linux-kernel - Re: [PATCH] irqchip/gic-v3: use dsb(ishst) to synchronize data to smp before issuing ipi

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Date:   Sat, 19 Feb 2022 09:56:26 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Barry Song <21cnbao@...il.com>
Cc:     tglx@...utronix.de, will@...nel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linuxarm@...wei.com,
        Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH] irqchip/gic-v3: use dsb(ishst) to synchronize data to smp
 before issuing ipi

On 2022-02-18 21:55, Barry Song wrote:
> dsb(ishst) should be enough here as we only need to guarantee the
> visibility of data to other CPUs in smp inner domain before we
> send the ipi.
> 
> Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
> ---
>  drivers/irqchip/irq-gic-v3.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3.c 
> b/drivers/irqchip/irq-gic-v3.c
> index 5e935d97207d..0efe1a9a9f3b 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -1211,7 +1211,7 @@ static void gic_ipi_send_mask(struct irq_data
> *d, const struct cpumask *mask)
>  	 * Ensure that stores to Normal memory are visible to the
>  	 * other CPUs before issuing the IPI.
>  	 */
> -	wmb();
> +	dsb(ishst);
> 
>  	for_each_cpu(cpu, mask) {
>  		u64 cluster_id = MPIDR_TO_SGI_CLUSTER_ID(cpu_logical_map(cpu));

I'm not opposed to that change, but I'm pretty curious whether this 
makes
any visible difference in practice. Could you measure the effect of this 
change
for any sort of IPI heavy workload?

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...