On Tue, Sep 06, 2022 at 03:05:46AM +0000, Joel Fernandes wrote:
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 4dc86274b3e8..b201606f7c4f 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -256,6 +256,31 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
> return __wake_nocb_gp(rdp_gp, rdp, force, flags);
> }
>
> +/*
> + * LAZY_FLUSH_JIFFIES decides the maximum amount of time that
> + * can elapse before lazy callbacks are flushed. Lazy callbacks
> + * could be flushed much earlier for a number of other reasons
> + * however, LAZY_FLUSH_JIFFIES will ensure no lazy callbacks are
> + * left unsubmitted to RCU after those many jiffies.
> + */
> +#define LAZY_FLUSH_JIFFIES (10 * HZ)
> +unsigned long jiffies_till_flush = LAZY_FLUSH_JIFFIES;
Still not static.
> @@ -293,12 +322,16 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
> * proves to be initially empty, just return false because the no-CB GP
> * kthread may need to be awakened in this case.
> *
> + * Return true if there was something to be flushed and it succeeded, otherwise
> + * false.
> + *
This kind of contradict the comment that follows. Not sure you need to add
that line because the existing comment seem to cover it.
> * Note that this function always returns true if rhp is NULL.
> */
> static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> - unsigned long j)
> + unsigned long j, unsigned long flush_flags)
> {
> struct rcu_cblist rcl;
> + bool lazy = flush_flags & FLUSH_BP_LAZY;
>
> WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp));
> rcu_lockdep_assert_cblist_protected(rdp);
> @@ -326,13 +372,20 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> * Note that this function always returns true if rhp is NULL.
> */
> static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> - unsigned long j)
> + unsigned long j, unsigned long flush_flags)
> {
> + bool ret;
> +
> if (!rcu_rdp_is_offloaded(rdp))
> return true;
> rcu_lockdep_assert_cblist_protected(rdp);
> rcu_nocb_bypass_lock(rdp);
> - return rcu_nocb_do_flush_bypass(rdp, rhp, j);
> + ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
> +
> + if (flush_flags & FLUSH_BP_WAKE)
> + wake_nocb_gp(rdp, true);
Why the true above?
Also should we check if the wake up is really necessary (otherwise it means we
force a wake up for all rdp's from rcu_barrier())?
was_alldone = rcu_segcblist_pend_cbs(&rdp->cblist);
ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
if (was_alldone && rcu_segcblist_pend_cbs(&rdp->cblist))
wake_nocb_gp(rdp, false);
> @@ -461,16 +521,29 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> // We need to use the bypass.
> rcu_nocb_wait_contended(rdp);
> rcu_nocb_bypass_lock(rdp);
> +
> ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
> rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
> rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
> +
> + if (IS_ENABLED(CONFIG_RCU_LAZY) && lazy)
> + WRITE_ONCE(rdp->lazy_len, rdp->lazy_len + 1);
> +
> if (!ncbs) {
> WRITE_ONCE(rdp->nocb_bypass_first, j);
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQ"));
> }
> +
> rcu_nocb_bypass_unlock(rdp);
> smp_mb(); /* Order enqueue before wake. */
> - if (ncbs) {
> +
> + // We had CBs in the bypass list before. There is nothing else to do if:
> + // There were only non-lazy CBs before, in this case, the bypass timer
Kind of misleading. I would replace "There were only non-lazy CBs before" with
"There was at least one non-lazy CBs before".
> + // or GP-thread will handle the CBs including any new lazy ones.
> + // Or, the new CB is lazy and the old bypass-CBs were also lazy. In this
> + // case the old lazy timer would have been setup. When that expires,
> + // the new lazy one will be handled.
> + if (ncbs && (!bypass_is_lazy || lazy)) {
> local_irq_restore(flags);
> } else {
> // No-CBs GP kthread might be indefinitely asleep, if so, wake.
> @@ -479,6 +552,10 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> TPS("FirstBQwake"));
> __call_rcu_nocb_wake(rdp, true, flags);
> + } else if (bypass_is_lazy && !lazy) {
> + trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> + TPS("FirstBQwakeLazy2Non"));
> + __call_rcu_nocb_wake(rdp, true, flags);
Not sure we need this chunk. Since there are pending callbacks anyway,
nocb_gp_wait() should be handling them and it will set the appropriate
timer on the next loop.
Thanks.
> } else {
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> TPS("FirstBQnoWake"));
