linux-kernel - Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll

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Date:   Wed, 05 Dec 2018 21:11:54 +0100
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Jason Baron <jbaron@...mai.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce
 ep_poll_callback() contention

On 2018-12-05 17:38, Jason Baron wrote:
> 
> I think it might be interesting for, at least testing, to see if not 
> grabbing
> wq.lock improves your benchmarks any further? fwiw, epoll only recently 
> started
> grabbing wq.lock bc lockdep required it.

That's easy!  I've just tested with the following hunk applied to my 
patch on top:

+++ b/fs/eventpoll.c
@@ -1228,7 +1228,7 @@ static int ep_poll_callback(wait_queue_entry_t 
*wait, unsigned mode, int sync, v
                                 break;
                         }
                 }
-               wake_up(&ep->wq);
+               wake_up_locked(&ep->wq);
         }

Run time:

threads   w/ wq.lock   w/o wq.lock
-------   ----------   -----------
      8        8581ms        8602ms
     16       13800ms       13715ms
     32       24167ms       23817ms

No big difference.  According to perf the contention is on read lock and
on try_to_wake_up(), the p->pi_lock, which serializes access exactly 
like
vanished wq.lock.


-   24.41%     5.39%  a.out    [kernel.kallsyms]   [k] ep_poll_callback
    - 19.02% ep_poll_callback
       + 11.88% _raw_read_lock_irqsave
       + 5.74% _raw_read_unlock_irqrestore
       - 1.39% __wake_up_common
          - 1.22% try_to_wake_up
             + 0.98% _raw_spin_lock_irqsave


--
Roman