Re: [RFC][PATCH 1/2] libnvdimm: Introduce ND_CMD_GET_STAT to retrieve nvdimm statistics

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 07, 2020 at 04:54:21PM -0800, Dan Williams wrote:
> [ add perf maintainers ]
> 
> On Sun, Nov 8, 2020 at 1:16 PM Vaibhav Jain <vaibhav@linux.ibm.com> wrote:
> >
> > Implement support for exposing generic nvdimm statistics via newly
> > introduced dimm-command ND_CMD_GET_STAT that can be handled by nvdimm
> > command handler function and provide values for these statistics back
> > to libnvdimm. Following generic nvdimm statistics are defined as an
> > enumeration in 'uapi/ndctl.h':
> >
> > * "media_reads" : Number of media reads that have occurred since reboot.
> > * "media_writes" : Number of media writes that have occurred since reboot.
> > * "read_requests" : Number of read requests that have occurred since reboot.
> > * "write_requests" : Number of write requests that have occurred since reboot.
> 
> Perhaps document these as "since device reset"? As I can imagine some
> devices might have a mechanism to reset the count outside of "reboot"
> which is a bit ambiguous.
> 
> > * "total_media_reads" : Total number of media reads that have occurred.
> > * "total_media_writes" : Total number of media writes that have occurred.
> > * "total_read_requests" : Total number of read requests that have occurred.
> > * "total_write_requests" : Total number of write requests that have occurred.
> >
> > Apart from ND_CMD_GET_STAT ioctl these nvdimm statistics are also
> > exposed via sysfs '<nvdimm-device>/stats' directory for easy user-space
> > access like below:
> >
> > /sys/class/nd/ndctl0/device/nmem0/stats # tail -n +1 *
> > ==> media_reads <==
> > 252197707602
> > ==> media_writes <==
> > 20684685172
> > ==> read_requests <==
> > 658810924962
> > ==> write_requests <==
> > 404464081574
> 
> Hmm, I haven't looked but how hard would it be to plumb these to be
> perf counter-events. So someone could combine these with other perf
> counters?
> 
> > In case a specific nvdimm-statistic is not supported than nvdimm
> > command handler function can simply return an error (e.g -ENOENT) for
> > request to read that nvdimm-statistic.
> 
> Makes sense, but I expect the perf route also has a way to enumerate
> which statistics / counters are supported. I'm not opposed to also
> having them in sysfs, but I think perf support should be a first class
> citizen.

arch/x86/events/msr.c might be a good starting point for a software pmu
delivering pure counters.