linux-kernel - Re: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data accesses.

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Date: Fri, 29 Dec 2023 22:08:03 -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>,
 "'xinhui.pan@...ux.vnet.ibm.com'" <xinhui.pan@...ux.vnet.ibm.com>,
 "'virtualization@...ts.linux-foundation.org'"
 <virtualization@...ts.linux-foundation.org>,
 'Zeng Heng' <zengheng4@...wei.com>
Subject: Re: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data
 accesses.

On 12/29/23 15:57, David Laight wrote:
> this_cpu_ptr() is rather more expensive than raw_cpu_read() since
> the latter can use an 'offset from register' (%gs for x86-84).
>
> Add a 'self' field to 'struct optimistic_spin_node' that can be
> read with raw_cpu_read(), initialise on first call.
>
> Signed-off-by: David Laight <david.laight@...lab.com>
> ---
>   kernel/locking/osq_lock.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 9bb3a077ba92..b60b0add0161 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -13,7 +13,7 @@
>    */
>   
>   struct optimistic_spin_node {
> -	struct optimistic_spin_node *next, *prev;
> +	struct optimistic_spin_node *self, *next, *prev;
>   	int locked; /* 1 if lock acquired */
>   	int cpu; /* encoded CPU # + 1 value */
>   };
> @@ -93,12 +93,16 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>   
>   bool osq_lock(struct optimistic_spin_queue *lock)
>   {
> -	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> +	struct optimistic_spin_node *node = raw_cpu_read(osq_node.self);

My gcc 11 compiler produces the following x86-64 code:

92        struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
    0x0000000000000029 <+25>:    mov    %rcx,%rdx
    0x000000000000002c <+28>:    add %gs:0x0(%rip),%rdx        # 0x34 
<osq_lock+36>

Which looks pretty optimized for me. Maybe older compiler may generate 
more complex code. However, I do have some doubt as to the benefit of 
this patch at the expense of making the code a bit more complex.

Cheers,
Longman