Messages in this thread | | | Subject | Re: [PATCH net-next v2 1/4] indirect call wrappers: helpers to speed-up indirect calls of builtin | From | Paolo Abeni <> | Date | Tue, 11 Dec 2018 23:28:32 +0100 |
| |
Hi,
I'm sorry for the long delay, I was (and still I am) diverted by some other duty.
On Fri, 2018-12-07 at 21:46 +0000, David Woodhouse wrote: > On Fri, 2018-12-07 at 21:46 +0100, Paolo Abeni wrote: > > > I wonder if we can declare the common case functions as 'weak' so that > > > the link failures don't happen when they're absent. > > > > I experimented a previous version with alias. I avoided weak alias > > usage, because I [mis?]understood not all compilers have a complete > > support for them (e.g. clang). > > Also, with weak ref, a coding error that is now discovered at build > > time will result in worse performance at runtime, likely with some > > uncommon configuration, possibly not as easily detected. I'm unsure > > that would be better ?!? > > I think everything supports weak linkage; we've been using it for > years.
Ok, I likely was confused by some old, non first-hand info.
Anyway weak alias will turn a compile time issue in a possible run-time (small) regression. I think the first option would be preferable.
> > I'm sorry, I don't follow here. I think static keys can't be used for > > the reported network case: we have different list elements each > > contaning a different function pointer and we access/use > > different ptr on a per packet basis. > > Yes, the alternatives would be used to change the "likely" case. > > We still do the "if (fn == default_fn) default_fn(); else (*fn)();" > part; or even the variant with two (or more) common cases. > > It's just that the value of 'default_fn' can be changed at runtime > (with patching like alternatives/static keys, since of course it has to > be a direct call).
Thanks for clarifying. If I understood correctly, you would like some helper for:
if (static_branch_likely(&use_default_fn_a)) INDIRECT_CALL_1(f, default_fn_a, <args>) else if (static_branch_likely(&use_default_fn_b)) INDIRECT_CALL_1(f, default_fn_b, <args>) // ...
if so, I think we can eventually add support for this kind of stuff on top of the proposed macros. WDYT?
Thanks,
Paolo
| |