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Message-ID: <30439f1e-cf75-0560-8bd4-5ec5cd692aef@oracle.com>
Date: Thu, 6 Dec 2018 22:33:32 -0800
From: Joe Jin <joe.jin@...cle.com>
To: Dongli Zhang <dongli.zhang@...cle.com>,
iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Cc: konrad.wilk@...cle.com, hch@....de, m.szyprowski@...sung.com,
robin.murphy@....com
Subject: Re: [PATCH RFC 1/1] swiotlb: add debugfs to track swiotlb buffer
usage
On 12/6/18 9:49 PM, Dongli Zhang wrote:
>
>
> On 12/07/2018 12:12 AM, Joe Jin wrote:
>> Hi Dongli,
>>
>> Maybe move d_swiotlb_usage declare into swiotlb_create_debugfs():
>
> I assume the call of swiotlb_tbl_map_single() might be frequent in some
> situations, e.g., when 'swiotlb=force'.
>
> That's why I declare the d_swiotlb_usage out of any functions and use "if
> (unlikely(!d_swiotlb_usage))".
This is reasonable.
Thanks,
Joe
>
> I think "if (unlikely(!d_swiotlb_usage))" incur less performance overhead than
> calling swiotlb_create_debugfs() every time to confirm if debugfs is created. I
> would declare d_swiotlb_usage statically inside swiotlb_create_debugfs() if the
> performance overhead is acceptable (it is trivial indeed).
>
>
> That is the reason I tag the patch with RFC because I am not sure if the
> on-demand creation of debugfs is fine with maintainers/reviewers. If swiotlb
> pages are never allocated, we would not be able to see the debugfs entry.
>
> I would prefer to limit the modification within swiotlb and to not taint any
> other files.
>
> The drawback is there is no place to create or delete the debugfs entry because
> swiotlb buffer could be initialized and uninitialized at very early stage.
>
>>
>> void swiotlb_create_debugfs(void)
>> {
>> #ifdef CONFIG_DEBUG_FS
>> static struct dentry *d_swiotlb_usage = NULL;
>>
>> if (d_swiotlb_usage)
>> return;
>>
>> d_swiotlb_usage = debugfs_create_dir("swiotlb", NULL);
>>
>> if (!d_swiotlb_usage)
>> return;
>>
>> debugfs_create_file("usage", 0600, d_swiotlb_usage,
>> NULL, &swiotlb_usage_fops);
>> #endif
>> }
>>
>> And for io_tlb_used, possible add a check at the begin of swiotlb_tbl_map_single(),
>> if there were not any free slots or not enough slots, return fail directly?
>
> This would optimize the slots allocation path. I will follow this in next
> version after I got more suggestions and confirmations from maintainers.
>
>
> Thank you very much!
>
> Dongli Zhang
>
>>
>> Thanks,
>> Joe
>> On 12/5/18 7:59 PM, Dongli Zhang wrote:
>>> The device driver will not be able to do dma operations once swiotlb buffer
>>> is full, either because the driver is using so many IO TLB blocks inflight,
>>> or because there is memory leak issue in device driver. To export the
>>> swiotlb buffer usage via debugfs would help the user estimate the size of
>>> swiotlb buffer to pre-allocate or analyze device driver memory leak issue.
>>>
>>> As the swiotlb can be initialized at very early stage when debugfs cannot
>>> register successfully, this patch creates the debugfs entry on demand.
>>>
>>> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
>>> ---
>>> kernel/dma/swiotlb.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 57 insertions(+)
>>>
>>> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
>>> index 045930e..d3c8aa4 100644
>>> --- a/kernel/dma/swiotlb.c
>>> +++ b/kernel/dma/swiotlb.c
>>> @@ -35,6 +35,9 @@
>>> #include <linux/scatterlist.h>
>>> #include <linux/mem_encrypt.h>
>>> #include <linux/set_memory.h>
>>> +#ifdef CONFIG_DEBUG_FS
>>> +#include <linux/debugfs.h>
>>> +#endif
>>>
>>> #include <asm/io.h>
>>> #include <asm/dma.h>
>>> @@ -73,6 +76,13 @@ static phys_addr_t io_tlb_start, io_tlb_end;
>>> */
>>> static unsigned long io_tlb_nslabs;
>>>
>>> +#ifdef CONFIG_DEBUG_FS
>>> +/*
>>> + * The number of used IO TLB block
>>> + */
>>> +static unsigned long io_tlb_used;
>>> +#endif
>>> +
>>> /*
>>> * This is a free list describing the number of free entries available from
>>> * each index
>>> @@ -100,6 +110,41 @@ static DEFINE_SPINLOCK(io_tlb_lock);
>>>
>>> static int late_alloc;
>>>
>>> +#ifdef CONFIG_DEBUG_FS
>>> +
>>> +static struct dentry *d_swiotlb_usage;
>>> +
>>> +static int swiotlb_usage_show(struct seq_file *m, void *v)
>>> +{
>>> + seq_printf(m, "%lu\n%lu\n", io_tlb_used, io_tlb_nslabs);
>>> + return 0;
>>> +}
>>> +
>>> +static int swiotlb_usage_open(struct inode *inode, struct file *filp)
>>> +{
>>> + return single_open(filp, swiotlb_usage_show, NULL);
>>> +}
>>> +
>>> +static const struct file_operations swiotlb_usage_fops = {
>>> + .open = swiotlb_usage_open,
>>> + .read = seq_read,
>>> + .llseek = seq_lseek,
>>> + .release = single_release,
>>> +};
>>> +
>>> +void swiotlb_create_debugfs(void)
>>> +{
>>> + d_swiotlb_usage = debugfs_create_dir("swiotlb", NULL);
>>> +
>>> + if (!d_swiotlb_usage)
>>> + return;
>>> +
>>> + debugfs_create_file("usage", 0600, d_swiotlb_usage,
>>> + NULL, &swiotlb_usage_fops);
>>> +}
>>> +
>>> +#endif
>>> +
>>> static int __init
>>> setup_io_tlb_npages(char *str)
>>> {
>>> @@ -449,6 +494,11 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
>>> pr_warn_once("%s is active and system is using DMA bounce buffers\n",
>>> sme_active() ? "SME" : "SEV");
>>>
>>> +#ifdef CONFIG_DEBUG_FS
>>> + if (unlikely(!d_swiotlb_usage))
>>> + swiotlb_create_debugfs();
>>> +#endif
>>> +
>>> mask = dma_get_seg_boundary(hwdev);
>>>
>>> tbl_dma_addr &= mask;
>>> @@ -528,6 +578,9 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
>>> dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes)\n", size);
>>> return SWIOTLB_MAP_ERROR;
>>> found:
>>> +#ifdef CONFIG_DEBUG_FS
>>> + io_tlb_used += nslots;
>>> +#endif
>>> spin_unlock_irqrestore(&io_tlb_lock, flags);
>>>
>>> /*
>>> @@ -588,6 +641,10 @@ void swiotlb_tbl_unmap_single(struct device *hwdev, phys_addr_t tlb_addr,
>>> */
>>> for (i = index - 1; (OFFSET(i, IO_TLB_SEGSIZE) != IO_TLB_SEGSIZE -1) && io_tlb_list[i]; i--)
>>> io_tlb_list[i] = ++count;
>>> +
>>> +#ifdef CONFIG_DEBUG_FS
>>> + io_tlb_used -= nslots;
>>> +#endif
>>> }
>>> spin_unlock_irqrestore(&io_tlb_lock, flags);
>>> }
>>>
>>
>>
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