Re: [PATCH v3] blk-cgroup: Replace u64 sync with spinlock for iostat update

[Waiman Long <longman@redhat.com>]

On 7/17/24 13:40, tj@kernel.org wrote:
> Hello, Waiman.
>
> On Wed, Jul 17, 2024 at 01:37:56PM -0400, Waiman Long wrote:
>> bis->sync is still being used in blk_cgroup_bio_start(). Replacing it with a
>> global lock may kill performance. We may have to use a per-cpu lock if we
>> want to go this route of eliminating bis->sync.
> So, the idea is to keep using u64_sync for blkg->iostat_cpu and use
> blkg_stat_lock for blkg->iostat. The former is the only one which is updated
> in hot path, right?

Well, it can be confusing whether we are dealing with blkg->iostat or 
blkg->iostat_cpu. In many cases, we are dealing with iostat_cpu instead 
of iostat like __blkcg_rstat_flush() and blkg_clear_stat(). So we can't 
eliminate the use of u64_stats_update_begin_irqsave() in those cases.

Cheers,
Longman