Re: [PATCH v9 3/3] blk-cgroup: Flush stats at blkgs destruction path

[Waiman Long <longman@redhat.com>]

On 11/4/22 16:00, Tejun Heo wrote:
> On Fri, Nov 04, 2022 at 02:20:50PM -0400, Waiman Long wrote:
>> +/**
>> + * cgroup_rstat_css_flush - flush stats for the given css and cpu
>> + * @css: target css to be flush
>> + * @cpu: the cpu that holds the stats to be flush
>> + *
>> + * A lightweight rstat flush operation for a given css and cpu.
>> + * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock
>> + * isn't used.
>> + */
>> +void cgroup_rstat_css_flush(struct cgroup_subsys_state *css, int cpu)
>> +{
>> +	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
>> +
>> +	raw_spin_lock_irq(cpu_lock);
>> +	rcu_read_lock();
>> +	css->ss->css_rstat_flush(css, cpu);
>> +	rcu_read_unlock();
>> +	raw_spin_unlock_irq(cpu_lock);
>> +}
> Would it make sense to itereate CPUs within the helper rather than asking
> the caller to do it? Also, in terms of patch sequencing, this introduces a
> bug and then fixes it. Prolly better to not introduce the bug in the first
> place.
>
> Thanks.

I should have named the function cgroup_rstat_css_cpu_flush() to 
indicate that the cpu is a needed parameter. We can have a 
cgroup_rstat_css_flush() in the future if the need arises.

It is an optimization to call this function only if the corresponding 
cpu has a pending lockless list. I could do cpu iteration here and call 
the flushing function for all the CPUs. It is less optimized this way. 
Since it is a slow path, I guess performance is not that critical. So I 
can go either way. Please let me know your preference.

Thanks,
Longman