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

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;

-#ifdef CONFIG_SCHEDSTATS
static void set_schedstats(bool enabled)
{
if (enabled)
@@ -2278,11 +2280,16 @@ static int __init setup_schedstats(char *str)
if (!str)
goto out;

+ /*
+ * This code is called before jump labels have been set up, so we can't
+ * change the static branch directly just yet. Instead set a temporary
+ * variable so init_schedstats() can do it later.
+ */
if (!strcmp(str, "enable")) {
- set_schedstats(true);
+ __sched_schedstats = true;
ret = 1;
} else if (!strcmp(str, "disable")) {
- set_schedstats(false);
+ __sched_schedstats = false;
ret = 1;
}
out:
@@ -2293,6 +2300,11 @@ out:
}
__setup("schedstats=", setup_schedstats);

+static void __init init_schedstats(void)
+{
+ set_schedstats(__sched_schedstats);
+}
+
#ifdef CONFIG_PROC_SYSCTL
int sysctl_schedstats(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos)
@@ -2313,8 +2325,10 @@ int sysctl_schedstats(struct ctl_table *table, int write,
set_schedstats(state);
return err;
}
-#endif
-#endif
+#endif /* CONFIG_PROC_SYSCTL */
+#else /* !CONFIG_SCHEDSTATS */
+static inline void init_schedstats(void) {}
+#endif /* CONFIG_SCHEDSTATS */

/*
* fork()/clone()-time setup:
@@ -7487,6 +7501,8 @@ void __init sched_init(void)
#endif
init_sched_fair_class();

+ init_schedstats();
+
scheduler_running = 1;
}

--
2.4.11
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