk_mq_init_queue with queue frozen */
-static void blk_mq_queue_reinit(struct request_queue *q,
- const struct cpumask *online_mask)
+static void blk_mq_queue_reinit(struct request_queue *q)
{
WARN_ON_ONCE(!atomic_read(&q->mq_freeze_depth));

@@ -2399,76 +2379,12 @@ static void blk_mq_queue_reinit(struct request_queue *q,
* involves free and re-allocate memory, worthy doing?)
*/

- blk_mq_map_swqueue(q, online_mask);
+ blk_mq_map_swqueue(q);

blk_mq_sysfs_register(q);
blk_mq_debugfs_register_hctxs(q);
}

-/*
- * New online cpumask which is going to be set in this hotplug event.
- * Declare this cpumasks as global as cpu-hotplug operation is invoked
- * one-by-one and dynamically allocating this could result in a failure.
- */
-static struct cpumask cpuhp_online_new;
-
-static void blk_mq_queue_reinit_work(void)
-{
- struct request_queue *q;
-
- mutex_lock(&all_q_mutex);
- /*
- * We need to freeze and reinit all existing queues. Freezing
- * involves synchronous wait for an RCU grace period and doing it
- * one by one may take a long time. Start freezing all queues in
- * one swoop and then wait for the completions so that freezing can
- * take place in parallel.
- */
- list_for_each_entry(q, &all_q_list, all_q_node)
- blk_freeze_queue_start(q);
- list_for_each_entry(q, &all_q_list, all_q_node)
- blk_mq_freeze_queue_wait(q);
-
- list_for_each_entry(q, &all_q_list, all_q_node)
- blk_mq_queue_reinit(q, &cpuhp_online_new);
-
- list_for_each_entry(q, &all_q_list, all_q_node)
- blk_mq_unfreeze_queue(q);
-
- mutex_unlock(&all_q_mutex);
-}
-
-static int blk_mq_queue_reinit_dead(unsigned int cpu)
-{
- cpumask_copy(&cpuhp_online_new, cpu_online_mask);
- blk_mq_queue_reinit_work();
- return 0;
-}
-
-/*
- * Before hotadded cpu starts handling requests, new mappings must be
- * established. Otherwise, these requests in hw queue might never be
- * dispatched.
- *
- * For example, there is a single hw queue (hctx) and two CPU queues (ctx0
- * for CPU0, and ctx1 for CPU1).
- *
- * Now CPU1 is just onlined and a request is inserted into ctx1->rq_list
- * and set bit0 in pending bitmap as ctx1->index_hw is still zero.
- *
- * And then while running hw queue, blk_mq_flush_busy_ctxs() finds bit0 is set
- * in pending bitmap and tries to retrieve requests in hctx->ctxs[0]->rq_list.
- * But htx->ctxs[0] is a pointer to ctx0, so the request in ctx1->rq_list is
- * ignored.
- */
-static int blk_mq_queue_reinit_prepare(unsigned int cpu)
-{
- cpumask_copy(&cpuhp_online_new, cpu_online_mask);
- cpumask_set_cpu(cpu, &cpuhp_online_new);
- blk_mq_queue_reinit_work();
- return 0;
-}
-
static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
{
int i;
@@ -2678,7 +2594,7 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)
blk_mq_update_queue_map(set);
list_for_each_entry(q, &set->tag_list, tag_set_list) {
blk_mq_realloc_hw_ctxs(set, q);
- blk_mq_queue_reinit(q, cpu_online_mask);
+ blk_mq_queue_reinit(q);
}

list_for_each_entry(q, &set->tag_list, tag_set_list)
@@ -2887,24 +2803,10 @@ bool blk_mq_poll(struct request_queue *q, blk_qc_t cookie)
}
EXPORT_SYMBOL_GPL(blk_mq_poll);

-void blk_mq_disable_hotplug(void)
-{
- mutex_lock(&all_q_mutex);
-}
-
-void blk_mq_enable_hotplug(void)
-{
- mutex_unlock(&all_q_mutex);
-}
-
static int __init blk_mq_init(void)
{
cpuhp_setup_state_multi(CPUHP_BLK_MQ_DEAD, "block/mq:dead", NULL,
blk_mq_hctx_notify_dead);
-
- cpuhp_setup_state_nocalls(CPUHP_BLK_MQ_PREPARE, "block/mq:prepare",
- blk_mq_queue_reinit_prepare,
- blk_mq_queue_reinit_dead);
return 0;
}
subsys_initcall(blk_mq_init);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index cc67b48e3551..558df56544d2 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -56,11 +56,6 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
bool at_head);
void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
struct list_head *list);
-/*
- * CPU hotplug helpers
- */
-void blk_mq_enable_hotplug(void);
-void blk_mq_disable_hotplug(void);

/*
* CPU -> queue mappings
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index c15f22c54535..7f815d915977 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -58,7 +58,6 @@ enum cpuhp_state {
CPUHP_XEN_EVTCHN_PREPARE,
CPUHP_ARM_SHMOBILE_SCU_PREPARE,
CPUHP_SH_SH3X_PREPARE,
- CPUHP_BLK_MQ_PREPARE,
CPUHP_NET_FLOW_PREPARE,
CPUHP_TOPOLOGY_PREPARE,
CPUHP_NET_IUCV_PREPARE,
--
2.11.0
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