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 [thread overview]
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
prev parent reply other threads:[~2019-02-13 15:35 UTC|newest]
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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