linux-kernel - Re: [RFC][PATCH 5/7] irq_work, smp: Allow irq_work on call_single

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Date:   Fri, 29 May 2020 01:40:32 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        cai@....pw, mgorman@...hsingularity.net
Subject: Re: [RFC][PATCH 5/7] irq_work, smp: Allow irq_work on
 call_single_queue

On Tue, May 26, 2020 at 06:11:02PM +0200, Peter Zijlstra wrote:
> Currently irq_work_queue_on() will issue an unconditional
> arch_send_call_function_single_ipi() and has the handler do
> irq_work_run().
> 
> This is unfortunate in that it makes the IPI handler look at a second
> cacheline and it misses the opportunity to avoid the IPI. Instead note
> that struct irq_work and struct __call_single_data are very similar in
> layout, so use a few bits in the flags word to encode a type and stick
> the irq_work on the call_single_queue list.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/irq_work.h |    7 ++
>  include/linux/smp.h      |   23 ++++++++-
>  kernel/irq_work.c        |   53 +++++++++++---------
>  kernel/smp.c             |  119 ++++++++++++++++++++++++++++-------------------
>  4 files changed, 130 insertions(+), 72 deletions(-)
> 
> --- a/include/linux/irq_work.h
> +++ b/include/linux/irq_work.h
> @@ -13,6 +13,8 @@
>   * busy      NULL, 2 -> {free, claimed} : callback in progress, can be claimed
>   */
>  
> +/* flags share CSD_FLAG_ space */
> +
>  #define IRQ_WORK_PENDING	BIT(0)
>  #define IRQ_WORK_BUSY		BIT(1)
>  
> @@ -23,9 +25,12 @@
>  
>  #define IRQ_WORK_CLAIMED	(IRQ_WORK_PENDING | IRQ_WORK_BUSY)
>  
> +/*
> + * structure shares layout with single_call_data_t.
> + */
>  struct irq_work {
> -	atomic_t flags;
>  	struct llist_node llnode;
> +	atomic_t flags;


We should probably have:

struct csd_node {
       atomic_t flags;
       struct llist_node;
}

embed inside struct irq_work and struct __call_single_data. Relying on
structure layout for things to work doesn't really clarify things :-)

Thanks.