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Date:   Tue, 20 Sep 2022 16:21:53 +0800
From:   "dust.li" <dust.li@linux.alibaba.com>
To:     Wen Gu <guwen@linux.alibaba.com>, kgraul@linux.ibm.com,
        wenjia@linux.ibm.com, davem@davemloft.net, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com
Cc:     linux-s390@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next 1/2] net/smc: Introduce a specific sysctl for
 TEST_LINK time
Message-ID: <20220920082153.GD108825@linux.alibaba.com>
Reply-To: dust.li@linux.alibaba.com
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On Tue, Sep 20, 2022 at 02:23:09PM +0800, Wen Gu wrote:
>
>
>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@linux.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.

Maybe 0 means disable the LLC TEST LINK ?


>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.

If proc_dointvec_jiffies_minmax is really the problem, maybe you can
write your own proc handler.


Thanks