Messages in this thread | | | Date | Tue, 25 May 2021 09:40:18 +0800 | From | Can Guo <> | Subject | Re: [PATCH v1 2/3] scsi: ufs: Optimize host lock on transfer requests send/compl paths |
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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.
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