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

[Avri Altman]

> > 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.
Btw, Is this performance improvement is on top of rq_affinity 2 or 1?

Thanks,
Avri

> >
> > 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
> >
> > --
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> > Forum,
> > Linux Foundation Collaborative Project