linux-kernel - Re: [PATCH] rcu: Latch normal synchronize

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Message-ID: <00e91ebc-0783-4519-9727-53dd3a625298@linux.ibm.com>
Date: Sat, 17 Jan 2026 17:48:56 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Joel Fernandes <joelagnelf@...dia.com>, Samir M <samir@...ux.ibm.com>
Cc: Uladzislau Rezki <urezki@...il.com>, Paul E McKenney
 <paulmck@...nel.org>,
        Vishal Chourasia <vishalc@...ux.ibm.com>,
        Neeraj upadhyay <Neeraj.Upadhyay@....com>, RCU <rcu@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH] rcu: Latch normal synchronize_rcu() path on flood



On 1/17/26 2:18 PM, Joel Fernandes wrote:
> 
> 
>> On Jan 17, 2026, at 1:17 AM, Samir M <samir@...ux.ibm.com> wrote:
>>
>> 
>>> On 15/01/26 12:04 am, Uladzislau Rezki (Sony) wrote:
>>> Currently, rcu_normal_wake_from_gp is only enabled by default
>>> on small systems(<= 16 CPUs) or when a user explicitly set it
>>> enabled.
>>>
>>> This patch introduces an adaptive latching mechanism:
>>>   * Tracks the number of in-flight synchronize_rcu() requests
>>>     using a new atomic_t counter(rcu_sr_normal_count);
>>>
>>>   * If the count exceeds RCU_SR_NORMAL_LATCH_THR(64), it sets
>>>     the rcu_sr_normal_latched, reverting new requests onto the
>>>     scaled wait_rcu_gp() path;
>>>
>>>   * The latch is cleared only when the pending requests are fully
>>>     drained(nr == 0);
>>>
>>>   * Enables rcu_normal_wake_from_gp by default for all systems,
>>>     relying on this dynamic throttling instead of static CPU
>>>     limits.
>>>
>>> Suggested-by: Joel Fernandes <joelagnelf@...dia.com>
>>> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
>>> ---
>>>   kernel/rcu/tree.c | 37 ++++++++++++++++++++++++++-----------
>>>   1 file changed, 26 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>> index 293bbd9ac3f4..c42d480d6e0b 100644
>>> --- a/kernel/rcu/tree.c
>>> +++ b/kernel/rcu/tree.c
>>> @@ -1631,17 +1631,21 @@ static void rcu_sr_put_wait_head(struct llist_node *node)
>>>       atomic_set_release(&sr_wn->inuse, 0);
>>>   }
>>>   -/* 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;
>>> +static int rcu_normal_wake_from_gp = 1;
>>>   module_param(rcu_normal_wake_from_gp, int, 0644);
>>>   static struct workqueue_struct *sync_wq;
>>>   +#define RCU_SR_NORMAL_LATCH_THR 64
>>> +
>>> +/* Number of in-flight synchronize_rcu() calls queued on srs_next. */
>>> +static atomic_long_t rcu_sr_normal_count;
>>> +static atomic_t rcu_sr_normal_latched;
>>> +
>>>   static void rcu_sr_normal_complete(struct llist_node *node)
>>>   {
>>>       struct rcu_synchronize *rs = container_of(
>>>           (struct rcu_head *) node, struct rcu_synchronize, head);
>>> +    long nr;
>>>         WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
>>>           !poll_state_synchronize_rcu_full(&rs->oldstate),
>>> @@ -1649,6 +1653,15 @@ static void rcu_sr_normal_complete(struct llist_node *node)
>>>         /* Finally. */
>>>       complete(&rs->completion);
>>> +    nr = atomic_long_dec_return(&rcu_sr_normal_count);
>>> +    WARN_ON_ONCE(nr < 0);
>>> +
>>> +    /*
>>> +     * Unlatch: switch back to normal path when fully
>>> +     * drained and if it has been latched.
>>> +     */
>>> +    if (nr == 0)
>>> +        (void)atomic_cmpxchg(&rcu_sr_normal_latched, 1, 0);
>>>   }
>>>     static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>>> @@ -1794,7 +1807,14 @@ static bool rcu_sr_normal_gp_init(void)
>>>     static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
>>>   {
>>> +    long nr;
>>> +
>>>       llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next);
>>> +    nr = atomic_long_inc_return(&rcu_sr_normal_count);
>>> +
>>> +    /* Latch: only when flooded and if unlatched. */
>>> +    if (nr >= RCU_SR_NORMAL_LATCH_THR)
>>> +        (void)atomic_cmpxchg(&rcu_sr_normal_latched, 0, 1);
>>>   }
>>>     /*
>>> @@ -3268,7 +3288,8 @@ 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) < 1) {
>>> +    if (READ_ONCE(rcu_normal_wake_from_gp) < 1 ||
>>> +            atomic_read(&rcu_sr_normal_latched)) {
>>>           wait_rcu_gp(call_rcu_hurry);
>>>           goto trace_complete_out;
>>>       }
>>> @@ -4892,12 +4913,6 @@ void __init rcu_init(void)
>>>       sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM | WQ_UNBOUND, 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)
>>
>>
>> Hi Uladzislau,
>>
>> I verified this patch using the configuration described below.
>> Configuration:
>>      •    Kernel version: 6.19.0-rc5
>>      •    Number of CPUs: 2048
>>
>> Using this setup, I evaluated the patch with both SMT enabled and SMT disabled. The results indicate that when SMT is enabled, the system time is noticeably higher. In contrast, with SMT disabled, no significant increase in system time is observed.
>>
>> SMT=ON  -> sys 31m22.922s
>> SMT=OFF -> sys 0m0.046s
>>
>>
>> SMT Mode    | Without Patch    | With Patch   | % Improvement    |
>> ------------------------------------------------------------------
>> SMT=off     | 30m 53.194s      | 26m 24.009s  | +14.53%          |
>> SMT=on      | 49m 5.920s       | 47m 5.513s   | +4.09%
> 
> So it takes you 47 minutes to offline CPUs and you are Ok with that?
> 
> - Joel
>


This is certainly quite long. IMO not worth the added complexity
of atomic inc/dec reads happening(even though till 64 CPUs)

Samir,
can you post number's with vishal's patch of using rcu expedited?


> 
> 
>>         |
>>
>>
>> Please add below tag: Tested-by: Samir M <samir@...ux.ibm.com>
>>
>> Regards,
>> Samir
>>
>>