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[34.85.230.48]) by smtp.gmail.com with ESMTPSA id l3-20020a05620a28c300b006b872b606b1sm2930436qkp.128.2022.09.21.16.52.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Sep 2022 16:52:45 -0700 (PDT) Date: Wed, 21 Sep 2022 23:52:45 +0000 From: Joel Fernandes To: Frederic Weisbecker Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com, urezki@gmail.com, neeraj.iitr10@gmail.com, paulmck@kernel.org, rostedt@goodmis.org, vineeth@bitbyteword.org, boqun.feng@gmail.com Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation Message-ID: References: <20220901221720.1105021-1-joel@joelfernandes.org> <20220901221720.1105021-7-joel@joelfernandes.org> <20220902152132.GA115525@lothringen> <20220906151757.GA183806@lothringen> <20220907094014.GA193994@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220907094014.GA193994@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 07, 2022 at 11:40:14AM +0200, Frederic Weisbecker wrote: > On Wed, Sep 07, 2022 at 12:06:26AM +0000, Joel Fernandes wrote: > > > > @@ -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); > > > > You mean something like the following right? Though I'm thinking if its > > better to call wake_nocb_gp() from tree.c in entrain() and let that handle > > the wake. That way, we can get rid of the extra FLUSH_BP flags as well and > > let the flush callers deal with the wakeups.. > > Ah yes that could make sense if only one caller cares. > > > > > Anyway, for testing this should be good... > > > > ---8<----------------------- > > > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > > index bd8f39ee2cd0..e3344c262672 100644 > > --- a/kernel/rcu/tree_nocb.h > > +++ b/kernel/rcu/tree_nocb.h > > @@ -382,15 +382,19 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp, > > unsigned long j, unsigned long flush_flags) > > { > > bool ret; > > + bool was_alldone; > > > > if (!rcu_rdp_is_offloaded(rdp)) > > return true; > > rcu_lockdep_assert_cblist_protected(rdp); > > rcu_nocb_bypass_lock(rdp); > > + if (flush_flags & FLUSH_BP_WAKE) > > + was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist); > > + > > You can check that outside bypass lock (but you still need nocb_lock). > > > ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags); > > > > - if (flush_flags & FLUSH_BP_WAKE) > > - wake_nocb_gp(rdp, true); > > + if (flush_flags & FLUSH_BP_WAKE && was_alldone) > > + wake_nocb_gp(rdp, false); > > That doesn't check if the bypass list was empty. I am ending up with something like the below for v6, after discussing with Paul on IRC he pointed out we only need to do the rcu_barrier() related wakeup when all the CBs are lazy in the bypass list. Otherwise timer goes off. I think Frederic mentioned something similar above in different words. I prefer to keep this logic in tree_nocb.h since rcu_barrier_entrain() shouldn't have to deal with nocb internals (in theory anyway). Looks Ok? thanks, - Joel ---8<----------------------- From: "Joel Fernandes (Google)" Subject: [PATCH for v6] fixup! rcu: Introduce call_rcu_lazy() API implementation Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/tree_nocb.h | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index c197534d0c99..fd056358f041 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -375,18 +375,26 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp, unsigned long j, unsigned long flush_flags) { bool ret; - bool was_alldone; + bool was_alldone = false; + bool bypass_all_lazy = false; if (!rcu_rdp_is_offloaded(rdp)) return true; rcu_lockdep_assert_cblist_protected(rdp); rcu_nocb_bypass_lock(rdp); - if (flush_flags & FLUSH_BP_WAKE) + + if (flush_flags & FLUSH_BP_WAKE) { was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist); + bypass_all_lazy = + (rcu_cblist_n_cbs(&rdp->nocb_bypass) == rdp->lazy_len); + } ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags); - if (flush_flags & FLUSH_BP_WAKE && was_alldone) + // Wake up the nocb GP thread if needed. GP thread could be sleeping + // while waiting for lazy timer to expire (otherwise rcu_barrier may + // end up waiting for the duration of the lazy timer). + if (flush_flags & FLUSH_BP_WAKE && was_alldone && bypass_all_lazy) wake_nocb_gp(rdp, false); return ret; -- 2.37.3.998.g577e59143f-goog