linux-kernel - Re: [PATCH v3 2/2] iommu: Remove cpu-local spinlock

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Date:	Wed, 01 Jun 2016 15:40:08 +0300
From:	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Joerg Roedel <joro@...tes.org>
Cc:	intel-gfx@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] iommu: Remove cpu-local spinlock

On ke, 2016-06-01 at 12:10 +0100, Chris Wilson wrote:
> By avoiding cross-CPU usage of the per-cpu iova cache, we can forgo
> having a spinlock inside the per-cpu struct. The only place where we
> actually may touch another CPU's data is when performing a cache flush
> after running out of memory. Here, we can instead schedule a task to run
> on the other CPU to do the flush before trying again.
> 
> Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: iommu@...ts.linux-foundation.org
> Cc: linux-kernel@...r.kernel.org
> ---
>  drivers/iommu/iova.c | 29 ++++++-----------------------
>  1 file changed, 6 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index e23001bfcfee..36cdc8eeab1c 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -390,6 +390,11 @@ free_iova(struct iova_domain *iovad, unsigned long pfn)
>  }
>  EXPORT_SYMBOL_GPL(free_iova);
>  
> +static void free_this_cached_iovas(void *info)
> +{
> +	free_cpu_cached_iovas(smp_processor_id(), info);
> +}
> +
>  /**
>   * alloc_iova_fast - allocates an iova from rcache
>   * @iovad: - iova domain in question
> @@ -413,17 +418,12 @@ alloc_iova_fast(struct iova_domain *iovad, unsigned long size,
>  retry:
>  	new_iova = alloc_iova(iovad, size, limit_pfn, true);
>  	if (!new_iova) {
> -		unsigned int cpu;
> -
>  		if (flushed_rcache)
>  			return 0;
>  
>  		/* Try replenishing IOVAs by flushing rcache. */
>  		flushed_rcache = true;
> -		preempt_disable();
> -		for_each_online_cpu(cpu)
> -			free_cpu_cached_iovas(cpu, iovad);
> -		preempt_enable();
> +		on_each_cpu(free_this_cached_iovas, iovad, true);

This is not on a hot path, so should be worthy change.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>

Regards, Joonas

>  		goto retry;
>  	}
>  
Joonas Lahtinen
Open Source Technology Center
Intel Corporation