Re: [PATCH] kfence: allow use of a deferrable timer

[Alexander Potapenko <glider@google.com>]

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On Tue, Mar 8, 2022 at 1:20 PM Marco Elver <elver@google.com> wrote:

> Allow the use of a deferrable timer, which does not force CPU wake-ups
> when the system is idle. A consequence is that the sample interval
> becomes very unpredictable, to the point that it is not guaranteed that
> the KFENCE KUnit test still passes.
>
> Nevertheless, on power-constrained systems this may be preferable, so
> let's give the user the option should they accept the above trade-off.
>
> Signed-off-by: Marco Elver <elver@google.com>
>
Reviewed-by: Alexander Potapenko <glider@google.com>


> ---
>  lib/Kconfig.kfence | 12 ++++++++++++
>  mm/kfence/core.c   | 15 +++++++++++++--
>  2 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
> index 912f252a41fc..1cf2ea2a1ac7 100644
> --- a/lib/Kconfig.kfence
> +++ b/lib/Kconfig.kfence
> @@ -45,6 +45,18 @@ config KFENCE_NUM_OBJECTS
>           pages are required; with one containing the object and two
> adjacent
>           ones used as guard pages.
>
> +config KFENCE_DEFERRABLE
> +       bool "Use a deferrable timer to trigger allocations" if EXPERT
> +       help
> +         Use a deferrable timer to trigger allocations. This avoids
> forcing
> +         CPU wake-ups if the system is idle, at the risk of a less
> predictable
> +         sample interval.
> +
> +         Warning: The KUnit test suite fails with this option enabled -
> due to
> +         the unpredictability of the sample interval!
> +
> +         Say N if you are unsure.
> +
>  config KFENCE_STATIC_KEYS
>         bool "Use static keys to set up allocations" if EXPERT
>         depends on JUMP_LABEL
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index f126b53b9b85..451277b41bfb 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -95,6 +95,10 @@ module_param_cb(sample_interval,
> &sample_interval_param_ops, &kfence_sample_inte
>  static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
>  module_param_named(skip_covered_thresh, kfence_skip_covered_thresh,
> ulong, 0644);
>
> +/* If true, use a deferrable timer at the risk of unpredictable sample
> intervals. */
> +static bool kfence_deferrable __read_mostly =
> IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
> +module_param_named(deferrable, kfence_deferrable, bool, 0444);
>
Could you please add a line or two to Documentation/dev-tools/kfence.rst as
well?


-- 
Alexander Potapenko
Software Engineer

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