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SubjectRe: [RFC PATCH] blk-cgroup: prevent rcu_sched detected stalls warnings in blkg_destroy_all()
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On 2020/11/25 20:32, Tejun Heo wrote:
> Hello,
>
> Thanks for the fix. A couple comments below.
>
> On Sat, Nov 21, 2020 at 04:34:20PM +0800, Yu Kuai wrote:
>> +#define BLKG_DESTROY_BATH 4096
>
> I think you meant BLKG_DESTROY_BATCH.
>
>> static void blkg_destroy_all(struct request_queue *q)
>> {
>> struct blkcg_gq *blkg, *n;
>> + int count = BLKG_DESTROY_BATH;
>
> But might as well just write 4096 here.
>
>> spin_lock_irq(&q->queue_lock);
>> list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
>> struct blkcg *blkcg = blkg->blkcg;
>>
>> + /*
>> + * If the list is too long, the loop can took a long time,
>> + * thus relese the lock for a while when a batch of blkcg
>> + * were destroyed.
>> + */
>> + if (!(--count)) {
>> + count = BLKG_DESTROY_BATH;
>> + spin_unlock_irq(&q->queue_lock);
>> + cond_resched();
>> + spin_lock_irq(&q->queue_lock);
>
> You can't continue iteration after dropping both locks. You'd have to jump
> out of loop and start list_for_each_entry_safe() again.

Thanks for your review, it's right. On the other hand
blkcg_activate_policy() and blkcg_deactivate_policy() might have the
same issue. My idea is that inserting a bookmark to the list, and
restard from here.

By the way, I found that blk_throtl_update_limit_valid() is called from
throtl_pd_offline(). If CONFIG_BLK_DEV_THROTTLING_LOW is off, lower
limit will always be zero, therefor a lot of time will be wasted to
iterate descendants to find a nonzero lower limit.

Do you think it's ok to do such modification:

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index b771c4299982..d52cac9f3a7c 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -587,6 +587,7 @@ static void throtl_pd_online(struct blkg_policy_data
*pd)
tg_update_has_rules(tg);
}

+#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
static void blk_throtl_update_limit_valid(struct throtl_data *td)
{
struct cgroup_subsys_state *pos_css;
@@ -607,6 +608,11 @@ static void blk_throtl_update_limit_valid(struct
throtl_data *td)

td->limit_valid[LIMIT_LOW] = low_valid;
}
+#else
+static inline void blk_throtl_update_limit_valid(struct throtl_data *td)
+{
+}
+#endif

static void throtl_upgrade_state(struct throtl_data *td);
static void throtl_pd_offline(struct blkg_policy_data *pd)
Thanks!
Yu Kuai

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