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Subject: Re: [PATCH -next 2/3] md/raid10: convert resync_lock to use seqlock
To:     Guoqing Jiang <guoqing.jiang@linux.dev>,
        Yu Kuai <yukuai1@huaweicloud.com>, song@kernel.org
Cc:     linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20220829131502.165356-1-yukuai1@huaweicloud.com>
 <20220829131502.165356-3-yukuai1@huaweicloud.com>
 <3d8859bc-80d6-08b0-fd40-8874df4d3419@linux.dev>
 <1891ec2c-0ccc-681e-31de-fdd28eebce82@huaweicloud.com>
 <82f11462-454c-4a5e-d3a2-e71479960eaf@linux.dev>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <01191595-f2f7-d953-fb00-e24803910515@huaweicloud.com>
Date:   Fri, 2 Sep 2022 18:53:56 +0800
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Hi,

在 2022/09/02 18:16, Guoqing Jiang 写道:
> 
> 
> On 9/2/22 6:02 PM, Yu Kuai wrote:
>> Hi,
>>
>> 在 2022/09/02 17:42, Guoqing Jiang 写道:
>>> Hi,
>>>
>>> On 8/29/22 9:15 PM, Yu Kuai wrote:
>>>> +static bool wait_barrier_nolock(struct r10conf *conf)
>>>> +{
>>>> +    unsigned int seq = raw_read_seqcount(&conf->resync_lock.seqcount);
>>>> +
>>>> +    if (seq & 1)
>>>> +        return false;
>>>> +
>>>> +    if (READ_ONCE(conf->barrier))
>>>> +        return false;
>>>> +
>>>> +    atomic_inc(&conf->nr_pending);
>>>> +    if (!read_seqcount_retry(&conf->resync_lock.seqcount, seq))
>>>
>>> I think 'seq' is usually get from read_seqcount_begin.
>>
>> read_seqcount_begin will loop untill "req & 1" failed, I'm afraid this
>> will cause high cpu usage in come cases.
>>
>> What I try to do here is just try once, and fall back to hold lock and
>> wait if failed.
> 
> Thanks for the explanation.
> 
> I'd suggest to try with read_seqcount_begin/read_seqcount_retry pattern
> because it is a common usage in kernel I think, then check whether the
> performance drops or not.  Maybe it is related to lockdep issue, but I am
> not sure.

I can try read_seqcount_begin/read_seqcount_retry.

Please take a look at another thread, lockdep issue is due to
inconsistent usage of lock and seqcount inside seqlock:

wait_event() only release lock, seqcount is not released.

Thansk,
Kuai
> 
> Thanks,
> Guoqing
> .
>