linux-kernel - Re: [RFC PATCH] x86: Kill notsc

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Message-ID: <20151021175803.GF3575@pd.tnic>
Date:	Wed, 21 Oct 2015 19:58:03 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	x86-ml <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86: Kill notsc

On Sun, Oct 18, 2015 at 04:20:07PM +0200, Borislav Petkov wrote:
> Ok,
> 
> let's try this and see where it takes us. Patch has been only lightly
> tested in kvm - I'll hammer on it for real once we agree about the
> general form.
> 
> Aanyway, this patch is something Peter and I have been talking about on
> IRC a couple of times already. I finally found some free time to poke at
> it, here's the result.
> 
> Thoughts?
> 
> ---
> 
> Kill "notsc" cmdline option and all the glue around it. The two boxes
> worldwide which don't have a TSC should disable X86_TSC. Thus, make
> native_sched_clock() use TSC unconditionally, even if the TSC is
> unstable because that's fine there. This gets rid of the static key
> too and makes the function even simpler and faster, which is a Good
> Thing(tm).
> 
> The jiffies-fallback is for the !X86_TSC case.
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>
> ---
>  Documentation/kernel-parameters.txt       |  2 -
>  Documentation/x86/x86_64/boot-options.txt |  5 ---
>  arch/x86/include/asm/tsc.h                |  3 +-
>  arch/x86/kernel/apic/apic.c               |  2 +-
>  arch/x86/kernel/tsc.c                     | 74 ++++++++-----------------------
>  5 files changed, 21 insertions(+), 65 deletions(-)

...

> +#ifdef CONFIG_X86_TSC
> +	/* return the value in ns */
> +	return cycles_2_ns(rdtsc());
> +#else

Ok, we have a problem here:

See the splat below. On the init path

start_kernel
|-> sched_init
    |-> init_idle

we're calling sched_clock() which does the cycles_2_ns() thing.

However, that cycles_2_ns() thing gets called only in tsc_init() which
comes later in start_kernel().

Which means, data in this line

      ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, data->cyc2ns_shift);

is NULL.

Now, the question is, can I push only the cyc2ns_init() call up, before
sched_init()?

I mean, it dummy initializes only the per-cpu variable and tsc_init()
will correct it anyway, as the comment says:

        /*
         * Secondary CPUs do not run through tsc_init(), so set up
         * all the scale factors for all CPUs, assuming the same
         * speed as the bootup CPU. (cpufreq notifiers will fix this
         * up if their speed diverges)
         */
        for_each_possible_cpu(cpu) {
                cyc2ns_init(cpu);
                set_cyc2ns_scale(cpu_khz, cpu);
        }


Thoughts?


[    0.000000] BUG: unable to handle kernel NULL pointer dereference at           (null)
[    0.000000] IP: [<ffffffff8100d19d>] native_sched_clock+0x2d/0x90
[    0.000000] PGD 0 
[    0.000000] Oops: 0000 [#1] PREEMPT SMP 
[    0.000000] Modules linked in:
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.3.0-rc6+ #4
[    0.000000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
[    0.000000] task: ffffffff819f7500 ti: ffffffff819e8000 task.ti: ffffffff819e8000
[    0.000000] RIP: 0010:[<ffffffff8100d19d>]  [<ffffffff8100d19d>] native_sched_clock+0x2d/0x90
[    0.000000] RSP: 0000:ffffffff819ebee0  EFLAGS: 00010046
[    0.000000] RAX: 0000000129f79c5a RBX: ffff88007b7d5c00 RCX: 0000000000000000
[    0.000000] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff819f76e0
[    0.000000] RBP: ffffffff819ebee0 R08: 0000000000000001 R09: 0000000000000000
[    0.000000] R10: 0000000000000001 R11: ffffffff819f7500 R12: 0000000000000000
[    0.000000] R13: 0000000000000000 R14: ffffffff819f7b30 R15: ffffffff819f7500
[    0.000000] FS:  0000000000000000(0000) GS:ffff88007b600000(0000) knlGS:0000000000000000
[    0.000000] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[    0.000000] CR2: 0000000000000000 CR3: 00000000019f2000 CR4: 00000000000406b0
[    0.000000] Stack:
[    0.000000]  ffffffff819ebf28 ffffffff8108622f 0000000000000000 0000000000000086
[    0.000000]  ffffffff819f7500 0000000000000008 0000000000000008 00000000001d5c00
[    0.000000]  ffffffff0000ffff ffffffff819ebf58 ffffffff81cace4f ffffffffffffffff
[    0.000000] Call Trace:
[    0.000000]  [<ffffffff8108622f>] init_idle+0x13f/0x2d0
[    0.000000]  [<ffffffff81cace4f>] sched_init+0x304/0x345
[    0.000000]  [<ffffffff81c97d6d>] start_kernel+0x25e/0x417
[    0.000000]  [<ffffffff81c972fe>] x86_64_start_reservations+0x2a/0x2c
[    0.000000]  [<ffffffff81c97468>] x86_64_start_kernel+0x168/0x176
[    0.000000] Code: 89 e5 48 83 e4 f0 0f 31 48 c1 e2 20 48 09 d0 65 ff 05 88 dd ff 7e 65 48 8b 35 e0 5b 1c 7f 65 48 8b 15 e0 5b 1c 7f 48 39 d6 75 2c <8b> 16 8b 4e 04 48 f7 e2 48 0f ad d0 48 d3 ea f6 c1 40 48 0f 45 
[    0.000000] RIP  [<ffffffff8100d19d>] native_sched_clock+0x2d/0x90
[    0.000000]  RSP <ffffffff819ebee0>
[    0.000000] CR2: 0000000000000000
[    0.000000] ---[ end trace aff58b1304cfecf1 ]---
[    0.000000] Kernel panic - not syncing: Attempted to kill the idle task!
[    0.000000] ---[ end Kernel panic - not syncing: Attempted to kill the idle task!

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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