netdev - Re: [PATCH net-next 1/2] net/smc: Introduce a specific sysctl for TEST

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Date:   Tue, 20 Sep 2022 14:23:09 +0800
From:   Wen Gu <guwen@...ux.alibaba.com>
To:     dust.li@...ux.alibaba.com, kgraul@...ux.ibm.com,
        wenjia@...ux.ibm.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] net/smc: Introduce a specific sysctl for
 TEST_LINK time



On 2022/9/20 12:55, dust.li wrote:

> On Tue, Sep 20, 2022 at 10:53:54AM +0800, Wen Gu wrote:
>> SMC-R tests the viability of link by sending out TEST_LINK LLC
>> messages over RoCE fabric when connections on link have been
>> idle for a time longer than keepalive interval (testlink time).
>>
>> But using tcp_keepalive_time as testlink time maybe not quite
>> suitable because it is default no less than two hours[1], which
>> is too long for single link to find peer dead. The active host
>> will still use peer-dead link (QP) sending messages, and can't
>> find out until get IB_WC_RETRY_EXC_ERR error CQEs, which takes
>> more time than TEST_LINK timeout (SMC_LLC_WAIT_TIME) normally.
>>
>> So this patch introduces a independent sysctl for SMC-R to set
>> link keepalive time, in order to detect link down in time. The
>> default value is 30 seconds.
>>
>> [1] https://www.rfc-editor.org/rfc/rfc1122#page-101
>>
>> Signed-off-by: Wen Gu <guwen@...ux.alibaba.com>
>> ---

>> /* called after lgr was removed from lgr_list */
>> diff --git a/net/smc/smc_llc.h b/net/smc/smc_llc.h
>> index 4404e52..1de9a29 100644
>> --- a/net/smc/smc_llc.h
>> +++ b/net/smc/smc_llc.h
>> @@ -19,6 +19,7 @@
>>
>> #define SMC_LLC_WAIT_FIRST_TIME		(5 * HZ)
>> #define SMC_LLC_WAIT_TIME		(2 * HZ)
>> +#define SMC_LLC_TESTLINK_DEFAULT_TIME	30
> 
> I'm wondering why we don't follow the upper to macros using (30 * HZ) ?
> 
Thanks for the reivew.

Because the value of sysctl_smcr_testlink_time is in seconds, and the value
of llc_testlink_time is jiffies.

I have thought about
1) using proc_dointvec_jiffies as sysctl's proc_handler just like TCP does.
    But proc_dointvec_jiffies has no minimum limit, value 0 makes no sense for SMC testlink.
2) using proc_dointvec_ms_jiffies_minmax as proc_handler. But millisecond interval
    seems expensive for SMC test link.

So, I choose to use proc_dointvec_minmax, make sysctl_smcr_testlink_time in
seconds, and convert to jiffies when assigning to llc_testlink_time.

Thanks,
Wen Gu.