Re: [PATCH 13/15] sched: Use a static_key for sched_clock_stable

From: Sasha Levin
Date: Wed Jan 22 2014 - 17:32:31 EST

On 01/22/2014 12:14 PM, Peter Zijlstra wrote:
On Tue, Jan 21, 2014 at 05:28:37PM -0500, Sasha Levin wrote:
[ 0.000000] Initmem setup node 30 [mem 0x12ee000000-0x138dffffff]
[ 0.000000] NODE_DATA [mem 0xcfa42000-0xcfa72fff]
[ 0.000000] NODE_DATA(30) on node 1
[ 0.000000] Initmem setup node 31 [mem 0x138e000000-0x142fffffff]
[ 0.000000] NODE_DATA [mem 0xcfa11000-0xcfa41fff]
[ 0.000000] NODE_DATA(31) on node 1
[ 0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
[ 0.000000] kvm-clock: cpu 0, msr 0:cf991001, boot clock
[133538.294040] Zone ranges:
[133538.294338] DMA [mem 0x00001000-0x00ffffff]
[133538.294804] DMA32 [mem 0x01000000-0xffffffff]
[133538.295223] Normal [mem 0x100000000-0x142fffffff]
[133538.295670] Movable zone start for each node

OK, took me a while to fiddle with KVM and all the various muck around
that to reproduce. But I can confirm the below does fix the issue for

I'm hoping to not have to re-introcude the kevents_up() check, but I
need to figure out what hardware triggered that and test again.

Subject: sched/clock: Fixup early sched_clock initialization
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed, 22 Jan 2014 12:59:18 +0100

The code would assume sched_clock_stable() and switch to !stable
later, this switch brings a discontinuity in time.

The discontinuity on switching from stable to unstable was always
present, but previously we would set stable/unstable before
initializing TSC and usually stick to the one we start out with.

So the static_key bits brought an extra switch where there previously
wasn't one.

Things are further complicated by the fact that we cannot use
static_key as early as we usually call set_sched_clock_stable().

Fix things by tracking the stable state in a regular variable and only
set the static_key to the right state on sched_clock_init(), which is
ran right after late_time_init->tsc_init().

Before this we would not be using the TSC anyway.

Fixes: 35af99e646c7 ("sched/clock, x86: Use a static_key for sched_clock_stable")
Cc: jacob.jun.pan@xxxxxxxxxxxxxxx
Cc: Mike Galbraith <bitbucket@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: hpa@xxxxxxxxx
Cc: paulmck@xxxxxxxxxxxxxxxxxx
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: lenb@xxxxxxxxxx
Cc: rjw@xxxxxxxxxxxxx
Cc: Eliezer Tamir <eliezer.tamir@xxxxxxxxxxxxxxx>
Cc: rui.zhang@xxxxxxxxx
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Reported-by: dyoung@xxxxxxxxxx
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

The patch fixes the problem for me.


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