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

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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




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