Re: per-cpu thoughts

["Björn Töpel" <bjorn.topel@gmail.com>]

On Mon, 11 Mar 2019 at 16:51, Christopher Lameter <cl@linux.com> wrote:
>
> On Mon, 11 Mar 2019, Bjï¿œrn Tï¿œpel wrote:
>
> > > > Hmm, you mean AMO *with* preempt_{dis,en}able, right? (No, interrupt
> > > > disable, only preemption.)
> > >
> > > If you disable preemption then you dont need AMO anymore. In fact that is
> > > the default fallback generic implementation. Just dont do anything and you
> > > already have that solution.
> >
> > But the generic one disables interrupts, right?
>
> No it doesnt.
>

Ok, please educate me what I'm missing here.

From asm-generic/percpu.h:

#define this_cpu_generic_to_op(pcp, val, op)                \
do {                                    \
    unsigned long __flags;                        \
    raw_local_irq_save(__flags);                    \
    raw_cpu_generic_to_op(pcp, val, op);                \
    raw_local_irq_restore(__flags);                    \
} while (0)

And the disassembly from my build does indeed have interrupt disabled regions.

Sorry for being a bit slow here. :-)


Björn

> > I believe the rational for RV is similar to ARM's; AMO+preemption
> > disable regions is *slightly* better than the generic, but not as good
> > as the IA one. Or am I missing something?>
>
> Looks to me that the better solution for  ARM may be to drop its
> implementation and revert to the generic one.
>
> The overhead of AMOs is likely too high to justify using those. And the
> LC/SC overhead is also generating useless overhead.
>
> If you benchmark those then please be aware that the kernel uses
> __this_xxx ops in places where the preempt disable is not needed. The
> savings come in the form of this_xxx ops where we avoid the overhead of
> enabling and disabling preemption (which can be significant especialy
> given the scheduler actions).

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv