Messages in this thread | | | From | Joel Fernandes <> | Date | Tue, 11 Oct 2022 19:47:07 -0400 | Subject | Re: [PATCH 2/2] rcu/nocb: Spare bypass locking upon normal enqueue |
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On Tue, Oct 11, 2022 at 3:21 PM Frederic Weisbecker <frederic@kernel.org> wrote: > > On Tue, Oct 11, 2022 at 02:00:40AM +0000, Joel Fernandes wrote: > > On Tue, Oct 11, 2022 at 12:39:56AM +0200, Frederic Weisbecker wrote: > > > When a callback is to be enqueued to the normal queue and not the bypass > > > one, a flush to the bypass queue is always tried anyway. This attempt > > > involves locking the bypass lock unconditionally. Although it is > > > guaranteed not to be contended at this point, because only call_rcu() > > > can lock the bypass lock without holding the nocb lock, it's still not > > > free and the operation can easily be spared most of the time by just > > > checking if the bypass list is empty. The check is safe as nobody can > > > queue nor flush the bypass concurrently. > > > > > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > > > --- > > > kernel/rcu/tree_nocb.h | 6 ++++-- > > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > > > index 094fd454b6c3..30c3d473ffd8 100644 > > > --- a/kernel/rcu/tree_nocb.h > > > +++ b/kernel/rcu/tree_nocb.h > > > @@ -423,8 +423,10 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp, > > > if (*was_alldone) > > > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, > > > TPS("FirstQ")); > > > - WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j)); > > > - WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass)); > > > + if (rcu_cblist_n_cbs(&rdp->nocb_bypass)) { > > > + WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j)); > > > + WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass)); > > > + } > > > return false; // Caller must enqueue the callback. > > > } > > > > Instead of this, since as you mentioned that the bypass lock is not contended > > in this path, isn't it unnecessary to even check or attempt to acquire the > > lock in call_rcu() path? So how about something like the following, or would > > this not work for some reason? > > You're right. But it's a bit error prone and it adds quite some code complication > just for a gain on a rare event (bypass is supposed to be flushed on rare > occasions by the caller).
But the "checking of whether to flush" which leads to "acquiring the bypass lock first" , is not a rare event as you pointed out (can be spared most of the time as you said). The alternative I proposed removes the need for the frequent locking (which is another way of implementing what you suggested).
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