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    Subject[PATCH 5.12 123/161] kfence: maximize allocation wait timeout duration
    Date
    From: Marco Elver <elver@google.com>

    commit 37c9284f6932b915043717703d6496dfd59c85f5 upstream.

    The allocation wait timeout was initially added because of warnings due to
    CONFIG_DETECT_HUNG_TASK=y [1]. While the 1 sec timeout is sufficient to
    resolve the warnings (given the hung task timeout must be 1 sec or larger)
    it may cause unnecessary wake-ups if the system is idle:

    https://lkml.kernel.org/r/CADYN=9J0DQhizAGB0-jz4HOBBh+05kMBXb4c0cXMS7Qi5NAJiw@mail.gmail.com

    Fix it by computing the timeout duration in terms of the current
    sysctl_hung_task_timeout_secs value.

    Link: https://lkml.kernel.org/r/20210421105132.3965998-3-elver@google.com
    Signed-off-by: Marco Elver <elver@google.com>
    Cc: Alexander Potapenko <glider@google.com>
    Cc: Dmitry Vyukov <dvyukov@google.com>
    Cc: Hillf Danton <hdanton@sina.com>
    Cc: Jann Horn <jannh@google.com>
    Cc: Mark Rutland <mark.rutland@arm.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    mm/kfence/core.c | 12 +++++++++++-
    1 file changed, 11 insertions(+), 1 deletion(-)

    --- a/mm/kfence/core.c
    +++ b/mm/kfence/core.c
    @@ -20,6 +20,7 @@
    #include <linux/moduleparam.h>
    #include <linux/random.h>
    #include <linux/rcupdate.h>
    +#include <linux/sched/sysctl.h>
    #include <linux/seq_file.h>
    #include <linux/slab.h>
    #include <linux/spinlock.h>
    @@ -620,7 +621,16 @@ static void toggle_allocation_gate(struc
    /* Enable static key, and await allocation to happen. */
    static_branch_enable(&kfence_allocation_key);

    - wait_event_timeout(allocation_wait, atomic_read(&kfence_allocation_gate), HZ);
    + if (sysctl_hung_task_timeout_secs) {
    + /*
    + * During low activity with no allocations we might wait a
    + * while; let's avoid the hung task warning.
    + */
    + wait_event_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
    + sysctl_hung_task_timeout_secs * HZ / 2);
    + } else {
    + wait_event(allocation_wait, atomic_read(&kfence_allocation_gate));
    + }

    /* Disable static key and reset timer. */
    static_branch_disable(&kfence_allocation_key);

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