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Message-ID: <4d29a5c75d1120d095f27e8e453da9b00c1336d5.camel@surriel.com>
Subject: Re: [PATCH] bounds: Support non-power-of-two CONFIG_NR_CPUS
From:   Rik van Riel <riel@surriel.com>
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Ingo Molnar <mingo@kernel.org>
Date:   Tue, 10 Oct 2023 11:19:34 -0400
In-Reply-To: <20231010145549.1244748-1-willy@infradead.org>
References: <20231010145549.1244748-1-willy@infradead.org>
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On Tue, 2023-10-10 at 15:55 +0100, Matthew Wilcox (Oracle) wrote:
> ilog2() rounds down, so for example when PowerPC 85xx sets
> CONFIG_NR_CPUS
> to 24, we will only allocate 4 bits to store the number of CPUs
> instead
> of 5.  Use bits_per() instead, which rounds up.  Found by code
> inspection.
> The effect of this would probably be a misaccounting when doing NUMA
> balancing, so to a user, it would only be a performance penalty.
> The effects may be more wide-spread; it's hard to tell.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Fixes: 90572890d202 ("mm: numa: Change page last {nid,pid} into
> {cpu,pid}")
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Rik van Riel <riel@surriel.com>

Reviewed-by: Rik van Riel <riel@surriel.com>

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All Rights Reversed.