Re: [PATCH] rcu: Reduce synchronize_rcu() delays when all wait heads are in use

[Neeraj Upadhyay]

On 3/13/2024 9:43 PM, Joel Fernandes wrote:
> 
> 
> On 3/13/2024 12:04 PM, Neeraj Upadhyay wrote:
>> Hi Joel,
>>
>> On 3/13/2024 8:10 PM, Joel Fernandes wrote:
>>> Hi Neeraj,
>>>
>>> On 3/13/2024 4:32 AM, Neeraj Upadhyay wrote:
>>>> When all wait heads are in use, which can happen when
>>>> rcu_sr_normal_gp_cleanup_work()'s callback processing
>>>> is slow, any new synchronize_rcu() user's rcu_synchronize
>>>> node's processing is deferred to future GP periods. This
>>>> can result in long list of synchronize_rcu() invocations
>>>> waiting for full grace period processing, which can delay
>>>> freeing of memory. Mitigate this problem by using first
>>>> node in the list as wait tail when all wait heads are in use.
>>>> While methods to speed up callback processing would be needed
>>>> to recover from this situation, allowing new nodes to complete
>>>> their grace period can help prevent delays due to a fixed
>>>> number of wait head nodes.
>>>>
>>>> Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
>>>> ---
>>>>  kernel/rcu/tree.c | 27 +++++++++++++--------------
>>>>  1 file changed, 13 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>> index 9fbb5ab57c84..bdccce1ed62f 100644
>>>> --- a/kernel/rcu/tree.c
>>>> +++ b/kernel/rcu/tree.c
>>>> @@ -1470,14 +1470,11 @@ static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
>>>>   * for this new grace period. Given that there are a fixed
>>>>   * number of wait nodes, if all wait nodes are in use
>>>>   * (which can happen when kworker callback processing
>>>> - * is delayed) and additional grace period is requested.
>>>> - * This means, a system is slow in processing callbacks.
>>>> - *
>>>> - * TODO: If a slow processing is detected, a first node
>>>> - * in the llist should be used as a wait-tail for this
>>>> - * grace period, therefore users which should wait due
>>>> - * to a slow process are handled by _this_ grace period
>>>> - * and not next.
>>>> + * is delayed), first node in the llist is used as wait
>>>> + * tail for this grace period. This means, the first node
>>>> + * has to go through additional grace periods before it is
>>>> + * part of the wait callbacks. This should be ok, as
>>>> + * the system is slow in processing callbacks anyway.
>>>>   *
>>>>   * Below is an illustration of how the done and wait
>>>>   * tail pointers move from one set of rcu_synchronize nodes
>>>> @@ -1725,15 +1722,17 @@ static bool rcu_sr_normal_gp_init(void)
>>>>  		return start_new_poll;
>>>>  
>>>>  	wait_head = rcu_sr_get_wait_head();
>>>> -	if (!wait_head) {
>>>> -		// Kick another GP to retry.
>>>> +	if (wait_head) {
>>>> +		/* Inject a wait-dummy-node. */
>>>> +		llist_add(wait_head, &rcu_state.srs_next);
>>>> +	} else {
>>>> +		// Kick another GP for first node.
>>>>  		start_new_poll = true;
>>>> -		return start_new_poll;
>>>> +		if (first == rcu_state.srs_done_tail)
>>>
>>> small nit:
>>> Does done_tail access here need smp_load_acquire() or READ_ONCE() to match the
>>> other users?
>>>
>>
>> As srs_done_tail is only updated in RCU GP thread context, I think it is not required.
>> Please correct me if I am wrong here.
> 
> But will KCSAN not scream that its a data race?
> 

For reads from the exclusive writer context? Interesting, let me check that...


Thanks
Neeraj

> thanks,
> 
>  - Joel
>