linux-kernel - Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

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Message-ID: <CA+55aFz6CfnVgGABpGZ4ywqaOyt2E3KFO9zY_H_nH1R=nria-A@mail.gmail.com>
Date:	Tue, 28 Apr 2015 16:16:18 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Rik van Riel <riel@...hat.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Dave Hansen <dave.hansen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, X86 ML <x86@...nel.org>
Subject: Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

On Tue, Apr 28, 2015 at 3:54 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> I had a totally different implementation idea in mind.  It goes
> something like this:
>
> For each CPU, we allocate a fixed number of PCIDs, e.g. 0-7.  We have
> a per-cpu array of the mm [1] that owns each PCID. [...]

We've done this before on other architectures.  See for example alpha.
Look up "__get_new_mm_context()" and friends. I think sparc does the
same (and I think sparc copied a lot of it from the alpha
implementation).

Iirc, the alpha version just generates a (per-cpu) asid one at a time,
and has a generation counter so that when you run out of ASID's you do
a global TLB invalidate on that CPU and start from 0 again. Actually,
I think the generation number is just the high bits of the asid
counter (alpha calls them "asn", intel calls them "pcid", and I tend
to prefer "asid", but it's all the same thing).

Then each thread just has a per-thread ASID. We don't try to make that
be per-thread and per-cpu, but instead just force a new allocation
when a thread moves to another CPU.

It's not obvious what alpha does, because we end up hiding the
per-thread ASN in the "struct pcb_struct" (in 'struct thread_info')
which is part the alpha pal-code interface. But it seemed to work and
is fairly simple.

I think something very similar should work with intel pcid's.

                        Linus
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