linux-kernel - Re: [PATCH v8 2/2] drivers/perf: hisi: Add driver for HiSilicon PCIe PMU

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Date:   Mon, 2 Aug 2021 11:03:43 +0100
From:   Will Deacon <will@...nel.org>
To:     Qi Liu <liuqi115@...wei.com>
Cc:     mark.rutland@....com, bhelgaas@...gle.com,
        linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        zhangshaokun@...ilicon.com
Subject: Re: [PATCH v8 2/2] drivers/perf: hisi: Add driver for HiSilicon PCIe
 PMU

On Wed, Jul 28, 2021 at 04:09:32PM +0800, Qi Liu wrote:
> PCIe PMU Root Complex Integrated End Point(RCiEP) device is supported
> to sample bandwidth, latency, buffer occupation etc.
> 
> Each PMU RCiEP device monitors multiple Root Ports, and each RCiEP is
> registered as a PMU in /sys/bus/event_source/devices, so users can
> select target PMU, and use filter to do further sets.
> 
> Filtering options contains:
> event        - select the event.
> subevent     - select the subevent.

Hmm, I was hoping that you would expose all the events as proper perf_events
and get rid of the subevents entirely.

Then userspace could do things like:

  // Count number of RX memory reads
  $ perf stat -e hisi_pcie0_0/rx_memory_read/

  // Count delay cycles
  $ perf stat -e hisi_pcie0_0/latency/

  // Count both of the above (events must be in the same group)
  $ perf stat -g -e hisi_pcie0_0/latency/ -e hisi_pcie0_0/rx_memory_read/

Note that in all three of these cases the hardware will be programmed in
the same way and both HISI_PCIE_CNT and HISI_PCIE_EXT_CNT are allocated!

So for example, doing this (i.e. without the '-g'):

  $ perf stat -e hisi_pcie0_0/latency/ -e hisi_pcie0_0/rx_memory_read/

would fail because the first event would allocate both of the counters.

All you need to do is check the counter scheduling constraints when
accepting an event group in the driver. No need for subevents at all.

Does that make sense?

Will