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

[Uladzislau Rezki]

On Tue, Mar 19, 2024 at 02:52:43PM -0400, Joel Fernandes wrote:
> 
> 
> On 3/19/2024 2:37 PM, Uladzislau Rezki wrote:
> > On Tue, Mar 19, 2024 at 01:33:11PM -0400, Joel Fernandes wrote:
> 
> >>> On 3/19/2024 1:26 PM, Uladzislau Rezki wrote:
> 
> >>>>>>>>>>>> /*
> >>>>>>>>>>>> @@ -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
> >>>>>
> >>>> Just in case, it is based on your patch:
> >>>>
> >>>> <snip>
> >>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >>>> index bd29fe3c76bf..98546afe7c21 100644
> >>>> --- a/kernel/rcu/tree.c
> >>>> +++ b/kernel/rcu/tree.c
> >>>> @@ -1711,29 +1711,25 @@ static void rcu_sr_normal_gp_cleanup(void)
> >>>>  	 * 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 &&
> >>>> -		/* Order atomic access with list manipulation. */
> >>>> -		!atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
> >>>> +	if (wait_tail->next && rcu_sr_is_wait_head(wait_tail->next) && !wait_tail->next->next &&
> >>>> +			!atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
> >>>
> >>>
> >>> Yes this also works. But also if wait_tail->next == NULL, then you do not need
> >>> to queue worker for that case as well. I sent this as v3.
> >>>
> >> Sorry, I see you did add that later in the patch ;-). I think we have converged
> >> on the final patch then, give or take the use of 'rcu' versus 'wait_tail->next'.
> >>
> > Just combine all parts into one place and resend :)
> 
> Yes sir ;)
> 
Ha-ha :)))))

--
Uladzislau Rezki