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

[Giovanni Gherdovich]

On Fri, 2020-05-01 at 15:30 +0200, Peter Zijlstra wrote:
> 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 ;-)

Right, I wasn't really checking for overflow, only for when the product
"mcnt * arch_max_freq_ratio" becomes zero.

Thanks for your edit (I took note of the macros check_*_overflow, didn't know
them). I fully subscribe to the paranoid approach.

I understand you've already edited the patches in your tree, so I am not
resending, just confirming my

Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>

> 
> --- 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);
>  }