Re: [PATCH v5 1/3] blkcg: Introduce blkg_root_lookup()

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On Thu, 2018-08-09 at 12:56 -0700, Tejun Heo wrote:
+AD4- Hello,
+AD4- 
+AD4- On Thu, Aug 09, 2018 at 07:53:36AM -0700, Bart Van Assche wrote:
+AD4- +AD4- +-/+ACoAKg-
+AD4- +AD4- +- +ACo- blkg+AF8-lookup - look up blkg for the specified request queue
+AD4- +AD4- +- +ACo- +AEA-q: request+AF8-queue of interest
+AD4- +AD4- +- +ACo-
+AD4- +AD4- +- +ACo- Lookup blkg for +AEA-q at the root level. See also blkg+AF8-lookup().
+AD4- +AD4- +- +ACo-/
+AD4- +AD4- +-static inline struct blkcg+AF8-gq +ACo-blkg+AF8-root+AF8-lookup(struct request+AF8-queue +ACo-q)
+AD4- +AD4- +-+AHs-
+AD4- +AD4- +-	struct blkcg+AF8-gq +ACo-blkg+ADs-
+AD4- +AD4- +-
+AD4- +AD4- +-	rcu+AF8-read+AF8-lock()+ADs-
+AD4- +AD4- +-	blkg +AD0- blkg+AF8-lookup(+ACY-blkcg+AF8-root, q)+ADs-
+AD4- +AD4- +-	rcu+AF8-read+AF8-unlock()+ADs-
+AD4- +AD4- +-
+AD4- +AD4- +-	return blkg+ADs-
+AD4- +AD4- +-+AH0-
+AD4- 
+AD4- So, root blkg is always available as long as the request+AF8-queue is
+AD4- alive.  Sth like the following would be simpler?
+AD4- 
+AD4- static inline struct blkcg+AF8-gq +ACo-blk+AF8-queue+AF8-root+AF8-blkg(struct request+AF8-queue +ACo-q)
+AD4- +AHs-
+AD4- 	return q-+AD4-root+AF8-blkg+ADs-
+AD4- +AH0-

That would not only be simpler, it would also avoid that blk+AF8-queue+AF8-root+AF8-blkg()
returns NULL if bypass mode is enabled and the request queue is still associated
with the block cgroup controller. How do you want to proceed with this change?

Bart.




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