From: Josef Bacik <josef@toxicpanda.com>
To: Dennis Zhou <dennisszhou@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
Johannes Weiner <hannes@cmpxchg.org>,
Josef Bacik <josef@toxicpanda.com>,
kernel-team@fb.com, linux-block@vger.kernel.org,
cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
Jiufei Xue <jiufei.xue@linux.alibaba.com>,
Joseph Qi <joseph.qi@linux.alibaba.com>
Subject: Re: [PATCH 02/15] blkcg: delay blkg destruction until after writeback has finished
Date: Fri, 31 Aug 2018 11:27:07 -0400 [thread overview]
Message-ID: <20180831152705.4mjm7xo6jq7ptdqn@destiny> (raw)
In-Reply-To: <20180831015356.69796-3-dennisszhou@gmail.com>
On Thu, Aug 30, 2018 at 09:53:43PM -0400, Dennis Zhou wrote:
> From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>
>
> Currently, blkcg destruction relies on a sequence of events:
> 1. Destruction starts. blkcg_css_offline() is called and blkgs
> release their reference to the blkcg. This immediately destroys
> the cgwbs (writeback).
> 2. With blkgs giving up their reference, the blkcg ref count should
> become zero and eventually call blkcg_css_free() which finally
> frees the blkcg.
>
> Jiufei Xue reported that there is a race between blkcg_bio_issue_check()
> and cgroup_rmdir(). To remedy this, blkg destruction becomes contingent
> on the completion of all writeback associated with the blkcg. A count of
> the number of cgwbs is maintained and once that goes to zero, blkg
> destruction can follow. This should prevent premature blkg destruction.
>
> The new process for blkcg cleanup is as follows:
> 1. Destruction starts. blkcg_css_offline() is called which offlines
> writeback. Blkg destruction is delayed on the nr_cgwbs count to
> avoid punting potentially large amounts of outstanding writeback
> to root while maintaining any ongoing policies.
> 2. When the nr_cgwbs becomes zero, blkcg_destroy_blkgs() is called and
> handles destruction of blkgs. This is where the css reference held
> by each blkg is released.
> 3. Once the blkcg ref count goes to zero, blkcg_css_free() is called.
> This finally frees the blkg.
>
> It seems in the past blk-throttle didn't do the most understandable
> things with taking data from a blkg while associating with current. So,
> the simplification and unification of what blk-throttle is doing caused
> this.
>
So the general approach is correct, but it's sort of confusing because you are
using nr_cgwbs as a reference counter, because it's set at 1 at blkg creation
time regardless of wether or not there's an assocated wb cg. So instead why not
just have a refcount_t ref, set it to 1 on creation and make the wb cg take a
ref when it's attached, and then just do the get/put like normal and cleanup as
you have below? What you are doing is a reference counter masquerading as a
count of the wb cg's, just add full ref counting to the blkcg and call it a day,
it'll be much less confusing. Thanks,
Josef
next prev parent reply other threads:[~2018-08-31 15:27 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-31 1:53 [PATCH 00/15] blkcg ref count refactor/cleanup + blkcg avg_lat Dennis Zhou
2018-08-31 1:53 ` [PATCH 01/15] Revert "blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()" Dennis Zhou
2018-08-31 1:53 ` [PATCH 02/15] blkcg: delay blkg destruction until after writeback has finished Dennis Zhou
2018-08-31 15:27 ` Josef Bacik [this message]
2018-08-31 20:19 ` Dennis Zhou
2018-08-31 1:53 ` [PATCH 03/15] blkcg: use tryget logic when associating a blkg with a bio Dennis Zhou
2018-08-31 15:30 ` Josef Bacik
2018-08-31 20:20 ` Dennis Zhou
2018-08-31 1:53 ` [PATCH 04/15] blkcg: fix ref count issue with bio_blkcg using task_css Dennis Zhou
2018-08-31 15:35 ` Josef Bacik
2018-08-31 23:04 ` Tejun Heo
2018-09-06 15:21 ` Dennis Zhou
2018-08-31 1:53 ` [PATCH 05/15] blkcg: update blkg_lookup_create to do locking Dennis Zhou
2018-08-31 15:37 ` Josef Bacik
2018-08-31 23:09 ` Tejun Heo
2018-08-31 1:53 ` [PATCH 06/15] blkcg: always associate a bio with a blkg Dennis Zhou
2018-08-31 9:01 ` kbuild test robot
2018-08-31 10:02 ` kbuild test robot
2018-08-31 23:16 ` Tejun Heo
2018-09-06 20:41 ` Dennis Zhou
2018-09-07 3:03 ` [LKP] [blkcg] c02c58dab2: WARNING:at_block/blk-throttle.c:#blk_throtl_bio kernel test robot
2018-08-31 1:53 ` [PATCH 07/15] blkcg: consolidate bio_issue_init and blkg association Dennis Zhou
2018-08-31 9:19 ` kbuild test robot
2018-08-31 11:11 ` kbuild test robot
2018-08-31 15:42 ` Josef Bacik
2018-09-06 20:43 ` Dennis Zhou
2018-08-31 23:45 ` Tejun Heo
2018-08-31 1:53 ` [PATCH 08/15] blkcg: associate a blkg for pages being evicted by swap Dennis Zhou
2018-08-31 15:44 ` Josef Bacik
2018-08-31 23:47 ` Tejun Heo
2018-08-31 1:53 ` [PATCH 09/15] blkcg: associate writeback bios with a blkg Dennis Zhou
2018-08-31 15:45 ` Josef Bacik
2018-08-31 23:53 ` Tejun Heo
2018-08-31 1:53 ` [PATCH 10/15] blkcg: remove bio->bi_css and instead use bio->bi_blkg Dennis Zhou
2018-08-31 15:46 ` Josef Bacik
2018-09-01 0:13 ` Tejun Heo
2018-08-31 1:53 ` [PATCH 11/15] blkcg: remove additional reference to the css Dennis Zhou
2018-09-01 0:26 ` Tejun Heo
2018-09-06 20:45 ` Dennis Zhou
2018-08-31 1:53 ` [PATCH 12/15] blkcg: cleanup and make blk_get_rl use blkg_lookup_create Dennis Zhou
2018-09-01 0:29 ` Tejun Heo
2018-09-11 2:37 ` [LKP] [blkcg] 22f657e287: general_protection_fault:#[##] kernel test robot
2018-08-31 1:53 ` [PATCH 13/15] blkcg: change blkg reference counting to use percpu_ref Dennis Zhou
2018-08-31 15:49 ` Josef Bacik
2018-09-01 0:31 ` Tejun Heo
2018-09-06 20:46 ` Dennis Zhou
2018-09-07 3:08 ` [LKP] [blkcg] 6ef69a3a0b: WARNING:suspicious_RCU_usage kernel test robot
2018-08-31 1:53 ` [PATCH 14/15] blkcg: rename blkg_try_get to blkg_tryget Dennis Zhou
2018-08-31 15:50 ` Josef Bacik
2018-09-01 0:32 ` Tejun Heo
2018-08-31 1:53 ` [PATCH 15/15] blkcg: add average latency tracking to blk-cgroup Dennis Zhou
2018-08-31 10:22 ` kbuild test robot
2018-08-31 11:38 ` kbuild test robot
2018-09-01 0:35 ` [PATCH 00/15] blkcg ref count refactor/cleanup + blkcg avg_lat Tejun Heo
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