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Date: Fri, 30 Aug 2013 15:20:48 -0400
From: Waiman Long <waiman.long@...com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Ingo Molnar <mingo@...nel.org>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Jeff Layton <jlayton@...hat.com>,
Miklos Szeredi <mszeredi@...e.cz>,
Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Andi Kleen <andi@...stfloor.org>,
"Chandramouleeswaran, Aswin" <aswin@...com>,
"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
update of refcount
On 08/30/2013 02:53 PM, Linus Torvalds wrote:
> So the perf data would be *much* more interesting for a more varied
> load. I know pretty much exactly what happens with my silly
> test-program, and as you can see it never really gets to the actual
> spinlock, because that test program will only ever hit the fast-path
> case. It would be much more interesting to see another load that may
> trigger the d_lock actually being taken. So:
>> For the other test cases that I am interested in, like the AIM7 benchmark,
>> your patch may not be as good as my original one. I got 1-3M JPM (varied
>> quite a lot in different runs) in the short workloads on a 80-core system.
>> My original one got 6M JPM. However, the test was done on 3.10 based kernel.
>> So I need to do more test to see if that has an effect on the JPM results.
> I'd really like to see a perf profile of that, particularly with some
> call chain data for the relevant functions (ie "what it is that causes
> us to get to spinlocks"). Because it may well be that you're hitting
> some of the cases that I didn't see, and thus didn't notice.
>
> In particular, I suspect AIM7 actually creates/deletes files and/or
> renames them too. Or maybe I screwed up the dget_parent() special case
> thing, which mattered because AIM7 did a lot of getcwd() calls or
> someting odd like that.
>
> Linus
Below is the perf data of my short workloads run in an 80-core DL980:
13.60% reaim [kernel.kallsyms] [k]
_raw_spin_lock_irqsave
|--48.79%-- tty_ldisc_try
|--48.58%-- tty_ldisc_deref
--2.63%-- [...]
11.31% swapper [kernel.kallsyms] [k] intel_idle
|--99.94%-- cpuidle_enter_state
--0.06%-- [...]
4.86% reaim [kernel.kallsyms] [k] lg_local_lock
|--59.41%-- mntput_no_expire
|--19.37%-- path_init
|--15.14%-- d_path
|--5.88%-- sys_getcwd
--0.21%-- [...]
3.00% reaim reaim [.] mul_short
2.41% reaim reaim [.] mul_long
|--87.21%-- 0xbc614e
--12.79%-- (nil)
2.29% reaim reaim [.] mul_int
2.20% reaim [kernel.kallsyms] [k] _raw_spin_lock
|--12.81%-- prepend_path
|--9.90%-- lockref_put_or_lock
|--9.62%-- __rcu_process_callbacks
|--8.77%-- load_balance
|--6.40%-- lockref_get
|--5.55%-- __mutex_lock_slowpath
|--4.85%-- __mutex_unlock_slowpath
|--4.83%-- inet_twsk_schedule
|--4.27%-- lockref_get_or_lock
|--2.19%-- task_rq_lock
|--2.13%-- sem_lock
|--2.09%-- scheduler_tick
|--1.88%-- try_to_wake_up
|--1.53%-- kmem_cache_free
|--1.30%-- unix_create1
|--1.22%-- unix_release_sock
|--1.21%-- process_backlog
|--1.11%-- unix_stream_sendmsg
|--1.03%-- enqueue_to_backlog
|--0.85%-- rcu_accelerate_cbs
|--0.79%-- unix_dgram_sendmsg
|--0.76%-- do_anonymous_page
|--0.70%-- unix_stream_recvmsg
|--0.69%-- unix_stream_connect
|--0.64%-- net_rx_action
|--0.61%-- tcp_v4_rcv
|--0.59%-- __do_fault
|--0.54%-- new_inode_pseudo
|--0.52%-- __d_lookup
--10.62%-- [...]
1.19% reaim [kernel.kallsyms] [k] mspin_lock
|--99.82%-- __mutex_lock_slowpath
--0.18%-- [...]
1.01% reaim [kernel.kallsyms] [k] lg_global_lock
|--51.62%-- __shmdt
--48.38%-- __shmctl
There are more contention in the lglock than I remember for the run in
3.10. This is an area that I need to look at. In fact, lglock is
becoming a problem for really large machine with a lot of cores. We have
a prototype 16-socket machine with 240 cores under development. The cost
of doing a lg_global_lock will be very high in that type of machine
given that it is already high in this 80-core machine. I have been
thinking about instead of per-cpu spinlocks, we could change the locking
to per-node level. While there will be more contention for
lg_local_lock, the cost of doing a lg_global_lock will be much lower and
contention within the local die should not be too bad. That will require
either a per-node variable infrastructure or simulated with the existing
per-cpu subsystem.
I will also need to look at ways reduce the need of taking d_lock in
existing code. One area that I am looking at is whether we can take out
the lock/unlock pair in prepend_path(). This function can only be called
with the rename_lock taken. So no filename change or deletion will be
allowed. It will only be a problem if somehow the dentry itself got
killed or dropped while the name is being copied out. The first dentry
referenced by the path structure should have a non-zero reference count,
so that shouldn't happen. I am not so sure about the parents of that
dentry as I am not so familiar with that part of the filesystem code.
Regards,
Longman
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