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From: Waiman Long <longman@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-ia64@vger.kernel.org,
	Linux-sh list <linux-sh@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	sparclinux@vger.kernel.org,
	linux-arch <linux-arch@vger.kernel.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	linux-hexagon@vger.kernel.org,
	the arch/x86 maintainers <x86@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	linux-xtensa@linux-xtensa.org, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org,
	linuxppc-dev@lists.ozlabs.org,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	linux-alpha@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock()
Date: Wed, 13 Feb 2019 10:33:58 -0500
Message-ID: <bdfc93cf-416a-ec17-4f9a-92e77652b490@redhat.com> (raw)
In-Reply-To: <20190213074544.GB62549@gmail.com>

On 02/13/2019 02:45 AM, Ingo Molnar wrote:
> * Waiman Long <longman@redhat.com> wrote:
>
>> I looked at the assembly code in arch/x86/include/asm/rwsem.h. For both
>> trylocks (read & write), the count is read first before attempting to
>> lock it. We did the same for all trylock functions in other locks.
>> Depending on how the trylock is used and how contended the lock is, it
>> may help or hurt performance. Changing down_read_trylock to do an
>> unconditional cmpxchg will change the performance profile of existing
>> code. So I would prefer keeping the current code.
>>
>> I do notice now that the generic down_write_trylock() code is doing an
>> unconditional compxchg. So I wonder if we should change it to read the
>> lock first like other trylocks or just leave it as it is.
> No, I think we should instead move the other trylocks to the 
> try-for-ownership model as well, like Linus suggested.
>
> That's the general assumption we make in locking primitives, that we 
> optimize for the common, expected case - which would be that the trylock 
> succeeds, and I don't see why trylock primitives should be different.
>
> In fact I can see more ways for read-for-sharing to perform suboptimally 
> on larger systems.

I don't mind changing to the try-for-ownership model for rwsem and
mutex. I do have some concern to do that for spinlock. Some of the lock
slowpath code do optimistic trylock. Making them unconditional cmpxchg
will impact lock contention performance.

I will update this rwsem patch to make the change while I am working on
it. For other locks, I will suggest we go slow and carefully evaluate
the performance implication before we make the changes.

Cheers,
Longman



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Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-11 19:31 [PATCH v2 0/2] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-02-11 19:31 ` [PATCH v2 1/2] " Waiman Long
2019-02-11 19:31 ` [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock() Waiman Long
2019-02-12 13:24   ` Peter Zijlstra
2019-02-12 13:25     ` Peter Zijlstra
2019-02-12 18:36       ` Waiman Long
2019-02-12 18:38         ` Waiman Long
2019-02-12 19:58   ` Linus Torvalds
2019-02-12 21:21     ` Waiman Long
2019-02-13  7:45       ` Ingo Molnar
2019-02-13 15:33         ` Waiman Long [this message]

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