Re: [PATCH] x86/fred: Optimize the FRED entry by prioritizing high-probability event dispatching

[Ethan Zhao <etzhao@xxxxxxxxxxx>]

On 1/16/2025 11:08 PM, Nikolay Borisov wrote:
> On 16.01.25 г. 8:51 ч., Ethan Zhao wrote:
> > External interrupts (EVENT_TYPE_EXTINT) and system calls 
> > (EVENT_TYPE_OTHER)
> > occur more frequently than other events in a typical system. 
> > Prioritizing
> > these events saves CPU cycles and optimizes the efficiency of 
> > performance-
> > critical paths.
> <snip>
>
> Can you also include some performance numbers?

Of coz will do when timing is good :)

Thanks,
Ethan

> > Signed-off-by: Ethan Zhao <haifeng.zhao@xxxxxxxxxxxxxxx>
> > ---
> > base commit: 619f0b6fad524f08d493a98d55bac9ab8895e3a6
> > ---
> >   arch/x86/entry/entry_fred.c | 25 +++++++++++++++++++------
> >   1 file changed, 19 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
> > index f004a4dc74c2..591f47771ecf 100644
> > --- a/arch/x86/entry/entry_fred.c
> > +++ b/arch/x86/entry/entry_fred.c
> > @@ -228,9 +228,18 @@ __visible noinstr void 
> > fred_entry_from_user(struct pt_regs *regs)
> >       /* Invalidate orig_ax so that syscall_get_nr() works correctly */
> >       regs->orig_ax = -1;
> >   -    switch (regs->fred_ss.type) {
> > -    case EVENT_TYPE_EXTINT:
> > +    if (regs->fred_ss.type == EVENT_TYPE_EXTINT)
> >           return fred_extint(regs);
> > +    else if (regs->fred_ss.type == EVENT_TYPE_OTHER)
> > +        return fred_other(regs);
> > +
> > +    /*
> > +     * Dispatch EVENT_TYPE_EXTINT and EVENT_TYPE_OTHER(syscall) type 
> > events
> > +     * first due to their high probability and let the compiler 
> > create binary search
> > +     * dispatching for the remaining events
> > +     */
>
> nit: At least to me it makes sense to have the comment above the 'if' 
> so that the flow is linear.
>
> > +
> > +    switch (regs->fred_ss.type) {
> >       case EVENT_TYPE_NMI:
> >           if (likely(regs->fred_ss.vector == X86_TRAP_NMI))
> >               return fred_exc_nmi(regs);
> > @@ -245,8 +254,6 @@ __visible noinstr void 
> > fred_entry_from_user(struct pt_regs *regs)
> >           break;
> >       case EVENT_TYPE_SWEXC:
> >           return fred_swexc(regs, error_code);
> > -    case EVENT_TYPE_OTHER:
> > -        return fred_other(regs);
> >       default: break;
> >       }
> >   @@ -260,9 +267,15 @@ __visible noinstr void 
> > fred_entry_from_kernel(struct pt_regs *regs)
> >       /* Invalidate orig_ax so that syscall_get_nr() works correctly */
> >       regs->orig_ax = -1;
> >   -    switch (regs->fred_ss.type) {
> > -    case EVENT_TYPE_EXTINT:
> > +    if (regs->fred_ss.type == EVENT_TYPE_EXTINT)
> >           return fred_extint(regs);
> > +
> > +    /*
> > +     * Dispatch EVENT_TYPE_EXTINT type event first due to its high 
> > probability
> > +     * and let the compiler do binary search dispatching for the 
> > other events
> > +     */
> > +
> > +    switch (regs->fred_ss.type) {
> >       case EVENT_TYPE_NMI:
> >           if (likely(regs->fred_ss.vector == X86_TRAP_NMI))
> >               return fred_exc_nmi(regs);