linux-kernel - Re: [PATCH] fs/resctrl: Slightly optimize cbm

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Message-ID: <aQjxrCa8t0TDc_pM@agluck-desk3>
Date: Mon, 3 Nov 2025 10:17:16 -0800
From: "Luck, Tony" <tony.luck@...el.com>
To: Dave Martin <Dave.Martin@....com>
CC: Christophe JAILLET <christophe.jaillet@...adoo.fr>, Reinette Chatre
	<reinette.chatre@...el.com>, James Morse <james.morse@....com>, Babu Moger
	<babu.moger@....com>, <linux-kernel@...r.kernel.org>,
	<kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH] fs/resctrl: Slightly optimize cbm_validate()

On Mon, Oct 27, 2025 at 11:43:49AM +0000, Dave Martin wrote:
> Hi,
> 
> [Tony, I have a side question on min_cbm_bits -- see below.]
> [...]
> 
> <aside>
> 
> Also, not directly related to this patch, but, looking at the final if
> statement:
> 
> 	if ((zero_bit - first_bit) < r->cache.min_cbm_bits) {
> 	        rdt_last_cmd_printf("Need at least %d bits in the mask\n",
> 	                            r->cache.min_cbm_bits);
> 	        return false;
> 	}
> 
> If min_cbm_bits is two or greater, this can fail if the bitmap has
> enough contiguous set bits but not in the first block of set bits,
> and it can succeed if there are blocks of set bits beyond the first
> block, that have fewer than min_cbm_bits.
> 
> Is that intended?  Do we ever expect arch_has_sparse_bitmasks alongside
> min_cbm_bits > 1, or should these be mutually exclusive?
> 
> </aside>

There's no enumeration for the minimium number of bits in a CBM mask.
Haswell (first to implemenent L3 cache allocation) got a quirk to
to set it to "2". I don't expect that we'd do that again.

So safe to assume that resctrl doesn't have to handle the combination
of min_cbm_bits > 1 with arch_has_sparse_bitmasks.

-Tony