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

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Message-ID: <d80ad697-ed71-6671-c4ea-a7ca5883f65e@gmail.com>
Date:   Thu, 23 Jun 2022 22:48:36 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     corbet@....net, 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

On 6/22/2022 6:54 PM, Christoph Hellwig wrote:
> Thanks,
> 
> this looks pretty good to me.  A few comments below:
> 

Thanks for your review.

> 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.

struct io_tlb_area is used in the struct io_tlb_mem.

> 
>> +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.

swiotlb_adjust_nareas() is exposed to platforms to set area number.
When swiotlb_init() is called, smp_init() isn't called at that point and
so standard API of checking cpu number (e.g, num_online_cpus()) doesn't
work. Platforms may have other ways to get cpu number(e.g x86 may ACPI
MADT table entries to get cpu nubmer) and set area number. I will post 
following patch to set cpu number via swiotlb_adjust_nareas(),

> 
>> +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.
> 

OK. Will update.