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From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: linux-arch@vger.kernel.org, linux-xtensa@linux-xtensa.org,
	Davidlohr Bueso <dave@stgolabs.net>,
	linux-ia64@vger.kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-sh@vger.kernel.org, linux-hexagon@vger.kernel.org,
	x86@kernel.org, Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	linuxppc-dev@lists.ozlabs.org,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH-tip 15/22] locking/rwsem: Merge owner into count on x86-64
Date: Thu, 7 Feb 2019 21:08:48 +0100
Message-ID: <20190207200848.GH32477@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1549566446-27967-16-git-send-email-longman@redhat.com>

On Thu, Feb 07, 2019 at 02:07:19PM -0500, Waiman Long wrote:
> On 32-bit architectures, there aren't enough bits to hold both.
> 64-bit architectures, however, can have enough bits to do that. For
> x86-64, the physical address can use up to 52 bits. That is 4PB of
> memory. That leaves 12 bits available for other use. The task structure
> pointer is also aligned to the L1 cache size. That means another 6 bits
> (64 bytes cacheline) will be available. Reserving 2 bits for status
> flags, we will have 16 bits for the reader count.  That can supports
> up to (64k-1) readers.

64k readers sounds like a number that is fairly 'easy' to reach, esp. on
64bit. These are preemptible locks after all, all we need to do is get
64k tasks nested on enough CPUs.

I'm sure there's some willing Java proglet around that spawns more than
64k threads just because it can. Run it on a big enough machine (ISTR
there's a number of >1k CPU systems out there) and voila.



  parent reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-07 19:07 [PATCH-tip 00/22] locking/rwsem: Rework rwsem-xadd & enable new rwsem features Waiman Long
2019-02-07 19:07 ` [PATCH-tip 01/22] locking/qspinlock_stat: Introduce a generic lockevent counting APIs Waiman Long
2019-02-07 19:07 ` [PATCH-tip 02/22] locking/lock_events: Make lock_events available for all archs & other locks Waiman Long
2019-02-07 19:07 ` [PATCH-tip 03/22] locking/rwsem: Relocate rwsem_down_read_failed() Waiman Long
2019-02-07 19:07 ` [PATCH-tip 04/22] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-02-07 19:36   ` Peter Zijlstra
2019-02-07 19:43     ` Waiman Long
2019-02-07 19:48     ` Peter Zijlstra
2019-02-07 19:07 ` [PATCH-tip 05/22] locking/rwsem: Move owner setting code from rwsem.c to rwsem.h Waiman Long
2019-02-07 19:07 ` [PATCH-tip 06/22] locking/rwsem: Rename kernel/locking/rwsem.h Waiman Long
2019-02-07 19:07 ` [PATCH-tip 07/22] locking/rwsem: Move rwsem internal function declarations to rwsem-xadd.h Waiman Long
2019-02-07 19:07 ` [PATCH-tip 08/22] locking/rwsem: Add debug check for __down_read*() Waiman Long
2019-02-07 19:07 ` [PATCH-tip 09/22] locking/rwsem: Enhance DEBUG_RWSEMS_WARN_ON() macro Waiman Long
2019-02-07 19:07 ` [PATCH-tip 10/22] locking/rwsem: Enable lock event counting Waiman Long
2019-02-07 19:07 ` [PATCH-tip 11/22] locking/rwsem: Implement a new locking scheme Waiman Long
2019-02-07 19:07 ` [PATCH-tip 12/22] locking/rwsem: Implement lock handoff to prevent lock starvation Waiman Long
2019-02-07 19:07 ` [PATCH-tip 13/22] locking/rwsem: Remove rwsem_wake() wakeup optimization Waiman Long
2019-02-07 19:07 ` [PATCH-tip 14/22] locking/rwsem: Add more rwsem owner access helpers Waiman Long
2019-02-07 19:07 ` [PATCH-tip 15/22] locking/rwsem: Merge owner into count on x86-64 Waiman Long
2019-02-07 19:45   ` Peter Zijlstra
2019-02-07 19:55     ` Waiman Long
2019-02-07 20:08   ` Peter Zijlstra [this message]
2019-02-07 20:54     ` Waiman Long
2019-02-08 14:19       ` Waiman Long
2019-02-07 19:07 ` [PATCH-tip 16/22] locking/rwsem: Remove redundant computation of writer lock word Waiman Long
2019-02-07 19:07 ` [PATCH-tip 17/22] locking/rwsem: Recheck owner if it is not on cpu Waiman Long
2019-02-07 19:07 ` [PATCH-tip 18/22] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value Waiman Long
2019-02-07 19:07 ` [PATCH-tip 19/22] locking/rwsem: Enable readers spinning on writer Waiman Long
2019-02-07 19:07 ` [PATCH-tip 20/22] locking/rwsem: Enable count-based spinning on reader Waiman Long
2019-02-07 19:07 ` [PATCH-tip 21/22] locking/rwsem: Wake up all readers in wait queue Waiman Long
2019-02-07 19:07 ` [PATCH-tip 22/22] locking/rwsem: Ensure an RT task will not spin on reader Waiman Long
2019-02-07 19:51 ` [PATCH-tip 00/22] locking/rwsem: Rework rwsem-xadd & enable new rwsem features Davidlohr Bueso
2019-02-07 20:00   ` Waiman Long
2019-02-11  7:38     ` Ingo Molnar
2019-02-08 19:50 ` Linus Torvalds
2019-02-08 20:31   ` Waiman Long
2019-02-09  0:03     ` Linus Torvalds
2019-02-14 13:23     ` Davidlohr Bueso
2019-02-14 15:22       ` Waiman Long
2019-02-13  9:19 ` Chen Rong
2019-02-13 19:56   ` Linus Torvalds
2019-04-10  8:15     ` huang ying
2019-04-10 16:08       ` Waiman Long
2019-04-12  0:49         ` huang ying

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