Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce global impact

[John Stultz]

On Tue, Aug 9, 2016 at 2:51 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Currently the percpu-rwsem switches to (global) atomic ops while a
> writer is waiting; which could be quite a while and slows down
> releasing the readers.
>
> This patch cures this problem by ordering the reader-state vs
> reader-count (see the comments in __percpu_down_read() and
> percpu_down_write()). This changes a global atomic op into a full
> memory barrier, which doesn't have the global cacheline contention.
>
> This also enables using the percpu-rwsem with rcu_sync disabled in order
> to bias the implementation differently, reducing the writer latency by
> adding some cost to readers.

So this by itself doesn't help us much, but including the following
from Oleg does help quite a bit:

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index db27804..9e9200b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5394,6 +5394,8 @@ int __init cgroup_init(void)
        BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
        BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));

+       rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);
+
        mutex_lock(&cgroup_mutex);

        /* Add init_css_set to the hash table */


thanks
-john