linux-kernel - Re: [PATCH v3] rcu: Reduce synchronize_rcu() latency by reporting GP kthread's CPU QS early

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Message-ID: <9B61E99F-0D5B-4811-B634-E86387EF4D61@nvidia.com>
Date: Sun, 11 Jan 2026 12:03:13 +0000
From: Joel Fernandes <joelagnelf@...dia.com>
To: Samir M <samir@...ux.ibm.com>
CC: Paul E McKenney <paulmck@...nel.org>, Frederic Weisbecker
	<frederic@...nel.org>, Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Josh
 Triplett <josh@...htriplett.org>, Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>, "rcu@...r.kernel.org"
	<rcu@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>, Mathieu
 Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan
	<jiangshanlai@...il.com>, Zqiang <qiang.zhang@...ux.dev>,
	"vishalc@...ux.ibm.com" <vishalc@...ux.ibm.com>
Subject: Re: [PATCH v3] rcu: Reduce synchronize_rcu() latency by reporting GP
 kthread's CPU QS early



> On Jan 11, 2026, at 5:13 AM, Samir M <samir@...ux.ibm.com> wrote:
> 
> 
>> On 30/12/25 6:11 am, Joel Fernandes wrote:
>> The RCU grace period mechanism uses a two-phase FQS (Force Quiescent
>> State) design where the first FQS saves dyntick-idle snapshots and
>> the second FQS compares them. This results in long and unnecessary latency
>> for synchronize_rcu() on idle systems (two FQS waits of ~3ms each with
>> 1000HZ) whenever one FQS wait sufficed.
>> 
>> Some investigations showed that the GP kthread's CPU is the holdout CPU
>> a lot of times after the first FQS as - it cannot be detected as "idle"
>> because it's actively running the FQS scan in the GP kthread.
>> 
>> Therefore, at the end of rcu_gp_init(), immediately report a quiescent
>> state for the GP kthread's CPU using rcu_qs() + rcu_report_qs_rdp(). The
>> GP kthread cannot be in an RCU read-side critical section while running
>> GP initialization, so this is safe and results in significant latency
>> improvements.
>> 
>> I benchmarked 100 synchronize_rcu() calls with 32 CPUs, 10 runs each
>> showing significant latency improvements (default settings for fqs jiffies):
>> 
>> Baseline (without fix):
>> | Run | Mean      | Min      | Max       |
>> |-----|-----------|----------|-----------|
>> | 1   | 10.088 ms | 9.989 ms | 18.848 ms |
>> | 2   | 10.064 ms | 9.982 ms | 16.470 ms |
>> | 3   | 10.051 ms | 9.988 ms | 15.113 ms |
>> | 4   | 10.125 ms | 9.929 ms | 22.411 ms |
>> | 5   |  8.695 ms | 5.996 ms | 15.471 ms |
>> | 6   | 10.157 ms | 9.977 ms | 25.723 ms |
>> | 7   | 10.102 ms | 9.990 ms | 20.224 ms |
>> | 8   |  8.050 ms | 5.985 ms | 10.007 ms |
>> | 9   | 10.059 ms | 9.978 ms | 15.934 ms |
>> | 10  | 10.077 ms | 9.984 ms | 17.703 ms |
>> 
>> With fix:
>> | Run | Mean     | Min      | Max       |
>> |-----|----------|----------|-----------|
>> | 1   | 6.027 ms | 5.915 ms |  8.589 ms |
>> | 2   | 6.032 ms | 5.984 ms |  9.241 ms |
>> | 3   | 6.010 ms | 5.986 ms |  7.004 ms |
>> | 4   | 6.076 ms | 5.993 ms | 10.001 ms |
>> | 5   | 6.084 ms | 5.893 ms | 10.250 ms |
>> | 6   | 6.034 ms | 5.908 ms |  9.456 ms |
>> | 7   | 6.051 ms | 5.993 ms | 10.000 ms |
>> | 8   | 6.057 ms | 5.941 ms | 10.001 ms |
>> | 9   | 6.016 ms | 5.927 ms |  7.540 ms |
>> | 10  | 6.036 ms | 5.993 ms |  9.579 ms |
>> 
>> Summary:
>> - Mean latency: 9.75 ms -> 6.04 ms (38% improvement)
>> - Max latency:  25.72 ms -> 10.25 ms (60% improvement)
>> 
>> Additional bridge setup/teardown testing by Uladzislau Rezki on x86_64
>> with 64 CPUs (100 iterations of bridge add/configure/delete):
>> 
>>                                real time
>> 1 - default:                   24.221s
>> 2 - this patch:                20.754s  (14% faster)
>> 3 - this patch + wake_from_gp: 15.895s  (34% faster)
>> 4 - wake_from_gp only:         18.947s  (22% faster)
>> 
>> Per-synchronize_rcu() latency (in usec):
>>               1         2         3       4
>> median: 37249.5   31540.5   15765   22480
>> min:    7881      7918      9803    7857
>> max:    63651     55639     31861   32040
>> 
>> This patch combined with rcu_normal_wake_from_gp reduces bridge
>> setup/teardown time from 24 seconds to 16 seconds.
>> 
>> Tested rcutorture TREE and SRCU configurations.
>> 
>> Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
>> Tested-by: Uladzislau Rezki (Sony) <urezki@...il.com>
>> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
>> ---
>>  kernel/rcu/tree.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>> 
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 78c045a5ef03..b7c818cabe44 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -160,6 +160,7 @@ static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
>>                    unsigned long gps, unsigned long flags);
>>  static void invoke_rcu_core(void);
>>  static void rcu_report_exp_rdp(struct rcu_data *rdp);
>> +static void rcu_report_qs_rdp(struct rcu_data *rdp);
>>  static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
>>  static bool rcu_rdp_is_offloaded(struct rcu_data *rdp);
>>  static bool rcu_rdp_cpu_online(struct rcu_data *rdp);
>> @@ -1983,6 +1984,17 @@ static noinline_for_stack bool rcu_gp_init(void)
>>      if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
>>          on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
>>  +    /*
>> +     * Immediately report QS for the GP kthread's CPU. The GP kthread
>> +     * cannot be in an RCU read-side critical section while running
>> +     * the FQS scan. This eliminates the need for a second FQS wait
>> +     * when all CPUs are idle.
>> +     */
>> +    preempt_disable();
>> +    rcu_qs();
>> +    rcu_report_qs_rdp(this_cpu_ptr(&rcu_data));
>> +    preempt_enable();
>> +
>>      return true;
>>  }
>>  
> 
> Hi,
> 
> 
> I verified this patch on ppc64 systems and observed consistent performance improvements.
> 
> The testing was conducted on Power LPARs using 20 cores (160 CPUs) with SMT enabled and disabled. All tests were performed on the latest upstream kernel (v6.19.0-rc3+), and the patch showed measurable improvements in both SMT configurations.
> 
> SMT Mode   |      With Patch (s)    |    Without Patch (s)  | Improvement (%)
> ————————————————————————————————————
> SMT ON.      |    51.662               |      75.540  |       31.61% faster
> SMT OFF      |    44.246               |      59.933  |       26.18% faster
> 
> Please add below tag: Tested-by: Samir M <samir@...ux.ibm.com>

Thanks! Boqun, do you mind adding the tag to the applied patch?

 - Joel


> 
> Thanks,
> Samir
>