linux-kernel - Re: [PATCH 0/5] x86/percpu semantics and fixes

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Message-ID: <20190310124650.GA6840@worktop.programming.kicks-ass.net>
Date:   Sun, 10 Mar 2019 13:46:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "luto@...nel.org" <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] x86/percpu semantics and fixes

On Fri, Mar 08, 2019 at 09:56:37PM +0100, Peter Zijlstra wrote:

> Anyway, can anybody explain percpu_stable_op() vs percpu_from_op() ?
> 
> I'm thinking of a variant of Linus' patch, but I'm confused about the
> above.

So whatever I tried with +m only made things worse and always affects
thousands of symbols.

Now, afaict the whole percpu_stable_op thing is an ugly hack becaues some
earlier compiler would not CSE the regular percpu_from_op. But since it
does do that today; esp. after my first patch, I tried implementing
this_cpu_read_stable() with percpu_from_op() (no volatile, obv).

That also affects _lots_ of sites, but also significantly shrinks the
kernel image.

It's 2307 symbols affected, but:

17642871        2157438  747808 20548117        1398a15 defconfig-build1/vmlinux.o (patch 1)
17639081        2157438  747808 20544327        1397b47 defconfig-build0/vmlinux.o (patch 1 - percpu_stable_op)

So I think I'll add a patch removing percpu_stable_op and all its users.