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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Alex Shi <alex.shi@linux.alibaba.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 2/2] locking/rtmutex: optimize rt_mutex_cmpxchgs
In-Reply-To: <1587135032-188866-2-git-send-email-alex.shi@linux.alibaba.com>
References: <1587135032-188866-1-git-send-email-alex.shi@linux.alibaba.com> <1587135032-188866-2-git-send-email-alex.shi@linux.alibaba.com>
Date:   Sun, 26 Apr 2020 18:31:55 +0200
Message-ID: <87k122ryx0.fsf@nanos.tec.linutronix.de>
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Alex Shi <alex.shi@linux.alibaba.com> writes:

> Checking l->owner first to skip time cost cmpxchgs.

I don't see what that buys.

It actually adds an extra conditional in the non-contended case, which
is the case we are optimizing for.

In the contended case, i.e. when l->owner != c the cmpxchg cost is
completely irrelevant compared to the slowpath costs.

>  #ifndef CONFIG_DEBUG_RT_MUTEXES
> -# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
> -# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_acquire(l, c, n)	\
> +			(l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_release(l, c, n)	\
> +			(l->owner == c && cmpxchg_release(&l->owner, c, n) == c)

This kind of micro-optimizing is more than dubious especially w/o
numbers backing up the benefit.

Thanks,

        tglx