Re: [PATCH] cpumask: avoid WARN in prefill_possible_map()

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, 12 Dec 2016, Dmitry Safonov wrote:

> Subject : [PATCH] cpumask: avoid WARN in prefill_possible_map()

'cpumask' is hardly the proper prefix for x86/smpboot related issues.

> With CONFIG_DEBUG_PER_CPU_MAPS and CONFIG_CPUMASK_OFFSTACK enabled
> fixes the following WARN_ON_ONCE() for booting with nr_cpus=1:

This sentence, aside of not qualifying as a sentence, makes no sense.

What has this to do with nr_cpus=1? If I boot with nr_cpus=2 then this
won't fail or what?

> [    0.000000] Linux version 4.9.0 (dsafonov@localhost.localdomain) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC) ) #36 SMP Mon Dec 12 18:05:46 MSK 2016
> [    0.000000] Command line: BOOT_IMAGE=/vmlinuz-4.9.0 root=/dev/mapper/vz-root ro crashkernel=auto rd.lvm.lv=vz/root rd.lvm.lv=vz/swap console=ttyS0,115200 vsyscall=none nr_cpus=1
> [    0.000000] smpboot: 4 Processors exceeds NR_CPUS limit of 1
> [    0.000000] smpboot: Allowing 1 CPUs, 0 hotplug CPUs
> [    0.000000] ------------[ cut here ]------------
> [    0.000000] WARNING: CPU: 0 PID: 0 at ./include/linux/cpumask.h:121 cpumask_check.part.2+0x1c/0x1e
> [    0.000000] Modules linked in:
> [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.9.0 #36
> [    0.000000] Call Trace:
> [    0.000000]  [<ffffffff8136664b>] dump_stack+0x67/0x9c
> [    0.000000]  [<ffffffff81063e21>] __warn+0xd1/0xf0
> [    0.000000]  [<ffffffff81063f0d>] warn_slowpath_null+0x1d/0x20
> [    0.000000]  [<ffffffff8105f4ca>] cpumask_check.part.2+0x1c/0x1e
> [    0.000000]  [<ffffffff8104008e>] cpumask_clear_cpu+0x2e/0x40
> [    0.000000]  [<ffffffff81f88a62>] prefill_possible_map+0x15c/0x16a
> [    0.000000]  [<ffffffff81f802c6>] setup_arch+0xba7/0xc33
> [    0.000000]  [<ffffffff81f78c59>] start_kernel+0x63/0x448
> [    0.000000]  [<ffffffff81f7858c>] x86_64_start_reservations+0x2a/0x2c
> [    0.000000]  [<ffffffff81f78678>] x86_64_start_kernel+0xea/0xed
> [    0.000000] ---[ end trace 5876da8d2ace83fb ]---

And that non-trimmed backtrace is useful because it takes so much room in
the changelog and looks nice? The callchain leading to
prefill_possible_map() is pretty much well known, i.e. the backtrace is
pointless.

> nr_cpu_ids is set to possible two lines futher - omit checking in
> set_cpu_possible() cycles.

-ENOPARSE.

You completely fail to explain the problem, i.e. how nr_cpu_ids gets
overwritten from it's initial compile time value NR_CPUS.

And of course you fail to explain why the "solution" is correct or
whatever you consider it to be.

> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 42f5eb7b4f6c..17167bec7c61 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1459,6 +1459,9 @@ __init void prefill_possible_map(void)
>  	pr_info("Allowing %d CPUs, %d hotplug CPUs\n",
>  		possible, max_t(int, possible - num_processors, 0));
>  
> +	/* Avoid WARN() in set_cpu_possible()=>cpumask_check() */
> +	nr_cpu_ids = NR_CPUS;
> +

If anything then this qualifies as a quick hack.

>  	for (i = 0; i < possible; i++)
>  		set_cpu_possible(i, true);
>  	for (; i < NR_CPUS; i++)

The underlying issue is not restricted to nr_cpus=1 at all. The problem
comes from the early_param setting nr_cpu_ids to the command line
parameter. If that one is smaller than NR_CPUS then the access to the
possible mask with a cpu number > nr_cpu_ids will trigger the warning.

So instead of playing completely non obvious hackery with nr_cpu_ids the
proper solution is to have a function which clears the underlying
__cpu_possible_map, which is sized NR_CPUS because it is compile time
allocated and then only set the possible bits. Does the untested patch
below fix the issue for you?

Thanks,

	tglx
8<--------------------
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1476,15 +1476,15 @@ early_param("possible_cpus", _setup_poss
 		possible = i;
 	}
 
+	nr_cpu_ids = possible;
+
 	pr_info("Allowing %d CPUs, %d hotplug CPUs\n",
 		possible, max_t(int, possible - num_processors, 0));
 
+	reset_cpu_possible_mask();
+
 	for (i = 0; i < possible; i++)
 		set_cpu_possible(i, true);
-	for (; i < NR_CPUS; i++)
-		set_cpu_possible(i, false);
-
-	nr_cpu_ids = possible;
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -112,6 +112,7 @@ extern struct cpumask __cpu_active_mask;
 #define cpu_possible(cpu)	((cpu) == 0)
 #define cpu_present(cpu)	((cpu) == 0)
 #define cpu_active(cpu)		((cpu) == 0)
+static inline void cpumask_reset_possible_mask(void) { }
 #endif
 
 /* verify cpu argument to cpumask_* operators */
@@ -722,6 +723,11 @@ void init_cpu_present(const struct cpuma
 void init_cpu_possible(const struct cpumask *src);
 void init_cpu_online(const struct cpumask *src);
 
+static inline void reset_cpu_possible_mask(void)
+{
+	bitmap_zero(cpumask_bits(&__cpu_possible_mask), NR_CPUS);
+}
+
 static inline void
 set_cpu_possible(unsigned int cpu, bool possible)
 {