Hi,
On Thu, Jul 11, 2024 at 05:56:29PM -0700, JP Kobryn wrote:
> Using a higher value for the initial gp sequence counter values allows for
> wrapping to occur faster. It can help with surfacing any issues that may be
> happening as a result of the wrap around.
>
> Signed-off-by: JP Kobryn <[email protected]>
> ---
> include/linux/rcupdate.h | 3 +++
> include/linux/srcutree.h | 15 ++++++++++++++-
> kernel/rcu/rcu.h | 3 ---
> kernel/rcu/srcutree.c | 6 ++++--
> 4 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index dfd2399f2cde..c148674b9e09 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -35,6 +35,9 @@
> #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
>
> +#define RCU_SEQ_CTR_SHIFT 2
> +#define RCU_SEQ_STATE_MASK ((1 << RCU_SEQ_CTR_SHIFT) - 1)
> +
> /* Exported common interfaces */
> void call_rcu(struct rcu_head *head, rcu_callback_t func);
> void rcu_barrier_tasks(void);
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index 8f3f72480e78..ed57598394de 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -129,10 +129,23 @@ struct srcu_struct {
> #define SRCU_STATE_SCAN1 1
> #define SRCU_STATE_SCAN2 2
>
> +/*
> + * Values for initializing gp sequence fields. Higher values allow wrap
> arounds to
> + * occur earlier.
> + * The second value with state is useful in the case of static
> initialization of
> + * srcu_usage where srcu_gp_seq_needed is expected to have some state value
> in its
> + * lower bits (or else it will appear to be already initialized within
> + * the call check_init_srcu_struct()).
> + */
> +#define SRCU_GP_SEQ_INITIAL_VAL ((0UL - 100UL) << RCU_SEQ_CTR_SHIFT)
> +#define SRCU_GP_SEQ_INITIAL_VAL_WITH_STATE (SRCU_GP_SEQ_INITIAL_VAL - 1)
> +
> #define __SRCU_USAGE_INIT(name)
> \
> {
> \
> .lock = __SPIN_LOCK_UNLOCKED(name.lock),
> \
> - .srcu_gp_seq_needed = -1UL,
> \
As srcu_gp_seq* counters are initialized before first use, if we do
not modify this file, we can move the SRCU_GP_SEQ_INITIAL_VAL and
SRCU_GP_SEQ_INITIAL_VAL_WITH_STATE macros to srcutree.c. We then
also don't need to move RCU_SEQ_CTR_SHIFT and RCU_SEQ_STATE_MASK
from rcu.h (keeping them internal to RCU). Is there a scenario
where that won't work?
> + .srcu_gp_seq = SRCU_GP_SEQ_INITIAL_VAL,
> \
> + .srcu_gp_seq_needed = SRCU_GP_SEQ_INITIAL_VAL_WITH_STATE,
> \
> + .srcu_gp_seq_needed_exp = SRCU_GP_SEQ_INITIAL_VAL,
> \
> .work = __DELAYED_WORK_INITIALIZER(name.work, NULL, 0),
> \
> }
>
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 38238e595a61..2bfed9855d67 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -54,9 +54,6 @@
> * grace-period sequence number.
> */
>
> -#define RCU_SEQ_CTR_SHIFT 2
> -#define RCU_SEQ_STATE_MASK ((1 << RCU_SEQ_CTR_SHIFT) - 1)
> -
> /* Low-order bit definition for polled grace-period APIs. */
> #define RCU_GET_STATE_COMPLETED 0x1
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index bc4b58b0204e..2209dd0589de 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -247,7 +247,7 @@ static int init_srcu_struct_fields(struct srcu_struct
> *ssp, bool is_static)
> mutex_init(&ssp->srcu_sup->srcu_cb_mutex);
> mutex_init(&ssp->srcu_sup->srcu_gp_mutex);
> ssp->srcu_idx = 0;
> - ssp->srcu_sup->srcu_gp_seq = 0;
> + ssp->srcu_sup->srcu_gp_seq = SRCU_GP_SEQ_INITIAL_VAL;
> ssp->srcu_sup->srcu_barrier_seq = 0;
> mutex_init(&ssp->srcu_sup->srcu_barrier_mutex);
> atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);
> @@ -258,7 +258,7 @@ static int init_srcu_struct_fields(struct srcu_struct
> *ssp, bool is_static)
> if (!ssp->sda)
> goto err_free_sup;
> init_srcu_struct_data(ssp);
> - ssp->srcu_sup->srcu_gp_seq_needed_exp = 0;
> + ssp->srcu_sup->srcu_gp_seq_needed_exp = SRCU_GP_SEQ_INITIAL_VAL;
> ssp->srcu_sup->srcu_last_gp_end = ktime_get_mono_fast_ns();
> if (READ_ONCE(ssp->srcu_sup->srcu_size_state) == SRCU_SIZE_SMALL &&
> SRCU_SIZING_IS_INIT()) {
> if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC))
> @@ -267,6 +267,8 @@ static int init_srcu_struct_fields(struct srcu_struct
> *ssp, bool is_static)
> }
> ssp->srcu_sup->srcu_ssp = ssp;
> smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed, 0); /* Init done.
> */
Remove this line?
- Neeraj
> + smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed,
> + SRCU_GP_SEQ_INITIAL_VAL); /* Init done. */
> return 0;
>
> err_free_sda:
> --
> 2.45.2
