From: "Uwe Kleine-König" <uwe@kleine-koenig.org>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
Alexei Starovoitov <ast@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>,
Linux-Sparse <linux-sparse@vger.kernel.org>,
Martin KaFai Lau <kafai@fb.com>,
Arthur Fabre <afabre@cloudflare.com>
Subject: Re: sparse problem with Linux kernel v5.5
Date: Fri, 7 Feb 2020 07:34:57 +0100 [thread overview]
Message-ID: <b6d24fd2-e5b8-7010-86b5-c46f65536eef@kleine-koenig.org> (raw)
In-Reply-To: <CAHk-=whEFcgPW8oD=rGGyDiML_7AaCP0DGdW=WXjvF9zAv6Vrg@mail.gmail.com>
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Hello,
On 2/7/20 12:47 AM, Linus Torvalds wrote:
> Instead, it could just calculate the nbuckets first, and then do the
> "log2()" on that:
>
> /* Use at least 2 buckets, select_bucket() is undefined
> behavior with 1 bucket */
> nbuckets = max_t(u32, 2, roundup_pow_of_two(num_possible_cpus()));
> smap->bucket_log = ilog2(buckets);
Isn't it kind of ineffective to first round to a power of two and then
take the ilog2 of it?
At a first glance I'd say that
ilog2(roundup_pow_of_two(x)) == ilog(x - 1) + 1
for x > 1. (Maybe even for x == 1? Didn't care to check, I think it
doesn't matter for the case at hand.)
This RHS might be easier to optimize for the compiler?!
Best regards
Uwe
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next prev parent reply other threads:[~2020-02-07 6:42 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-06 4:49 sparse problem with Linux kernel v5.5 Randy Dunlap
2020-02-06 4:58 ` Randy Dunlap
2020-02-06 7:18 ` Linus Torvalds
2020-02-06 11:46 ` Luc Van Oostenryck
2020-02-06 18:25 ` Randy Dunlap
2020-02-06 20:06 ` Luc Van Oostenryck
2020-02-06 23:47 ` Linus Torvalds
2020-02-07 0:34 ` Martin KaFai Lau
2020-02-07 10:35 ` Luc Van Oostenryck
2020-02-07 6:34 ` Uwe Kleine-König [this message]
2020-02-07 8:29 ` Martin KaFai Lau
2020-02-07 11:34 ` Uwe Kleine-König
2020-02-07 12:43 ` Luc Van Oostenryck
2020-02-07 10:31 ` Luc Van Oostenryck
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