Re: [PATCH] enforce function inlining for hot functions

[Hagen Paul Pfeifer]

On 25 April 2015 at 12:31, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> I am not arguing either way on the wisdom or lack thereof of gcc's
> inlining decisions.  But PROVE_RCU=n and CONFIG_DEBUG_LOCK_ALLOC=n should
> make rcu_read_lock() and rcu_read_unlock() both be empty functions in
> a CONFIG_PREEMPT=n, which should hopefully trivialize gcc's inlining
> decisions in that particular case.

Hey Paul,

yes, with DEBUG_LOCK_ALLOC disabled all rcu_read_lock and unlock
functions are perfectly inlined. So now we have the following
situation: depending on the gcc version and the particular kernel
configuration some hot functions are not inlined - they are duplicated
hundred times. Which is bad no matter how you consider
gcc/kernel-configuration. I think this should *never* happened.

With the patch we can make sure that hot functions are *always*
inlined - no matter what gcc version and kernel configuration is used.

Furthermore, as Markus already noted: compiled with -O2 this do not
happened. Duplicates are only generated for -Os[1]. Ok, but now the
question: should this happened for Os? I don't think so. I think we
can do it better and mark these few functions as always inline. For
the remaining inlined marked function we should provide gcc the
flexibility and do not artificially enforce inlining. The current
situation is bad: OPTIMIZE_INLINING is default no, which defacto
enforces inlining for *all* inlined marked functions. GCC inlining
mechanism is defacto disabled, which is also bad. Last but not least:
the patch do not change anything for the current user, because we will
still disable OPTIMIZE_INLINING (resulting in __always_inline for all
inlined marked functions). The patch effects users who enable
OPTIMIZE_INLINING and trust the compiler.

Hagen

PS: thank you Markus for the comment.

[1] which is nonsense: the functions are not inlined yet, but are
copied hundred times for "size optimized builds". gcc should rather
redeclare the functions global, define it one time and call this
function every time. But implementing such a scheme is probably a
monster of itself and LTO is required so solve all issues with such a
concept.
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