Re: [PATCH] irqchip/gic-v3: do runtime cpu cap check only when necessary

[Puyou Lu]

On Sat, Aug 27, 2022 at 04:13:00PM +0100, Marc Zyngier wrote:
> On Sat, 27 Aug 2022 06:19:27 +0100,
> Puyou Lu <puyou.lu@xxxxxxxxx> wrote:
> > 
> > Now cpu cap check is done every exception happens on every arm64 platform,
> > but this check is necessary on just few of then, so we can drop this
> > check at compile time on others. This can decrease exception handle time
> > on most cases.
> > 
> > Fixes: 6d4e11c5e2e8 ("irqchip/gicv3: Workaround for Cavium ThunderX erratum 23154")
> > Signed-off-by: Puyou Lu <puyou.lu@xxxxxxxxx>
> > ---
> >  drivers/irqchip/irq-gic-v3.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> > index 262658fd5f9e..3f08c2ef1251 100644
> > --- a/drivers/irqchip/irq-gic-v3.c
> > +++ b/drivers/irqchip/irq-gic-v3.c
> > @@ -237,9 +237,11 @@ static void gic_redist_wait_for_rwp(void)
> >  
> >  static u64 __maybe_unused gic_read_iar(void)
> >  {
> > +#ifdef CONFIG_CAVIUM_ERRATUM_23154
> >  	if (cpus_have_const_cap(ARM64_WORKAROUND_CAVIUM_23154))
> >  		return gic_read_iar_cavium_thunderx();
> >  	else
> > +#endif
> >  		return gic_read_iar_common();
> >  }
> >  #endif
> 
> You realise that cpus_have_const_cap() results purely in a couple of
> branches once the caps have been finalised, right?
> 
> Please provide data showing that it actually "can decrease exception
> handle time on most cases", because I'm pretty sure you cannot measure
> the difference in any meaningful way.
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.

Hi Marc,
Thank you for the reply. Actually I did no test, just from the disassemble
code of vmlinux, I saw about 6 instruction generated by
cpus_have_const_cap, and about 36 by gic_read_iar_cavium_thunderx, which
is useless for most CPUs. I think this will waste some cpu cycles, as
exceptions can occur hunderds or thousands times per second. Also
(6+36)*4=168 bytes of icache is wasted, and icache misses increase
somewhere else.
If I got things wrong, please correct me.