Re: [PATCH v4] blk-cgroup: Replace u64 sync with spinlock for iostat

Boy Wu (吳勃誼)
2024-08-09T07:50:55Z

On Tue, 2024-07-30 at 09:49 -1000, tj@kernel.org wrote:
>
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
> Hello, Boy.
>
> On Fri, Jul 26, 2024 at 03:43:27AM +0000, Boy Wu (吳勃誼) wrote:
> ...
> > The use of a spinlock with u64 sync is suggested in
> > include/linux/u64_stats_sync.h:33.
> >
> > * Usage :
> > *
> > * Stats producer (writer) should use following template granted it
> > already got
> > * an exclusive access to counters (a lock is already taken, or per
> cpu
> > * data is used [in a non preemptable context])
> > *
> > * spin_lock_bh(...) or other synchronization to get exclusive
> access
> > * ...
> > * u64_stats_update_begin(&stats->syncp);
> > * u64_stats_add(&stats->bytes64, len); // non atomic operation
> > * u64_stats_inc(&stats->packets64); // non atomic operation
> > * u64_stats_update_end(&stats->syncp);
> >
> > Is this a incorrect statment?
>
> That's not incorrect and it'd make sense if we really want to use
> u64_sync -
> e.g. the reader is hot path. Here, just a spinlock would be simpler
> and do
> fine.
>
> Thanks.
>
> --
> tejun

u64_sync with spin lock has the benefit of locking only when writing
iostat, but replacing u64_sync with spin lock will lock not only when
writing iostat but also when reading iostat. Does it have enough
benefit to replace u64_sync and add the cost of locking when reading
iostat?

--
Boy.Wu