linux-kernel - Re: [PATCH v3 2/2] blk-cgroup: Optimize blkcg_rstat

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Date:   Wed, 1 Jun 2022 17:30:53 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH v3 2/2] blk-cgroup: Optimize blkcg_rstat_flush()


On 6/1/22 17:26, Tejun Heo wrote:
> On Wed, Jun 01, 2022 at 05:18:24PM -0400, Waiman Long wrote:
>> @@ -59,6 +59,57 @@ static struct workqueue_struct *blkcg_punt_bio_wq;
>>   
>>   #define BLKG_DESTROY_BATCH_SIZE  64
>>   
>> +/*
>> + * lnode.next of the last entry in a lockless list is NULL. To make it
>> + * always non-NULL for lnode's, a sentinel node has to be put at the
>> + * end of the lockless list. So all the percpu lhead's are initialized
>> + * to point to that sentinel node.
>> + */
> Can you please add why we want all entries to have non-NULL next?

As said elsewhere, lnode->next is used as a flag to indicate its 
presence in a lockless list. Sorry for not being explicit here.


>
>> +static inline bool blkcg_llist_empty(struct llist_head *lhead)
>> +{
>> +	return lhead->first == &llist_last;
>> +}
>> +
>> +static inline void init_blkcg_llists(struct blkcg *blkcg)
>> +{
>> +	int cpu;
>> +
>> +	for_each_possible_cpu(cpu)
>> +		per_cpu_ptr(blkcg->lhead, cpu)->first = &llist_last;
>> +}
>> +
>> +static inline struct llist_node *
>> +fetch_delete_blkcg_llist(struct llist_head *lhead)
>> +{
>> +	return xchg(&lhead->first, &llist_last);
>> +}
>> +
>> +static inline struct llist_node *
>> +fetch_delete_lnode_next(struct llist_node *lnode)
>> +{
>> +	struct llist_node *next = READ_ONCE(lnode->next);
>> +	struct blkcg_gq *blkg = llist_entry(lnode, struct blkg_iostat_set,
>> +					    lnode)->blkg;
>> +
>> +	WRITE_ONCE(lnode->next, NULL);
>> +	percpu_ref_put(&blkg->refcnt);
>> +	return next;
>> +}
> It's not a strong opinion but I'm not too fond of using inlines to mark
> trivial functions. The compiler should be able to make these decisions,
> right?
>
> Other than the above two bikesheddings,

Sure, I can remove the inline keywords. I think I do it out of habit:-)

Regards,
Longman

>
>   Acked-by: Tejun Heo <tj@...nel.org>
>
> Thanks.
>