On Mon, Jul 07, 2025 at 09:19:03AM +0530, Neeraj Upadhyay wrote: > > > On 7/3/2025 9:05 PM, Uladzislau Rezki wrote: > > On Wed, Jul 02, 2025 at 04:59:36PM +0200, Uladzislau Rezki (Sony) wrote: > >> Automatically enable the rcu_normal_wake_from_gp parameter on > >> systems with a small number of CPUs. The activation threshold > >> is set to 16 CPUs. > >> > >> This helps to reduce a latency of normal synchronize_rcu() API > >> by waking up GP-waiters earlier and decoupling synchronize_rcu() > >> callers from regular callback handling. > >> > >> A benchmark running 64 parallel jobs(system with 64 CPUs) invoking > >> synchronize_rcu() demonstrates a notable latency reduction with the > >> setting enabled. > >> > >> Latency distribution (microseconds): > >> > >> <default> > >> 0 - 9999 : 1 > >> 10000 - 19999 : 4 > >> 20000 - 29999 : 399 > >> 30000 - 39999 : 3197 > >> 40000 - 49999 : 10428 > >> 50000 - 59999 : 17363 > >> 60000 - 69999 : 15529 > >> 70000 - 79999 : 9287 > >> 80000 - 89999 : 4249 > >> 90000 - 99999 : 1915 > >> 100000 - 109999 : 922 > >> 110000 - 119999 : 390 > >> 120000 - 129999 : 187 > >> ... > >> <default> > >> > >> <rcu_normal_wake_from_gp> > >> 0 - 9999 : 1 > >> 10000 - 19999 : 234 > >> 20000 - 29999 : 6678 > >> 30000 - 39999 : 33463 > >> 40000 - 49999 : 20669 > >> 50000 - 59999 : 2766 > >> 60000 - 69999 : 183 > >> ... > >> <rcu_normal_wake_from_gp> > >> > >> Reviewed-by: Joel Fernandes <[email protected]> > >> Signed-off-by: Uladzislau Rezki (Sony) <[email protected]> > >> --- > >> kernel/rcu/tree.c | 14 +++++++++++--- > >> 1 file changed, 11 insertions(+), 3 deletions(-) > >> > >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > >> index e8a4b720d7d2..b88ceb35cebd 100644 > >> --- a/kernel/rcu/tree.c > >> +++ b/kernel/rcu/tree.c > >> @@ -1625,8 +1625,10 @@ static void rcu_sr_put_wait_head(struct llist_node > >> *node) > >> atomic_set_release(&sr_wn->inuse, 0); > >> } > >> > >> -/* Disabled by default. */ > >> -static int rcu_normal_wake_from_gp; > >> +/* Enable rcu_normal_wake_from_gp automatically on small systems. */ > >> +#define WAKE_FROM_GP_CPU_THRESHOLD 16 > >> + > >> +static int rcu_normal_wake_from_gp = -1; > >> module_param(rcu_normal_wake_from_gp, int, 0644); > >> static struct workqueue_struct *sync_wq; > >> > >> @@ -3239,7 +3241,7 @@ static void synchronize_rcu_normal(void) > >> > >> trace_rcu_sr_normal(rcu_state.name, &rs.head, TPS("request")); > >> > >> - if (!READ_ONCE(rcu_normal_wake_from_gp)) { > >> + if (READ_ONCE(rcu_normal_wake_from_gp) < 1) { > >> wait_rcu_gp(call_rcu_hurry); > >> goto trace_complete_out; > >> } > >> @@ -4843,6 +4845,12 @@ void __init rcu_init(void) > >> sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0); > >> WARN_ON(!sync_wq); > >> > >> + /* Respect if explicitly disabled via a boot parameter. */ > >> + if (rcu_normal_wake_from_gp < 0) { > >> + if (num_possible_cpus() <= WAKE_FROM_GP_CPU_THRESHOLD) > >> + rcu_normal_wake_from_gp = 1; > >> + } > >> + > >> /* Fill in default value for rcutree.qovld boot parameter. */ > >> /* -After- the rcu_node ->lock fields are initialized! */ > >> if (qovld < 0) > >> -- > >> 2.39.5 > >> > > Neeraj, are you planning to take this for next merge window? > > > > Yes, I have queued v2 of these for testing here: > > https://git.kernel.org/pub/scm/linux/kernel/git/neeraj.upadhyay/linux-rcu.git/log/?h=dev.01.07.2025 > > Will include them in PR for next merge window. > Thank you! I asked because i did not see both in our common RCU tree.
-- Uladzislau Rezki
