Re: [PATCH v2 rcu/dev 1/2] rcu/tree: Reduce wake up for synchronize_rcu() common case

[Joel Fernandes]

On Tue, Mar 19, 2024 at 12:02 PM Joel Fernandes <joel@joelfernandesorg> wrote:
>
> 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@gmailcom> 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@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> > > >>>> ---
> > > >>>> 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;
> +

Ugh, should be "if (!wait_head->next)"  instead of "if (!rcu)".  But
in any case, the original patch except the warning should hold.
Still, I am testing the above diff now.

 - Joel