Re: [PATCH v6 2/2] drivers/perf: hisi: Add driver for HiSilicon PCIe PMU

[Will Deacon <will@kernel.org>]

Hi,

On Wed, Jun 16, 2021 at 09:54:23AM +0800, liuqi (BA) wrote:
> On 2021/6/15 17:35, Will Deacon wrote:
> > On Tue, Jun 15, 2021 at 04:57:09PM +0800, liuqi (BA) wrote:
> > > On 2021/6/12 0:23, Will Deacon wrote:
> > > > On Mon, May 31, 2021 at 09:32:31PM +0800, Qi Liu wrote:
> > > > > +	/* Process data to set unit of latency as "us". */
> > > > > +	if (is_latency_event(idx))
> > > > > +		return div64_u64(data * us_per_cycle, data_ext);
> > > > > +
> > > > > +	if (is_bus_util_event(idx))
> > > > > +		return div64_u64(data * us_per_cycle, data_ext);
> > > > > +
> > > > > +	if (is_buf_util_event(idx))
> > > > > +		return div64_u64(data, data_ext * us_per_cycle);
> > > > 
> > > > Why do we need to do all this division in the kernel? Can't we just expose
> > > > the underlying values and let userspace figure out what it wants to do with
> > > > the numbers?
> > > > 
> > > Our PMU hardware support 8 sets of counters to count bandwidth, latency and
> > > utilization events.
> > > 
> > > For example, when users set latency event, common counter will count delay
> > > cycles, and extern counter count number of PCIe packets automaticly. And we
> > > do not have a event number for counting number of PCIe packets.
> > > 
> > > So this division cannot move to userspace tool.
> > 
> > Why can't you expose the packet counter as an extra event to userspace?
> > 
> Maybe I didn’t express it clearly.
> 
> As there is no hardware event number for PCIe packets counting, extern
> counter count packets *automaticly* when latency events is selected by
> users.
> 
> This means users cannot set "config=0xXX" to start packets counting event.
> So we can only get the value of counter and extern counter in driver and do
> the division, then pass the result to userspace.

I still think it would be ideal if we could expose both values to userspace
rather than combine them somehow. Hmm. Anyway...

I struggled to figure out exactly what's being counted from the
documentation patch (please update that). Please can you explain exactly
what appears in the HISI_PCIE_CNT and HISI_PCIE_EXT_CNT registers for the
different modes of operation? Without that, the ratios you've chosen to
report seem rather arbitrary.

I also couldn't figure out how the latency event works. For example, I was
assuming it would be a filter (a bit like the length), so you could say
things like "I'm only interested in packets with a latency higher than x"
but it doesn't look like it works that way.

Thanks,

Will