Re: [GIT PULL] Introduce try_alloc_pages for 6.15

[Alexei Starovoitov]

On Sun, Mar 30, 2025 at 3:08 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sun, 30 Mar 2025 at 14:30, Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > But to avoid being finger pointed, I'll switch to checking alloc_flags
> > first. It does seem a better trade off to avoid cache bouncing because
> > of 2nd cmpxchg. Though when I wrote it this way I convinced myself and
> > others that it's faster to do trylock first to avoid branch misprediction.
>
> Yes, the really hot paths (ie core locking) do the "trylock -> read
> spinning" for that reason. Then for the normal case, _only_ the
> trylock is in the path, and that's the best of both worlds.
>
> And in practice, the "do two compare-and-exchange" operations actually
> does work fine, because the cacheline will generally be sticky enough
> that you don't actually get many extra cachline bouncing.

Right, but I also realized that in the contended case there is
an unnecessary irq save/restore pair.
Posted the fix:
https://lore.kernel.org/bpf/20250331002809.94758-1-alexei.starovoitov@xxxxxxxxx/

maybe apply directly?

I'll send the renaming fix once we converge on a good name.