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

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Message-ID: <CALAqxLXLf_hB0xQ_vnBYoHyDF2-Bkh9GznLQK=ikYFdcw5u3WQ@mail.gmail.com>
Date:	Tue, 9 Aug 2016 16:47:38 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Rom Lemarchand <romlem@...gle.com>,
	Colin Cross <ccross@...gle.com>, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce
 global impact

On Tue, Aug 9, 2016 at 2:51 AM, Peter Zijlstra <peterz@...radead.org> 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