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Subject: Re: [PATCH v1 2/3] scsi: ufs: Optimize host lock on transfer requests
 send/compl paths
To:     Bart Van Assche <bvanassche@acm.org>,
        Can Guo <cang@codeaurora.org>, nguyenb@codeaurora.org,
        hongwus@codeaurora.org, linux-scsi@vger.kernel.org,
        kernel-team@android.com
Cc:     Stanley Chu <stanley.chu@mediatek.com>,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Avri Altman <avri.altman@wdc.com>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Bean Huo <beanhuo@micron.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Kiwoong Kim <kwmad.kim@samsung.com>,
        Satya Tangirala <satyat@google.com>,
        open list <linux-kernel@vger.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-arm-kernel@lists.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@lists.infradead.org>
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 <1621845419-14194-3-git-send-email-cang@codeaurora.org>
 <41a08b3e-122d-4f1a-abbd-4b5730f880b2@acm.org>
From:   "Asutosh Das (asd)" <asutoshd@codeaurora.org>
Message-ID: <d4ff8e1a-f368-6720-798a-a2a31a4d41fb@codeaurora.org>
Date:   Mon, 24 May 2021 18:34:04 -0700
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On 5/24/2021 1:10 PM, Bart Van Assche wrote:
> On 5/24/21 1:36 AM, Can Guo wrote:
>> Current UFS IRQ handler is completely wrapped by host lock, and because
>> ufshcd_send_command() is also protected by host lock, when IRQ handler
>> fires, not only the CPU running the IRQ handler cannot send new requests,
>> the rest CPUs can neither. Move the host lock wrapping the IRQ handler into
>> specific branches, i.e., ufshcd_uic_cmd_compl(), ufshcd_check_errors(),
>> ufshcd_tmc_handler() and ufshcd_transfer_req_compl(). Meanwhile, to further
>> reduce occpuation of host lock in ufshcd_transfer_req_compl(), host lock is
>> no longer required to call __ufshcd_transfer_req_compl(). As per test, the
>> optimization can bring considerable gain to random read/write performance.
> 

> An additional question is whether it is necessary for v3.0 UFS devices
> to serialize the submission path against the completion path? Multiple
> high-performance SCSI LLDs support hardware with separate submission and
> completion queues and hence do not need any serialization between the
> submission and the completion path. I'm asking this because it is likely
> that sooner or later multiqueue support will be added in the UFS
> specification. Benefiting from multiqueue support will require to rework
> locking in the UFS driver anyway.
> 
Hi Bart,
No it's not necessary to serialize both the paths. I think this series 
attempts to remove this serialization to a certain degree, which is 
what's giving the performance improvement.

Even if multiqueue support would be available in the future, I think 
this change is apt now for the current available specification.

> Thanks,
> 
> Bart.
> 


Thanks,
-asd

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