linux-kernel - Re: [PATCH -next 2/3] md/raid10: convert resync

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Date:   Fri, 2 Sep 2022 18:53:56 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Yu Kuai <yukuai1@...weicloud.com>, song@...nel.org
Cc:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next 2/3] md/raid10: convert resync_lock to use seqlock

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
> .
>