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From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org,
	linux-s390@vger.kernel.org, linux-arch@vger.kernel.org,
	Davidlohr Bueso <dave@stgolabs.net>,
	Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
	Scott J Norton <scott.norton@hpe.com>,
	Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation
Date: Wed, 15 Jun 2016 23:57:16 +0200	[thread overview]
Message-ID: <20160615215716.GX30921@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <5761AD27.9020106@hpe.com>

On Wed, Jun 15, 2016 at 03:31:51PM -0400, Waiman Long wrote:
> On 06/15/2016 01:43 PM, Peter Zijlstra wrote:
> >On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote:
> >>even the reduced maximum of about 16k (32-bit) or 1G (64-bit) should
> >>be more than enough for the foreseeable future.
> >So what happens if I manage to create 16k+ threads on my 32bit kernel
> >and get them all to do mmap() or so at the same time.
> >
> >That doesn't seem too far fetched.
> >
> >Then again, with double that (the current limit) that doesn't seem
> >impossible either.
> 
> To hit the limit, we need to have all the threads calling down_write() at
> exactly the same instance in time which, I think, is pretty hard to do.
> Also, I don't believe you will ever see a 16k-cpu massive SMP system running
> on 32-bit kernel. I can imagine such a system running on 64-bit kernel, but
> certainly not 32-bit.

Ah, so I thought we kept the WRITE_BIAS while blocking, which we don't.

But if they all get preempted before we undo the WRITE_BIAS then 1 CPU
will be able to trigger this. However utterly unlikely.

  reply	other threads:[~2016-06-15 21:57 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-14 22:48 [RFC PATCH-tip v2 0/6] locking/rwsem: Enable reader optimistic spinning Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier Waiman Long
2016-06-15  8:04   ` Boqun Feng
2016-06-15 17:18     ` Peter Zijlstra
2016-06-15 19:01     ` Waiman Long
2016-06-16  2:19       ` Boqun Feng
2016-06-16 10:16         ` Will Deacon
2016-06-16 21:35         ` Waiman Long
2016-06-17  0:48           ` Boqun Feng
2016-06-17 15:26             ` Waiman Long
2016-06-17 15:45               ` Will Deacon
2016-06-17 18:17                 ` Waiman Long
2016-06-18  8:46                   ` Boqun Feng
2016-06-20  7:59                     ` Will Deacon
2016-06-15 16:56   ` Davidlohr Bueso
2016-06-15 17:12     ` Peter Zijlstra
2016-06-15 18:27       ` Davidlohr Bueso
2016-06-15 18:40         ` Peter Zijlstra
2016-06-15 18:56           ` Davidlohr Bueso
2016-06-17  1:11           ` Davidlohr Bueso
2016-06-17 14:28             ` Waiman Long
2016-06-17 16:29               ` Davidlohr Bueso
2016-06-17 16:46                 ` Davidlohr Bueso
2016-06-15 19:08       ` Waiman Long
2016-06-15 20:04         ` Waiman Long
2016-06-15 21:59           ` Peter Zijlstra
2016-06-14 22:48 ` [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP Waiman Long
2016-06-15 17:22   ` Peter Zijlstra
2016-06-15 19:17     ` Waiman Long
2016-06-16  2:14       ` Davidlohr Bueso
2016-06-16 21:25         ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 3/6] locking/rwsem: Enable count-based spinning on reader Waiman Long
2016-06-15 17:38   ` Peter Zijlstra
2016-06-15 19:28     ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 4/6] locking/rwsem: move down rwsem_down_read_failed function Waiman Long
2016-06-15 17:40   ` Peter Zijlstra
2016-06-15 19:21     ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation Waiman Long
2016-06-15 17:43   ` Peter Zijlstra
2016-06-15 19:31     ` Waiman Long
2016-06-15 21:57       ` Peter Zijlstra [this message]
2016-06-15 17:45   ` Peter Zijlstra
2016-06-15 19:35     ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 6/6] locking/rwsem: Enable spinning readers Waiman Long

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