linux-kernel - Re: [PATCH] blk-cgroup: Use cond

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Date:   Wed, 27 Jan 2021 09:37:46 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     axboe@...nel.dk, joseph.qi@...ux.alibaba.com,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-cgroup: Use cond_resched() when destroy blkgs

Hello, Baolin.

On Tue, Jan 26, 2021 at 09:33:25PM +0800, Baolin Wang wrote:
> On !PREEMPT kernel, we can get below softlockup when doing stress
> testing with creating and destroying block cgroup repeatly. The
> reason is it may take a long time to acquire the queue's lock in
> the loop of blkcg_destroy_blkgs(), thus we can use cond_resched()
> instead of cpu_relax() to avoid this issue, since the
> blkcg_destroy_blkgs() is not called from atomic contexts.
> 
> [ 4757.010308] watchdog: BUG: soft lockup - CPU#11 stuck for 94s!
> [ 4757.010698] Call trace:
> [ 4757.010700]  blkcg_destroy_blkgs+0x68/0x150
> [ 4757.010701]  cgwb_release_workfn+0x104/0x158
> [ 4757.010702]  process_one_work+0x1bc/0x3f0
> [ 4757.010704]  worker_thread+0x164/0x468
> [ 4757.010705]  kthread+0x108/0x138
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

* Can you please add might_sleep() at the top of the function?

* Given that the system can accumulate a huge number of blkgs in
  pathological cases, I wonder whether a better way to go about it is
  explicitly testing need_resched() on each loop and release locks and do
  cond_resched() if true?

Thanks.

-- 
tejun