Re: [PATCH v2 1/3] x86, sched: check for counters overflow in frequency invariant accounting

["Rafael J. Wysocki" <rafael@kernel.org>]

On Sun, May 31, 2020 at 8:26 PM Giovanni Gherdovich <ggherdovich@suse.cz> wrote:
>
> The product mcnt * arch_max_freq_ratio can 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.
>
> Under similar conditions, the difference acnt of two consecutive APERF
> readings can overflow as well.
>
> In these circumstances is 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.
>
> A note on i386: prior to version 5.1, the GCC compiler didn't have the
> builtin function __builtin_mul_overflow. In these GCC versions the macro
> check_mul_overflow needs __udivdi3() to do (u64)a/b, which the kernel
> doesn't provide. For this reason this change fails to build on i386 if
> GCC<5.1, and we protect the entire frequency invariant code behind
> CONFIG_X86_64 (special thanks to "kbuild test robot" <lkp@intel.com>).
>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
>  arch/x86/include/asm/topology.h |  2 +-
>  arch/x86/kernel/smpboot.c       | 33 ++++++++++++++++++++++++++++-----
>  2 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
> index 79d8d5496330..f4234575f3fd 100644
> --- a/arch/x86/include/asm/topology.h
> +++ b/arch/x86/include/asm/topology.h
> @@ -193,7 +193,7 @@ static inline void sched_clear_itmt_support(void)
>  }
>  #endif /* CONFIG_SCHED_MC_PRIO */
>
> -#ifdef CONFIG_SMP
> +#if defined(CONFIG_SMP) && defined(CONFIG_X86_64)
>  #include <asm/cpufeature.h>
>
>  DECLARE_STATIC_KEY_FALSE(arch_scale_freq_key);
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 2f24c334a938..d660966d7de7 100644
> --- 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>
> @@ -1777,6 +1778,7 @@ void native_play_dead(void)
>
>  #endif
>
> +#ifdef CONFIG_X86_64
>  /*
>   * APERF/MPERF frequency ratio computation.
>   *
> @@ -2047,11 +2049,19 @@ static void init_freq_invariance(bool secondary)
>         }
>  }
>
> +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;
>
> @@ -2063,19 +2073,32 @@ 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);
> +}
> +#else
> +static inline void init_freq_invariance(bool secondary)
> +{
>  }
> +#endif /* CONFIG_X86_64 */
> --
> 2.16.4
>