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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Sat, 30 Dec 2023 12:09:20 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Waiman Long <longman@redhat.com>
Cc: David Laight <David.Laight@aculab.com>,
	"'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>,
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	"'virtualization@lists.linux-foundation.org'" <virtualization@lists.linux-foundation.org>,
	'Zeng Heng' <zengheng4@huawei.com>
Subject: Re: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data
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* Waiman Long <longman@redhat.com> wrote:

> 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@aculab.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.

GCC-11 is plenty of a look-back window in terms of compiler efficiency: 
latest enterprise distros use GCC-11 or newer, while recent desktop 
distros use GCC-13. Anything older won't matter, because no major 
distribution is going to use new kernels with old compilers.

Thanks,

	Ingo