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From: Andi Kleen <andi@firstfloor.org>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-arch@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michel Lespinasse <walken@google.com>,
	Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	George Spelvin <linux@horizon.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Daniel J Blueman <daniel@numascale.com>,
	Alexander Fyodorov <halcy@yandex.ru>,
	Aswin Chandramouleeswaran <aswin@hp.com>,
	Scott J Norton <scott.norton@hp.com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.>
Subject: Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation
Date: Wed, 29 Jan 2014 01:20:48 +0100	[thread overview]
Message-ID: <20140129002048.GE11821@two.firstfloor.org> (raw)
In-Reply-To: <1390933151-1797-2-git-send-email-Waiman.Long@hp.com>


So the 1-2 threads case is the standard case on a small
system, isn't it? This may well cause regressions.

> In the extremely unlikely case that all the queue node entries are
> used up, the current code will fall back to busy spinning without
> waiting in a queue with warning message.

Traditionally we had some code which could take thousands
of locks in rare cases (e.g. all locks in a hash table or all locks of
a big reader lock) 

The biggest offender was the mm for changing mmu 
notifiers, but I believe that's a mutex now.
lglocks presumably still can do it on large enough
systems.  I wouldn't be surprised if there is 
other code which e.g. make take all locks in a table.

I don't think the warning is valid and will
likely trigger in some obscure cases.

-Andi

WARNING: multiple messages have this Message-ID (diff)
From: Andi Kleen <andi@firstfloor.org>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-arch@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michel Lespinasse <walken@google.com>,
	Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	George Spelvin <linux@horizon.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Daniel J Blueman <daniel@numascale.com>,
	Alexander Fyodorov <halcy@yandex.ru>,
	Aswin Chandramouleeswaran <aswin@hp.com>,
	Scott J Norton <scott.norton@hp.com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>
Subject: Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation
Date: Wed, 29 Jan 2014 01:20:48 +0100	[thread overview]
Message-ID: <20140129002048.GE11821@two.firstfloor.org> (raw)
Message-ID: <20140129002048.XXNYaWbdZbLamlrnPvzDoLNtZJXb5amJDX1Eahxbuis@z> (raw)
In-Reply-To: <1390933151-1797-2-git-send-email-Waiman.Long@hp.com>


So the 1-2 threads case is the standard case on a small
system, isn't it? This may well cause regressions.

> In the extremely unlikely case that all the queue node entries are
> used up, the current code will fall back to busy spinning without
> waiting in a queue with warning message.

Traditionally we had some code which could take thousands
of locks in rare cases (e.g. all locks in a hash table or all locks of
a big reader lock) 

The biggest offender was the mm for changing mmu 
notifiers, but I believe that's a mutex now.
lglocks presumably still can do it on large enough
systems.  I wouldn't be surprised if there is 
other code which e.g. make take all locks in a table.

I don't think the warning is valid and will
likely trigger in some obscure cases.

-Andi

  reply	other threads:[~2014-01-29  0:20 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-01-28 18:19 [PATCH v3 0/2] qspinlock: Introducing a 4-byte queue spinlock Waiman Long
2014-01-28 18:19 ` [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation Waiman Long
2014-01-29  0:20   ` Andi Kleen [this message]
2014-01-29  0:20     ` Andi Kleen
2014-01-29  2:57     ` George Spelvin
2014-01-29  2:57       ` George Spelvin
2014-01-29 17:57     ` Waiman Long
2014-01-30 17:43   ` Rik van Riel
2014-01-30 19:00   ` Tim Chen
2014-01-30 19:28     ` Peter Zijlstra
2014-01-30 22:27       ` Tim Chen
2014-01-31 18:26       ` Waiman Long
2014-01-31 19:14         ` George Spelvin
2014-01-31 19:28           ` Waiman Long
2014-01-31 19:45         ` Peter Zijlstra
2014-01-31 18:16     ` Waiman Long
2014-01-30 19:35   ` Peter Zijlstra
2014-01-31 18:28     ` Waiman Long
2014-01-31 15:08   ` Peter Zijlstra
2014-01-31 19:24     ` Waiman Long
2014-01-31 19:51       ` Peter Zijlstra
2014-02-03 11:40       ` Peter Zijlstra
2014-02-06  3:10         ` Waiman Long
2014-02-07 18:17           ` Paul E. McKenney
2014-02-03  8:51   ` Raghavendra K T
2014-01-28 18:19 ` [PATCH v3 2/2] qspinlock, x86: Enable x86-64 to use queue spinlock Waiman Long
2014-01-30 17:45   ` Rik van Riel
2014-01-30  8:55 ` [PATCH v3 0/2] qspinlock: Introducing a 4-byte " Raghavendra K T
2014-01-30 15:38   ` Waiman Long
2014-01-30 18:49     ` Raghavendra K T
2014-02-03  8:51       ` Raghavendra K T
2014-02-06  3:09         ` Waiman Long

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