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Message-ID: <20240319160252.GA186534@joelbox2>
Date: Tue, 19 Mar 2024 12:02:52 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Uladzislau Rezki <urezki@...il.com>
Cc: linux-kernel@...r.kernel.org, frederic@...nel.org, boqun.feng@...il.com,
neeraj.iitr10@...il.com, rcu@...r.kernel.org, rostedt@...dmis.org,
"Paul E. McKenney" <paulmck@...nel.org>,
Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
Josh Triplett <josh@...htriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH v2 rcu/dev 1/2] rcu/tree: Reduce wake up for
synchronize_rcu() common case
On Tue, Mar 19, 2024 at 03:48:46PM +0100, Uladzislau Rezki wrote:
> On Tue, Mar 19, 2024 at 10:29:59AM -0400, Joel Fernandes wrote:
> >
> >
> > > On Mar 19, 2024, at 5:53 AM, Uladzislau Rezki <urezki@...il.com> wrote:
> > >
> > > On Mon, Mar 18, 2024 at 05:05:31PM -0400, Joel Fernandes wrote:
> > >>
> > >>
> > >>>> On Mar 18, 2024, at 2:58 PM, Uladzislau Rezki <urezki@...il.com> wrote:
> > >>>
> > >>> Hello, Joel!
> > >>>
> > >>> Sorry for late checking, see below few comments:
> > >>>
> > >>>> In the synchronize_rcu() common case, we will have less than
> > >>>> SR_MAX_USERS_WAKE_FROM_GP number of users per GP. Waking up the kworker
> > >>>> is pointless just to free the last injected wait head since at that point,
> > >>>> all the users have already been awakened.
> > >>>>
> > >>>> Introduce a new counter to track this and prevent the wakeup in the
> > >>>> common case.
> > >>>>
> > >>>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > >>>> ---
> > >>>> Rebased on paul/dev of today.
> > >>>>
> > >>>> kernel/rcu/tree.c | 36 +++++++++++++++++++++++++++++++-----
> > >>>> kernel/rcu/tree.h | 1 +
> > >>>> 2 files changed, 32 insertions(+), 5 deletions(-)
> > >>>>
> > >>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > >>>> index 9fbb5ab57c84..bd29fe3c76bf 100644
> > >>>> --- a/kernel/rcu/tree.c
> > >>>> +++ b/kernel/rcu/tree.c
> > >>>> @@ -96,6 +96,7 @@ static struct rcu_state rcu_state = {
> > >>>> .ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
> > >>>> .srs_cleanup_work = __WORK_INITIALIZER(rcu_state.srs_cleanup_work,
> > >>>> rcu_sr_normal_gp_cleanup_work),
> > >>>> + .srs_cleanups_pending = ATOMIC_INIT(0),
> > >>>> };
> > >>>>
> > >>>> /* Dump rcu_node combining tree at boot to verify correct setup. */
> > >>>> @@ -1642,8 +1643,11 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > >>>> * the done tail list manipulations are protected here.
> > >>>> */
> > >>>> done = smp_load_acquire(&rcu_state.srs_done_tail);
> > >>>> - if (!done)
> > >>>> + if (!done) {
> > >>>> + /* See comments below. */
> > >>>> + atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
> > >>>> return;
> > >>>> + }
> > >>>>
> > >>>> WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
> > >>>> head = done->next;
> > >>>> @@ -1666,6 +1670,9 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > >>>>
> > >>>> rcu_sr_put_wait_head(rcu);
> > >>>> }
> > >>>> +
> > >>>> + /* Order list manipulations with atomic access. */
> > >>>> + atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
> > >>>> }
> > >>>>
> > >>>> /*
> > >>>> @@ -1673,7 +1680,7 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > >>>> */
> > >>>> static void rcu_sr_normal_gp_cleanup(void)
> > >>>> {
> > >>>> - struct llist_node *wait_tail, *next, *rcu;
> > >>>> + struct llist_node *wait_tail, *next = NULL, *rcu = NULL;
> > >>>> int done = 0;
> > >>>>
> > >>>> wait_tail = rcu_state.srs_wait_tail;
> > >>>> @@ -1699,16 +1706,35 @@ static void rcu_sr_normal_gp_cleanup(void)
> > >>>> break;
> > >>>> }
> > >>>>
> > >>>> - // concurrent sr_normal_gp_cleanup work might observe this update.
> > >>>> - smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> > >>>> + /*
> > >>>> + * Fast path, no more users to process. Remove the last wait head
> > >>>> + * if no inflight-workers. If there are in-flight workers, let them
> > >>>> + * remove the last wait head.
> > >>>> + */
> > >>>> + WARN_ON_ONCE(!rcu);
> > >>>>
> > >>> This assumption is not correct. An "rcu" can be NULL in fact.
> > >>
> > >> Hmm I could never trigger that. Are you saying that is true after Neeraj recent patch or something else?
> > >> Note, after Neeraj patch to handle the lack of heads availability, it could be true so I requested
> > >> him to rebase his patch on top of this one.
> > >>
> > >> However I will revisit my patch and look for if it could occur but please let me know if you knew of a sequence of events to make it NULL.
> > >>>
> > > I think we should agree on your patch first otherwise it becomes a bit
> > > messy or go with a Neeraj as first step and then work on youth. So, i
> > > reviewed this patch based on latest Paul's dev branch. I see that Neeraj
> > > needs further work.
> >
> > You are right. So the only change is to drop the warning and those braces. Agreed?
> >
> Let me check a bit. Looks like correct but just in case.
>
Thanks. I was also considering improving it for the rcu == NULL case, as
below. I will test it more before re-sending.
On top of my patch:
---8<-----------------------
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0df659a878ee..a5ef844835d4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1706,15 +1706,18 @@ static void rcu_sr_normal_gp_cleanup(void)
break;
}
+
+ /* Last head stays. No more processing to do. */
+ if (!rcu)
+ return;
+
/*
* Fast path, no more users to process. Remove the last wait head
* if no inflight-workers. If there are in-flight workers, let them
* remove the last wait head.
*/
- WARN_ON_ONCE(!rcu);
- ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
- if (rcu && rcu_sr_is_wait_head(rcu) && rcu->next == NULL &&
+ if (rcu_sr_is_wait_head(rcu) && rcu->next == NULL &&
/* Order atomic access with list manipulation. */
!atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
wait_tail->next = NULL;
@@ -1724,6 +1727,7 @@ static void rcu_sr_normal_gp_cleanup(void)
}
/* Concurrent sr_normal_gp_cleanup work might observe this update. */
+ ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
smp_store_release(&rcu_state.srs_done_tail, wait_tail);
/*
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