Re: [PATCH v2 3/8] cxl/mem: Register CXL memX devices

[Dan Williams]

On Thu, Feb 11, 2021 at 2:19 AM Jonathan Cameron
<Jonathan.Cameron@xxxxxxxxxx> wrote:
>
> On Wed, 10 Feb 2021 18:17:25 +0000
> Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
>
> > On Tue, 9 Feb 2021 16:02:54 -0800
> > Ben Widawsky <ben.widawsky@xxxxxxxxx> wrote:
> >
> > > From: Dan Williams <dan.j.williams@xxxxxxxxx>
> > >
> > > Create the /sys/bus/cxl hierarchy to enumerate:
> > >
> > > * Memory Devices (per-endpoint control devices)
> > >
> > > * Memory Address Space Devices (platform address ranges with
> > >   interleaving, performance, and persistence attributes)
> > >
> > > * Memory Regions (active provisioned memory from an address space device
> > >   that is in use as System RAM or delegated to libnvdimm as Persistent
> > >   Memory regions).
> > >
> > > For now, only the per-endpoint control devices are registered on the
> > > 'cxl' bus. However, going forward it will provide a mechanism to
> > > coordinate cross-device interleave.
> > >
> > > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> >
> > One stray header, and a request for a tiny bit of reordering to
> > make it easier to chase through creation and destruction.
> >
> > Either way with the header move to earlier patch I'm fine with this one.
> >
> > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
>
> Actually thinking more on this, what is the justification for the
> complexity + overhead of a percpu_refcount vs a refcount

A typical refcount does not have the block and drain semantics of a
percpu_ref. I'm planning to circle back and make this a first class
facility of the cdev interface borrowing the debugfs approach [1], but
for now percpu_ref fits the bill locally.

> I don't think this is a high enough performance path for it to matter.
> Perhaps I'm missing a usecase where it does?

It's less about percpu_ref performance and more about the
percpu_ref_tryget_live() facility.

[1]: http://lore.kernel.org/r/CAPcyv4jEYPsyh0bhbtKGRbK3bgp=_+=2rjx4X0gLi5-25VvDyg@xxxxxxxxxxxxxx