Received-SPF: pass (google.com: domain of linux-kernel+bounces-14208-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Tue, 2 Jan 2024 10:49:32 +0100
From: Ingo Molnar <mingo@kernel.org>
To: David Laight <David.Laight@aculab.com>
Cc: "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>,
	"'peterz@infradead.org'" <peterz@infradead.org>,
	"'longman@redhat.com'" <longman@redhat.com>,
	"'mingo@redhat.com'" <mingo@redhat.com>,
	"'will@kernel.org'" <will@kernel.org>,
	"'boqun.feng@gmail.com'" <boqun.feng@gmail.com>,
	'Linus Torvalds' <torvalds@linux-foundation.org>,
	"'virtualization@lists.linux-foundation.org'" <virtualization@lists.linux-foundation.org>,
	'Zeng Heng' <zengheng4@huawei.com>
Subject: Re: [PATCH next v2 2/5] locking/osq_lock: Optimise the
 vcpu_is_preempted() check.
Message-ID: <ZZPcLJSbfab8pweu@gmail.com>
References: <2b4e8a5816a742d2bd23fdbaa8498e80@AcuMS.aculab.com>
 <3a9d1782cd50436c99ced8c10175bae6@AcuMS.aculab.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <3a9d1782cd50436c99ced8c10175bae6@AcuMS.aculab.com>


* David Laight <David.Laight@ACULAB.COM> wrote:

> The vcpu_is_preempted() test stops osq_lock() spinning if a virtual
> cpu is no longer running.
> 
> Although patched out for bare-metal the code still needs the cpu number.

Comma missing.

> Reading this from 'prev->cpu' is a pretty much guaranteed have a cache miss
> when osq_unlock() is waking up the next cpu.
> 
> Instead save 'prev->cpu' in 'node->prev_cpu' and use that value instead.
> Update in the osq_lock() 'unqueue' path when 'node->prev' is changed.
> 
> This is simpler than checking for 'node->prev' changing and caching
> 'prev->cpu'.

Throughout the series, in changelogs and comments, please do:

 s/cpu
  /CPU

Please be more careful about changelog quality.

>  struct optimistic_spin_node {
>  	struct optimistic_spin_node *next, *prev;
> -	int locked; /* 1 if lock acquired */
> -	int cpu; /* encoded CPU # + 1 value */
> +	int locked;    /* 1 if lock acquired */
> +	int cpu;       /* encoded CPU # + 1 value */
> +	int prev_cpu;  /* encoded CPU # + 1 value */
>  };

s/ encoded CPU # + 1 value
 / encoded value: CPU+1

Thanks,

	Ingo