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

[Can Guo]

On 2021-05-25 04:10, 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.
>
> Hi Can,
>
> Using the host lock to serialize the completion path against the
> submission path was a common practice 11 years ago, before the host 
> lock
> push-down (see also
> https://linux-scsi.vger.kernel.narkive.com/UEmGgwAc/rfc-patch-scsi-host-lock-push-down).
> Modern SCSI LLDs should not use the SCSI host lock. Please consider
> introducing one or more new synchronization objects in struct ufs_hba
> and to use these instead of the SCSI host lock. That will save multiple
> pointer dereferences in the hot path since hba->host->host_lock will
> become hba->new_spin_lock.
>
> 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,

Agree with all above, and what you ask is right what we are doing in the
3rd change - get rid of host lock on dispatch and completion paths.

I agree with using dedicated spin locks for dedicated purposes in UFS 
driver,
e.g., clk gating has its own gating_lock and clk scaling has its own 
scaling_lock.
But this specific series is only for improving performance. We will take 
your
comments into consideration and address it in future.

Thanks,

Can Guo.

> Thanks,
>
> Bart.