Re: [PATCH v1 08/11] rcu: Wake NOCB rcuog kthreads on expedited grace period completion

Le Wed, Jun 24, 2026 at 06:23:50AM -0700, Puranjay Mohan a écrit :

When an expedited grace period completes, rcu_exp_wait_wake() wakes
waiters on rnp->exp_wq[] but does not notify the NOCB rcuog kthreads.  An
rcuog kthread that is waiting for a grace period sleeps on the leaf
rcu_node's ->nocb_gp_wq[] with a wait condition based on the grace-period
state, so without a wakeup, callbacks on offloaded CPUs that could
benefit from the expedited GP wait until the rcuog kthread wakes for some
other reason (e.g. the next normal GP or a timer).

Make the rcuog grace-period wait honour expedited GPs and wake it when
one completes:

 - nocb_gp_wait() now records the grace period to wait for as a struct
   rcu_gp_seq (both normal and expedited), tracks the earliest pending
   normal and expedited sequence across the group, and releases the wait
   via poll_state_synchronize_rcu_full() so it wakes for whichever
   completes first.  ->nocb_gp_seq is widened to struct rcu_gp_seq
   accordingly.

 - rcu_exp_wait_wake() calls the new rcu_nocb_exp_cleanup() on leaf
   nodes, which wakes both ->nocb_gp_wq[0] and ->nocb_gp_wq[1] (the
   expedited sequence does not share parity with the normal ->gp_seq the
   waiter indexed with).  Both this path and rcu_nocb_gp_cleanup() use
   the shared rcu_nocb_cleanup_wake() helper, which checks swait_active()
   first; the smp_mb() in rcu_gp_cleanup()/rcu_exp_wait_wake() orders the
   grace-period state update before that check.

A stub rcu_nocb_exp_cleanup() is provided for CONFIG_RCU_NOCB_CPU=n.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

-- 
Frederic Weisbecker
SUSE Labs