Re: [PATCH] kernfs: Change kernfs_rwsem to a per-cpu rwsem.

[Tejun Heo <tj@kernel.org>]

On Mon, Jun 20, 2022 at 01:26:34PM +1000, Imran Khan wrote:
> On large systems when few hundred CPUs simulateously acquire kernfs_rwsem,
> for reading we see performance degradation due to bouncing of cache line
> that contains kernfs_rwsem. Changing kernfs_rwsem into a per-fs, per-cpu
> rwsem can fix this degradation.
...
> Moreover this run of 200 applications take more than 32 secs to finish on
> this system.
> 
> After changing kernfs_rwsem to a per-cpu rwsem, I can see that contention
> for kernfs_rwsem is no longer visible in perf data and the test execution
> time has reduced to almost half (17 secs or less from 32 secs or more).
> 
> The overhead involving write operations with per-cpu rwsem will be higher
> but frequency of creation/deletion of kernfs files is much less than
> frequency at which kernfs (cgroup, sysfs) files are read.

The problem with percpu_rwsem is that write locking requires going
through a RCU grace period, which can easily add two or more digit
millisec latencies. I'm pretty sure there are code paths which are
pretty heavy on write - e.g. during boot, depending on the machine
configuration, we could be write-acquiring the rwsem hundreds of
thousands of times and we'd be constantly doing RCU grace periods.

So, I don't think kernfs_rwsem is a good candidate for percpu rwsem.
There likely are plenty of cases where write path isn't cold enough.

Thanks.

-- 
tejun