linux-kernel - Re: [PATCH] xfs: optimise xfs_mod

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Date:   Tue, 5 Nov 2019 11:08:01 +0800
From:   Shaokun Zhang <zhangshaokun@...ilicon.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     <linux-xfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Yang Guo <guoyang2@...wei.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Subject: Re: [PATCH] xfs: optimise xfs_mod_icount/ifree when delta < 0

Hi Christoph,

On 2019/11/4 23:25, Christoph Hellwig wrote:
> On Mon, Nov 04, 2019 at 07:29:40PM +0800, Shaokun Zhang wrote:
>> From: Yang Guo <guoyang2@...wei.com>
>>
>> percpu_counter_compare will be called by xfs_mod_icount/ifree to check
>> whether the counter less than 0 and it is a expensive function.
>> let's check it only when delta < 0, it will be good for xfs's performance.
> 
> How much overhead do you see?  In the end the compare is just a debug

Thanks your reply, sorry for my not clear description.
__percpu_counter_compare itself is not expensive, but __percpu_counter_sum
called by __percpu_counter_compare is high load, I will list it in next thread.

> check, so if it actually shows up we should remove it entirely.
> 

I'm not sure about it, so I check the delta to do less modification.

Thanks,
Shaokun

>