linux-kernel - Re: [PATCH next v2 5/5] locking/osq_lock: Optimise decode_cpu() and per_cpu

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Date: Sun, 31 Dec 2023 23:14:21 -0500
From: Waiman Long <longman@...hat.com>
To: David Laight <David.Laight@...LAB.COM>,
 "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
 "'peterz@...radead.org'" <peterz@...radead.org>
Cc: "'mingo@...hat.com'" <mingo@...hat.com>,
 "'will@...nel.org'" <will@...nel.org>,
 "'boqun.feng@...il.com'" <boqun.feng@...il.com>,
 'Linus Torvalds' <torvalds@...ux-foundation.org>,
 "'virtualization@...ts.linux-foundation.org'"
 <virtualization@...ts.linux-foundation.org>,
 'Zeng Heng' <zengheng4@...wei.com>
Subject: Re: [PATCH next v2 5/5] locking/osq_lock: Optimise decode_cpu() and
 per_cpu_ptr().


On 12/31/23 16:55, David Laight wrote:
> per_cpu_ptr() indexes __per_cpu_offset[] with the cpu number.
> This requires the cpu number be 64bit.
> However the value is osq_lock() comes from a 32bit xchg() and there
> isn't a way of telling gcc the high bits are zero (they are) so
> there will always be an instruction to clear the high bits.
>
> The cpu number is also offset by one (to make the initialiser 0)
> It seems to be impossible to get gcc to convert __per_cpu_offset[cpu_p1 - 1]
> into (__per_cpu_offset - 1)[cpu_p1] (transferring the offset to the address).
>
> Converting the cpu number to 32bit unsigned prior to the decrement means
> that gcc knows the decrement has set the high bits to zero and doesn't
> add a register-register move (or cltq) to zero/sign extend the value.
>
> Not massive but saves two instructions.
>
> Signed-off-by: David Laight <david.laight@...lab.com>
> ---
>   kernel/locking/osq_lock.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 35bb99e96697..37a4fa872989 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -29,11 +29,9 @@ static inline int encode_cpu(int cpu_nr)
>   	return cpu_nr + 1;
>   }
>   
> -static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
> +static inline struct optimistic_spin_node *decode_cpu(unsigned int encoded_cpu_val)
>   {
> -	int cpu_nr = encoded_cpu_val - 1;
> -
> -	return per_cpu_ptr(&osq_node, cpu_nr);
> +	return per_cpu_ptr(&osq_node, encoded_cpu_val - 1);
>   }
>   
>   /*

You really like micro-optimization.

Anyway,

Reviewed-by: Waiman Long <longman@...hat.com>