linux-kernel - Re: [RFC PATCH 1/3] irqchip/gic-v3: Make use of ICC

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Date:   Mon, 29 Mar 2021 10:55:23 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Jingyi Wang <wangjingyi11@...wei.com>
Cc:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <tglx@...utronix.de>,
        <wanghaibin.wang@...wei.com>, <yuzenghui@...wei.com>,
        <zhukeqian1@...wei.com>
Subject: Re: [RFC PATCH 1/3] irqchip/gic-v3: Make use of ICC_SGI1R IRM bit

On Mon, 29 Mar 2021 09:52:08 +0100,
Jingyi Wang <wangjingyi11@...wei.com> wrote:
> 
> IRM, bit[40] in ICC_SGI1R, determines how the generated SGIs
> are distributed to PEs. If the bit is set, interrupts are routed
> to all PEs in the system excluding "self". We use cpumask to
> determine if this bit should be set and make use of that.
> 
> This will reduce vm trap when broadcast IPIs are sent.

I remember writing similar code about 4 years ago, only to realise
what:

- the cost of computing the resulting mask is pretty high for large
machines
- Linux almost never sends broadcast IPIs, so the complexity was all
in vain

What changed? Please provide supporting data showing how many IPIs we
actually save, and for which workload.

> 
> Signed-off-by: Jingyi Wang <wangjingyi11@...wei.com>
> ---
>  drivers/irqchip/irq-gic-v3.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index eb0ee356a629..8ecc1b274ea8 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -1127,6 +1127,7 @@ static void gic_send_sgi(u64 cluster_id, u16 tlist, unsigned int irq)
>  static void gic_ipi_send_mask(struct irq_data *d, const struct cpumask *mask)
>  {
>  	int cpu;
> +	cpumask_t tmp;
>  
>  	if (WARN_ON(d->hwirq >= 16))
>  		return;
> @@ -1137,6 +1138,17 @@ static void gic_ipi_send_mask(struct irq_data *d, const struct cpumask *mask)
>  	 */
>  	wmb();
>  
> +	if (!cpumask_and(&tmp, mask, cpumask_of(smp_processor_id()))) {

Are you sure this does the right thing? This is checking that the
current CPU is not part of the mask. But it not checking that the mask
is actually "all but self".

This means you are potentially sending IPIs to CPUs that are not part
of the mask, making performance potentially worse.

Thanks,

	M.

> +		/* Set Interrupt Routing Mode bit */
> +		u64 val;
> +		val = (d->hwirq) << ICC_SGI1R_SGI_ID_SHIFT;
> +		val |= BIT_ULL(ICC_SGI1R_IRQ_ROUTING_MODE_BIT);
> +		gic_write_sgi1r(val);
> +
> +		isb();
> +		return;
> +	}
> +
>  	for_each_cpu(cpu, mask) {
>  		u64 cluster_id = MPIDR_TO_SGI_CLUSTER_ID(cpu_logical_map(cpu));
>  		u16 tlist;
> -- 
> 2.19.1
> 
> 

-- 
Without deviation from the norm, progress is not possible.