Re: [PATCH] fs/writeback: Attach inode's wb to root if needed

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Hello,

On Thu, May 09, 2019 at 04:03:53PM +0800, zhangliguang wrote:
> There might have tons of files queued in the writeback, awaiting for
> writing back. Unfortunately, the writeback's cgroup has been dead. In
> this case, we reassociate the inode with another writeback cgroup, but
> we possibly can't because the writeback associated with the dead cgroup
> is the only valid one. In this case, the new writeback is allocated,
> initialized and associated with the inode. It causes unnecessary high
> system load and latency.
> 
> This fixes the issue by enforce moving the inode to root cgroup when the
> previous binding cgroup becomes dead. With it, no more unnecessary
> writebacks are created, populated and the system load decreased by about
> 6x in the online service we encounted:
>     Without the patch: about 30% system load
>     With the patch:    about  5% system load

Can you please describe the scenario with more details?  I'm having a
bit of hard time understanding the amount of cpu cycles being
consumed.

Thanks.

-- 
tejun



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