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.
> 
> 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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