Hi Bart On 09/21/2018 01:06 AM, Bart Van Assche wrote: > On Thu, 2018-09-20 at 18:18 +0800, Jianchao Wang wrote: >> The current queue freeze depending on percpu_ref_kil/reinit has a limit that >> we have drain the requests before unfreeze the queue. >> >> Let's rework the queue freeze feature as following: >> 1. introduce __percpu_ref_get_many. >> It is same with original percpu_ref_get_many, but just need callers to provide >> sched rcu critical section. We will put the __percpu_ref_get_many and our own >> condition checking under rcu_read_lock_sched. With this new helper interface, >> we could save an extra rcu_read_lock_sched. >> 2. rework the blk_queue_enter as: >> >> rcu_read_lock_sched() >> if condition check true >> __percpu_ref_get_many(&q->q_usage_counter, 1) >> else >> goto wait >> rcu_read_unlock_sched() >> 3. use percpu_ref_switch_to_atomic/percpu to switch mode directly. >> >> Then we could unfreeze the queue w/o draining requests. >> In addition, preempt-only mode code could be simplified. > Hello Jianchao, > = > Thanks for having taken a look. However, the approach of this patch series > may be more complicated than necessary. Had you considered something like > the patch below? This patch is just a trying that let our life easier w/o changing anything in percpu-ref code. :) In fact, the 1st patch which introduces a new non-functional-changing helper interface in percpu-ref code is not necessary. This patchset just implement our own condition checking in blk_queue_enter instead of depending on __PERCPU_REF_DEAD checking in percpu_ref_tryget_live. Then we could do more thing here, like: - unfreeze the queue without draining requests. - check whether q->q_usage_counter is zero - add other gate conditions into blk_queue_enter, such as preempt-only, even any others. So why not try it ? Thanks Jianchao
