Re: [Qemu-devel] [PATCH v5 1/6] libnvdimm: nd_region flush callback support

[Pankaj Gupta]

> 
> Dan Williams <address@hidden> writes:
> 
> > On Mon, Apr 22, 2019 at 8:59 AM Jeff Moyer <address@hidden> wrote:
> >>
> >> Dan Williams <address@hidden> writes:
> >>
> >> > On Thu, Apr 18, 2019 at 9:18 AM Christoph Hellwig <address@hidden>
> >> > wrote:
> >> >>
> >> >> On Thu, Apr 18, 2019 at 09:05:05AM -0700, Dan Williams wrote:
> >> >> > > > I'd either add a comment about avoiding retpoline overhead here
> >> >> > > > or just
> >> >> > > > make ->flush == NULL mean generic_nvdimm_flush(). Just so that
> >> >> > > > people don't
> >> >> > > > get confused by the code.
> >> >> > >
> >> >> > > Isn't this premature optimization?  I really don't like adding
> >> >> > > things
> >> >> > > like this without some numbers to show it's worth it.
> >> >> >
> >> >> > I don't think it's premature given this optimization technique is
> >> >> > already being deployed elsewhere, see:
> >> >> >
> >> >> > https://lwn.net/Articles/774347/
> >> >>
> >> >> For one this one was backed by numbers, and second after feedback
> >> >> from Linux we switched to the NULL pointer check instead.
> >> >
> >> > Ok I should have noticed the switch to NULL pointer check. However,
> >> > the question still stands do we want everyone to run numbers to
> >> > justify this optimization, or make it a new common kernel coding
> >> > practice to do:
> >> >
> >> >     if (!object->op)
> >> >         generic_op(object);
> >> >     else
> >> >         object->op(object);
> >> >
> >> > ...in hot paths?
> >>
> >> I don't think nvdimm_flush is a hot path.  Numbers of some
> >> representative workload would prove one of us right.
> >
> > I'd rather say that the if "if (!op) do_generic()" pattern is more
> > readable in the general case, saves grepping for who set the op in the
> > common case. The fact that it has the potential to be faster is gravy
> > at that point.
> 
> If the primary motivation is performance, then I'd expect performance
> numbers to back it up.  If that isn't the primary motivation, then
> choose whichever way you feel is appropriate.

Agree. This change enhances the code readability. Will add this change in
v6 with other changes.

Thank you! 

Pankaj

> 
> Cheers,
> Jeff
>