linux-kernel - Re: [patch 05/24] perfmon: X86 generic code (x86)

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Message-ID: <7c86c4470811270138l5501f53dte1e6d8d6b0817e0e@mail.gmail.com>
Date:	Thu, 27 Nov 2008 10:38:08 +0100
From:	"stephane eranian" <eranian@...glemail.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	"Andi Kleen" <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...e.hu, x86@...nel.org,
	sfr@...b.auug.org.au
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

Thomas,

On Thu, Nov 27, 2008 at 12:16 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 26 Nov 2008, stephane eranian wrote:
>> > What a nonsense. We have a bitmask already. Why not iterate over the
>> > bitmask and be done ?
>> >
>>
>> Bitmask can be sparsed. Num represents the number of bits we have to find.
>> The idea is that we don't need to scan the entire bitmask, we stop as soon as
>> we have found all the bits we care about (i.e., all the bits that are set).
>>
>> Example:
>>          num = 3
>>      bitmask=0000000010001001
>>                                     ^ we will iterate until we are
>> done with that bit.
>
> Errm.
>
> #define for_each_bit(bit, addr, size) \
>        for ((bit) = find_first_bit((addr), (size)); \
>             (bit) < (size); \
>             (bit) = find_next_bit((addr), (size), (bit) + 1))
>
> find_first_bit() and find_next_bit() are single instructions on most
> architectures. "size" is known upfront at setup time of the
> context/set and can be cached.
>
> This takes exactly 3 iterations, while your method needs 8. And it
> gets worse with the following example:
>
> Example:
>  num = 1
>  bitmask=1000 0000 0000 0000 0000 0000 0000 00000
>
>          ^ you will iterate until we are done with that bit (32 times)
>
>  for_each_bit() will iterate exactly _once_.
>
Ok, you've convinced me. I will make the change.
Thanks.
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