Re: [PATCH 1/2] sched/debug: fix 'schedstats=enable' cmdline option

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Jun 07, 2016 at 02:43:16PM -0500, Josh Poimboeuf wrote:
> The 'schedstats=enable' option doesn't work, and also produces the
> following warning during boot:
> 
>   WARNING: CPU: 0 PID: 0 at /home/jpoimboe/git/linux/kernel/jump_label.c:61 static_key_slow_inc+0x8c/0xa0
>   static_key_slow_inc used before call to jump_label_init
>   Modules linked in:
>   CPU: 0 PID: 0 Comm: swapper Not tainted 4.7.0-rc1+ #25
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
>    0000000000000086 3ae3475a4bea95d4 ffffffff81e03da8 ffffffff8143fc83
>    ffffffff81e03df8 0000000000000000 ffffffff81e03de8 ffffffff810b1ffb
>    0000003d00000096 ffffffff823514d0 ffff88007ff197c8 0000000000000000
>   Call Trace:
>    [<ffffffff8143fc83>] dump_stack+0x85/0xc2
>    [<ffffffff810b1ffb>] __warn+0xcb/0xf0
>    [<ffffffff810b207f>] warn_slowpath_fmt+0x5f/0x80
>    [<ffffffff811e9c0c>] static_key_slow_inc+0x8c/0xa0
>    [<ffffffff810e07c6>] static_key_enable+0x16/0x40
>    [<ffffffff8216d633>] setup_schedstats+0x29/0x94
>    [<ffffffff82148a05>] unknown_bootoption+0x89/0x191
>    [<ffffffff810d8617>] parse_args+0x297/0x4b0
>    [<ffffffff82148d61>] start_kernel+0x1d8/0x4a9
>    [<ffffffff8214897c>] ? set_init_arg+0x55/0x55
>    [<ffffffff82148120>] ? early_idt_handler_array+0x120/0x120
>    [<ffffffff821482db>] x86_64_start_reservations+0x2f/0x31
>    [<ffffffff82148427>] x86_64_start_kernel+0x14a/0x16d
> 
> The problem is that it tries to update the 'sched_schedstats' static key
> before jump labels have been initialized.
> 

Odd, I thought that particular one had been addressed. At least I saw
errors like that in other areas before and they got fixed later by ftrace.

> Changing jump_label_init() to be called earlier before
> parse_early_param() wouldn't fix it: it would still fail trying to
> poke_text() because mm isn't yet initialized.
> 
> Instead, just create a temporary '__sched_schedstats' variable which can
> be copied to the static key later during sched_init() after jump labels
> have been initialized.
> 
> Fixes: cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
> Cc: stable@vger.kernel.org
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
>  kernel/sched/core.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7f2cae4..385c947 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2253,9 +2253,11 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
>  #endif
>  #endif
>  
> +#ifdef CONFIG_SCHEDSTATS
> +
>  DEFINE_STATIC_KEY_FALSE(sched_schedstats);
> +static bool __initdata __sched_schedstats = false;
>  

Initialisation is unnecessary. Otherwise;

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs