linux-kernel - Re: [RFC PATCH V4 1/1] swiotlb: Split up single swiotlb lock

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Date:   Wed, 22 Jun 2022 03:54:19 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     corbet@....net, hch@...radead.org, m.szyprowski@...sung.com,
        robin.murphy@....com, paulmck@...nel.org,
        akpm@...ux-foundation.org, bp@...e.de, tglx@...utronix.de,
        songmuchun@...edance.com, rdunlap@...radead.org,
        damien.lemoal@...nsource.wdc.com, michael.h.kelley@...rosoft.com,
        kys@...rosoft.com, Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        iommu@...ts.linux-foundation.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        wei.liu@...nel.org, parri.andrea@...il.com,
        thomas.lendacky@....com, linux-hyperv@...r.kernel.org,
        kirill.shutemov@...el.com, andi.kleen@...el.com,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [RFC PATCH V4 1/1] swiotlb: Split up single swiotlb lock

Thanks,

this looks pretty good to me.  A few comments below:

On Fri, Jun 17, 2022 at 10:47:41AM -0400, Tianyu Lan wrote:
> +/**
> + * struct io_tlb_area - IO TLB memory area descriptor
> + *
> + * This is a single area with a single lock.
> + *
> + * @used:	The number of used IO TLB block.
> + * @index:	The slot index to start searching in this area for next round.
> + * @lock:	The lock to protect the above data structures in the map and
> + *		unmap calls.
> + */
> +struct io_tlb_area {
> +	unsigned long used;
> +	unsigned int index;
> +	spinlock_t lock;
> +};

This can go into swiotlb.c.

> +void __init swiotlb_adjust_nareas(unsigned int nareas);

And this should be marked static.

> +#define DEFAULT_NUM_AREAS 1

I'd drop this define, the magic 1 and a > 1 comparism seems to
convey how it is used much better as the checks aren't about default
or not, but about larger than one.

I also think that we want some good way to size the default, e.g.
by number of CPUs or memory size.

> +void __init swiotlb_adjust_nareas(unsigned int nareas)
> +{
> +	if (!is_power_of_2(nareas)) {
> +		pr_err("swiotlb: Invalid areas parameter %d.\n", nareas);
> +		return;
> +	}
> +
> +	default_nareas = nareas;
> +
> +	pr_info("area num %d.\n", nareas);
> +	/* Round up number of slabs to the next power of 2.
> +	 * The last area is going be smaller than the rest if
> +	 * default_nslabs is not power of two.
> +	 */

Please follow the normal kernel comment style with a /* on its own line.