Re: [PATCH 16/23] sched: Use lightweight hazard pointers to grab lazy mms

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On Sat, Jan 8, 2022 at 9:56 PM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>
> Just wondering: In a world of ASID/PCID - does the “lazy TLB” really
> have a worthy advantage?
>
> Considering the fact that with PTI anyhow address spaces are switched
> all the time, can’t we just get rid of it?

Hmm.. That may indeed be the right thing to do.

I think arm64 already hardcodes ASID 0 to init_mm, and that kernel
threads (and the idle threads in particular) might as well just use
that. In that kind of situation, there's likely little advantage to
reusing a user address space ID, and quite possibly any potential
advantage is overshadowed by the costs.

The lazy tlb thing goes back a *looong* way, and lots of things have
changed since. Maybe it's not worth it any more.

Or maybe it's only worth it on platforms where it's free (UP, possibly
other situations - like if you have IPI and it's "free").

If I read the history correctly, it looks like PF_LAZY_TLB was
introduced in 2.3.11-pre4 or something. Back in summer of 1999. The
"active_mm" vs "mm" model came later.

                  Linus




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