Linux Virtualization - Apr 2019 - [PATCH v5 1/6] libnvdimm: nd_region flush callback support

Jan Kara <jack at suse.cz> writes:
> On Thu 11-04-19 07:51:48, Dan Williams wrote:
>> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta at
redhat.com> wrote:
>> > +       } else {
>> > +               if (nd_region->flush(nd_region))
>> > +                       rc = -EIO;
>> 
>> Given the common case wants to be fast and synchronous I think we
>> should try to avoid retpoline overhead by default. So something like
>> this:
>> 
>> if (nd_region->flush == generic_nvdimm_flush)
>>     rc = generic_nvdimm_flush(...);
>
> 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.

-Jeff

Hello,

Thank you for the suggestions on this.
> 
> > On Thu 11-04-19 07:51:48, Dan Williams wrote:
> >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta at
redhat.com> wrote:
> >> > +       } else {
> >> > +               if (nd_region->flush(nd_region))
> >> > +                       rc = -EIO;
> >> 
> >> Given the common case wants to be fast and synchronous I think we
> >> should try to avoid retpoline overhead by default. So something
like
> >> this:
> >> 
> >> if (nd_region->flush == generic_nvdimm_flush)
> >>     rc = generic_nvdimm_flush(...);
> >
> > 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.
Can we add the optimization after this version is merged.

Best regards,
Pankaj 
> 
> -Jeff
> 
>

On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer <jmoyer at redhat.com>
wrote:
>
> Jan Kara <jack at suse.cz> writes:
>
> > On Thu 11-04-19 07:51:48, Dan Williams wrote:
> >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta at
redhat.com> wrote:
> >> > +       } else {
> >> > +               if (nd_region->flush(nd_region))
> >> > +                       rc = -EIO;
> >>
> >> Given the common case wants to be fast and synchronous I think we
> >> should try to avoid retpoline overhead by default. So something
like
> >> this:
> >>
> >> if (nd_region->flush == generic_nvdimm_flush)
> >>     rc = generic_nvdimm_flush(...);
> >
> > 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/

Dan Williams <dan.j.williams at intel.com> writes:
> On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer <jmoyer at redhat.com>
wrote:
>>
>> Jan Kara <jack at suse.cz> writes:
>>
>> > On Thu 11-04-19 07:51:48, Dan Williams wrote:
>> >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta at
redhat.com> wrote:
>> >> > +       } else {
>> >> > +               if (nd_region->flush(nd_region))
>> >> > +                       rc = -EIO;
>> >>
>> >> Given the common case wants to be fast and synchronous I think
we
>> >> should try to avoid retpoline overhead by default. So
something like
>> >> this:
>> >>
>> >> if (nd_region->flush == generic_nvdimm_flush)
>> >>     rc = generic_nvdimm_flush(...);
>> >
>> > 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/
The technique is fine, but that doesn't mean it should be applied
everywhere.  Is *this* code path really going to benefit from the
optimization?

-Jeff

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.