linux-kernel - RE: [PATCH v1 2/3] scsi: ufs: Optimize host lock on transfer requests send/compl paths

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Message-ID: <DM6PR04MB65752DD2F442C178B2D0233AFC259@DM6PR04MB6575.namprd04.prod.outlook.com>
Date:   Tue, 25 May 2021 08:24:30 +0000
From:   Avri Altman <Avri.Altman@....com>
To:     "Asutosh Das (asd)" <asutoshd@...eaurora.org>,
        Bart Van Assche <bvanassche@....org>,
        Can Guo <cang@...eaurora.org>,
        "nguyenb@...eaurora.org" <nguyenb@...eaurora.org>,
        "hongwus@...eaurora.org" <hongwus@...eaurora.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>
CC:     Stanley Chu <stanley.chu@...iatek.com>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Bean Huo <beanhuo@...ron.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kiwoong Kim <kwmad.kim@...sung.com>,
        Satya Tangirala <satyat@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>
Subject: RE: [PATCH v1 2/3] scsi: ufs: Optimize host lock on transfer requests
 send/compl paths

> 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.
I agree - this looks like the harbinger of a major change,
And going further with respect of hw queues,
will need the spec support - e.g. doorbell per lane, etc.

Thanks,
Avri
 
> > Thanks,
> >
> > Bart.
> >
> 
> 
> Thanks,
> -asd
> 
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
> Forum,
> Linux Foundation Collaborative Project