[RFC PATCH 2/2] powerpc: nop trap instruction after WARN_ONCE fires

[Christophe Leroy]

Le 23/09/2022 à 17:41, Nicholas Piggin a écrit :
> WARN_ONCE and similar are often used in frequently executed code, and
> should not crash the system. The program check interrupt caused by
> WARN_ON_ONCE can be a significant overhead even when nothing is being
> printed. This can cause performance to become unacceptable, having the
> same effective impact to the user as a BUG_ON().
> 
> Avoid this overhead by patching the trap with a nop instruction after a
> "once" trap fires. Conditional warnings that return a result must have
> equivalent compare and branch instructions after the trap, so when it is
> nopped the statement will behave the same way. It's possible the asm
> goto should be removed entirely and this comparison just done in C now.

You mean, just like PPC32 ? (Since db87a7199229 ("powerpc/bug: Remove 
specific powerpc BUG_ON() and WARN_ON() on PPC32"))

But I'm having hard time with your change.

You change only WARN_ON()
But WARN_ON_ONCE() calls __WARN_FLAGS()
And WARN_ONCE() calls WARN() via DO_ONCE_LITE_IF()

So I don't see any ..._ONCE something going with WARN_ON().

Am I missing something ?

Otherwise, what you want to do is to patch the 'twi' in __WARN_FLAGS() 
by a non conditional branch to __label_warn_on .



Christophe