linux-kernel - Re: [tip:locking/core] refcount_t: Introduce a special purpose refcount type

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Date:   Wed, 15 Feb 2017 10:02:51 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dwindsor@...il.com" <dwindsor@...il.com>,
        "ishkamiel@...il.com" <ishkamiel@...il.com>
Subject: Re: [tip:locking/core] refcount_t: Introduce a special purpose
 refcount type


* Reshetova, Elena <elena.reshetova@...el.com> wrote:

> > Subject: refcount: Out-of-line everything
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Fri Feb 10 16:27:52 CET 2017
> > 
> > Linus asked to please make this real C code.
> 
> Perhaps a completely stupid question, but I am going to ask anyway since only 
> this way I can learn. What a real difference it makes? Or are we talking more 
> about styling and etc.? Is it because of size concerns? This way it is certainly 
> now done differently than rest of atomic and kref...

It's about generated code size mostly.

This inline function is way too large to be inlined:

static inline __refcount_check
bool refcount_add_not_zero(unsigned int i, refcount_t *r)
{
	unsigned int old, new, val = atomic_read(&r->refs);

	for (;;) {
		if (!val)
			return false;

		if (unlikely(val == UINT_MAX))
			return true;

		new = val + i;
		if (new < val)
			new = UINT_MAX;
		old = atomic_cmpxchg_relaxed(&r->refs, val, new);
		if (old == val)
			break;

		val = old;
	}

	REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

	return true;
}

When used then this function generates this much code on x86-64 defconfig:

00000000000084d0 <test>:
    84d0:	8b 0f                	mov    (%rdi),%ecx
    84d2:	55                   	push   %rbp
    84d3:	48 89 e5             	mov    %rsp,%rbp

    84d6:	85 c9                	test   %ecx,%ecx                |
    84d8:	74 30                	je     850a <test+0x3a>         |
    84da:	83 f9 ff             	cmp    $0xffffffff,%ecx         |
    84dd:	be ff ff ff ff       	mov    $0xffffffff,%esi         |
    84e2:	75 04                	jne    84e8 <test+0x18>         |
    84e4:	eb 1d                	jmp    8503 <test+0x33>         |
    84e6:	89 c1                	mov    %eax,%ecx                |
    84e8:	8d 51 01             	lea    0x1(%rcx),%edx           |
    84eb:	89 c8                	mov    %ecx,%eax                |
    84ed:	39 ca                	cmp    %ecx,%edx                |
    84ef:	0f 42 d6             	cmovb  %esi,%edx                |
    84f2:	f0 0f b1 17          	lock cmpxchg %edx,(%rdi)        |
    84f6:	39 c8                	cmp    %ecx,%eax                |
    84f8:	74 09                	je     8503 <test+0x33>         |
    84fa:	85 c0                	test   %eax,%eax                |
    84fc:	74 0c                	je     850a <test+0x3a>         |
    84fe:	83 f8 ff             	cmp    $0xffffffff,%eax         |
    8501:	75 e3                	jne    84e6 <test+0x16>         |
    8503:	b8 01 00 00 00       	mov    $0x1,%eax                |

    8508:	5d                   	pop    %rbp
    8509:	c3                   	retq   
    850a:	31 c0                	xor    %eax,%eax
    850c:	5d                   	pop    %rbp
    850d:	c3                   	retq   


(I've annotated the fastpath impact with '|'. Out of line code generally does not 
count.)

It's about ~50 bytes of code per usage site. It might be better in some cases, but 
not by much.

This is way above any sane inlining threshold. The 'unconditionally good' inlining 
threshold is at 1-2 instructions and below ~10 bytes of code.

So for example refcount_set() and refcount_read() can stay inlined:

static inline void refcount_set(refcount_t *r, unsigned int n)
{
	atomic_set(&r->refs, n);
}

static inline unsigned int refcount_read(const refcount_t *r)
{
	return atomic_read(&r->refs);
}


... beacuse they compile into a single instruction with 2-5 bytes I$ overhead.

Thanks,

	Ingo