Re: [RFC perf/core 05/11] uprobes: Add mapping for optimized uprobe trampolines

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 21, 2024 at 08:02:12AM -0800, Alexei Starovoitov wrote:
> On Thu, Nov 21, 2024 at 4:17 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Nov 20, 2024 at 04:07:38PM -0800, Andrii Nakryiko wrote:
> >
> > > USDTs are meant to be "transparent" to the surrounding code and they
> > > don't mark any clobbered registers. Technically it could be added, but
> > > I'm not a fan of this.
> >
> > Sure. Anyway, another thing to consider is FRED, will all of this still
> > matter once that lands? If FRED gets us INT3 performance close to what
> > SYSCALL has, then all this work will go unused.
> 
> afaik not a single cpu in the datacenter supports FRED while
> uprobe overhead is real.
> imo it's worth improving performance today for existing cpus.

I understand, but OTOH adding a syscall now, that we'll have to maintain
for years and years, even through we know it'll not be used much is a
bit annoying.

> I suspect arm64 might benefit too. Even if arm hw does the same
> amount of work for trap vs syscall the sw overhead of handling
> trap is different.

Well, the RISC CPUs have a much harder time using this, their immediate
range is typically puny and they end up needing multiple instructions
and some register in order to set up a call.

Elsewhere in the thread Mark Rutland already noted that arm64 really
doesn't need or want this.