linux-kernel - Re: [PATCH 11/17] find: micro-optimize for_each_{set,clear}

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Date:   Thu, 26 Aug 2021 14:09:55 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
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        kvm@...r.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Alexander Lobakin <alobakin@...me>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Alexey Klimov <aklimov@...hat.com>,
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        Will Deacon <will@...nel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 11/17] find: micro-optimize for_each_{set,clear}_bit()

On Thu, Aug 26, 2021 at 03:57:13PM +0200, Petr Mladek wrote:
> On Sat 2021-08-14 14:17:07, Yury Norov wrote:
> > The macros iterate thru all set/clear bits in a bitmap. They search a
> > first bit using find_first_bit(), and the rest bits using find_next_bit().
> > 
> > Since find_next_bit() is called shortly after find_first_bit(), we can
> > save few lines of I-cache by not using find_first_bit().
> 
> Is this only a speculation or does it fix a real performance problem?
> 
> The macro is used like:
> 
> 	for_each_set_bit(bit, addr, size) {
> 		fn(bit);
> 	}
> 
> IMHO, the micro-opimization does not help when fn() is non-trivial.
 
The effect is measurable:

Start testing for_each_bit()
for_each_set_bit:                15296 ns,   1000 iterations
for_each_set_bit_from:           15225 ns,   1000 iterations

Start testing for_each_bit() with cash flushing
for_each_set_bit:               547626 ns,   1000 iterations
for_each_set_bit_from:          497899 ns,   1000 iterations

Refer this:

https://www.mail-archive.com/dri-devel@lists.freedesktop.org/msg356151.html

Thanks,
Yury
 
> > --- a/include/linux/find.h
> > +++ b/include/linux/find.h
> > @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
> >  #endif
> >  
> >  #define for_each_set_bit(bit, addr, size) \
> > -	for ((bit) = find_first_bit((addr), (size));		\
> > +	for ((bit) = find_next_bit((addr), (size), 0);		\
> >  	     (bit) < (size);					\
> >  	     (bit) = find_next_bit((addr), (size), (bit) + 1))
> >  
> 
> It is not a big deal. I just think that the original code is slightly
> more self-explaining.
> 
> Best Regards,
> Petr