linux-kernel - Re: [PATCH v2 2/2] locking/rwsem: Optimize down_read

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Date:   Wed, 13 Feb 2019 10:33:58 -0500
From:   Waiman Long <longman@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch <linux-arch@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock()

On 02/13/2019 02:45 AM, Ingo Molnar wrote:
> * Waiman Long <longman@...hat.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