linux-kernel - Re: [PATCH] net/mlx4_en: ensure rx_desc updating reaches HW before prod db updating

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Message-ID: <918db4ec-8c3c-aafa-4be6-0e00a99632e2@mellanox.com>
Date:   Thu, 25 Jan 2018 11:54:31 +0200
From:   Tariq Toukan <tariqt@...lanox.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Cc:     junxiao.bi@...cle.com, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: [PATCH] net/mlx4_en: ensure rx_desc updating reaches HW before
 prod db updating



On 25/01/2018 8:25 AM, jianchao.wang wrote:
> Hi Eric
> 
> Thanks for you kindly response and suggestion.
> That's really appreciated.
> 
> Jianchao
> 
> On 01/25/2018 11:55 AM, Eric Dumazet wrote:
>> On Thu, 2018-01-25 at 11:27 +0800, jianchao.wang wrote:
>>> Hi Tariq
>>>
>>> On 01/22/2018 10:12 AM, jianchao.wang wrote:
>>>>>> On 19/01/2018 5:49 PM, Eric Dumazet wrote:
>>>>>>> On Fri, 2018-01-19 at 23:16 +0800, jianchao.wang wrote:
>>>>>>>> Hi Tariq
>>>>>>>>
>>>>>>>> Very sad that the crash was reproduced again after applied the patch.
>>>>>
>>>>> Memory barriers vary for different Archs, can you please share more details regarding arch and repro steps?
>>>> The hardware is HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 12/27/2015
>>>> The xen is installed. The crash occurred in DOM0.
>>>> Regarding to the repro steps, it is a customer's test which does heavy disk I/O over NFS storage without any guest.
>>>>
>>>
>>> What is the finial suggestion on this ?
>>> If use wmb there, is the performance pulled down ?

I want to evaluate this effect.
I agree with Eric, expected impact is restricted, especially after 
batching the allocations.

>>
>> Since https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_pub_scm_linux_kernel_git_davem_net-2Dnext.git_commit_-3Fid-3Ddad42c3038a59d27fced28ee4ec1d4a891b28155&d=DwICaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=c0oI8duFkyFBILMQYDsqRApHQrOlLY_2uGiz_utcd7s&s=E4_XKmSI0B63qB0DLQ1EX_fj1bOP78ZdeYADBf33B-k&e=
>>
>> we batch allocations, so mlx4_en_refill_rx_buffers() is not called that often.
>>
>> I doubt the additional wmb() will have serious impact there.
>>

I will test the effect (it'll be beginning of next week).
I'll update so we can make a more confident decision.

Thanks,
Tariq

>>
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