Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

[Peter Zijlstra]

On Tue, Apr 28, 2020 at 03:24:49PM +0200, Giovanni Gherdovich wrote:
> The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.
> 
> For context, a large value for arch_max_freq_ratio would be 5000,
> corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
> 1500-2000). A large increment frequency for the MPERF counter would be 5GHz
> (the base clock of all CPUs on the market today is less than that). With
> these figures, a CPU would need to go without a scheduler tick for around 8
> days for the u64 overflow to happen. It is unlikely, but the check is
> warranted.
> 
> In that case it's also appropriate to disable frequency invariant
> accounting: the feature relies on measures of the clock frequency done at
> every scheduler tick, which need to be "fresh" to be at all meaningful.
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

>  	acnt <<= 2*SCHED_CAPACITY_SHIFT;
>  	mcnt *= arch_max_freq_ratio;
> +	if (!mcnt) {

The problem is; this doesn't do what you claim it does.

> +		pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n");
> +		/* static_branch_disable() acquires a lock and may sleep */
> +		schedule_work(&disable_freq_invariance_work);
> +		return;
> +	}
>  
>  	freq_scale = div64_u64(acnt, mcnt);

I've changed the patch like so.. OK?

(ok, perhaps I went a little overboard with the paranoia ;-)

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -55,6 +55,7 @@
 #include <linux/gfp.h>
 #include <linux/cpuidle.h>
 #include <linux/numa.h>
+#include <linux/overflow.h>
 
 #include <asm/acpi.h>
 #include <asm/desc.h>
@@ -2057,11 +2058,19 @@ static void init_freq_invariance(bool se
 	}
 }
 
+static void disable_freq_invariance_workfn(struct work_struct *work)
+{
+	static_branch_disable(&arch_scale_freq_key);
+}
+
+static DECLARE_WORK(disable_freq_invariance_work,
+		    disable_freq_invariance_workfn);
+
 DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE;
 
 void arch_scale_freq_tick(void)
 {
-	u64 freq_scale;
+	u64 freq_scale = SCHED_CAPACITY_SCALE;
 	u64 aperf, mperf;
 	u64 acnt, mcnt;
 
@@ -2073,19 +2082,27 @@ void arch_scale_freq_tick(void)
 
 	acnt = aperf - this_cpu_read(arch_prev_aperf);
 	mcnt = mperf - this_cpu_read(arch_prev_mperf);
-	if (!mcnt)
-		return;
 
 	this_cpu_write(arch_prev_aperf, aperf);
 	this_cpu_write(arch_prev_mperf, mperf);
 
-	acnt <<= 2*SCHED_CAPACITY_SHIFT;
-	mcnt *= arch_max_freq_ratio;
+	if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
+		goto error;
+
+	if (check_mul_overflow(mcnt, arch_max_freq_ratio, &mcnt) || !mcnt)
+		goto error;
 
 	freq_scale = div64_u64(acnt, mcnt);
+	if (!freq_scale)
+		goto error;
 
 	if (freq_scale > SCHED_CAPACITY_SCALE)
 		freq_scale = SCHED_CAPACITY_SCALE;
 
 	this_cpu_write(arch_freq_scale, freq_scale);
+	return;
+
+error:
+	pr_warn("Scheduler frequency invariance went wobbly, disabling!\n");
+	schedule_work(&disable_freq_invariance_work);
 }