linux-kernel - RE: [PATCH v6 09/24] x86/resctrl: Use set_bit()/clear

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Message-ID: <6f7b411db77846b2a305b93d0cf0ee7b@AcuMS.aculab.com>
Date:   Sun, 17 Sep 2023 21:00:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'James Morse' <james.morse@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        "carl@...amperecomputing.com" <carl@...amperecomputing.com>,
        "lcherian@...vell.com" <lcherian@...vell.com>,
        "bobo.shaobowang@...wei.com" <bobo.shaobowang@...wei.com>,
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        Xin Hao <xhao@...ux.alibaba.com>,
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        "dfustini@...libre.com" <dfustini@...libre.com>,
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Subject: RE: [PATCH v6 09/24] x86/resctrl: Use set_bit()/clear_bit() instead
 of open coding

From: James Morse
> Sent: 14 September 2023 18:21
> 
> The resctrl CLOSID allocator uses a single 32bit word to track which
> CLOSID are free. The setting and clearing of bits is open coded.
> 
> A subsequent patch adds resctrl_closid_is_free(), which adds more open
> coded bitmaps operations. These will eventually need changing to use
> the bitops helpers so that a CLOSID bitmap of the correct size can be
> allocated dynamically.
> 
> Convert the existing open coded bit manipulations of closid_free_map
> to use set_bit() and friends.
> 
>  int closids_supported(void)
> @@ -126,7 +126,7 @@ static void closid_init(void)
>  	closid_free_map = BIT_MASK(rdt_min_closid) - 1;
> 
>  	/* CLOSID 0 is always reserved for the default group */
> -	closid_free_map &= ~1;
> +	clear_bit(0, &closid_free_map);

Don't the clear_bit() etc functions use locked accesses?
These are always measurably more expensive than the C operators.

	David

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