linux-kernel - Re: [PATCH 3/4] dma-debug: Dynamically expand the dma_debug

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Date:   Tue, 4 Dec 2018 17:38:12 +0000
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>, <hch@....de>
CC:     <m.szyprowski@...sung.com>, <iommu@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>, <cai@....us>,
        <salil.mehta@...wei.com>
Subject: Re: [PATCH 3/4] dma-debug: Dynamically expand the dma_debug_entry
 pool


>
>>> In fact, having got this far in, what I'd quite like to do is to get rid
>>> of dma_debug_resize_entries() such that we never need to free things at
>>> all, since then we could allocate whole pages as blocks of entries to
>>> save on masses of individual slab allocations.
>>>
>>
>> On a related topic, is it possible for the user to learn the total
>> entries created at a given point in time? If not, could we add a file
>> in the debugfs folder for this?
>

Hi Robin,

> I did get as far as pondering that you effectively lose track of
> utilisation once the low-water-mark of min_free_entries hits 0 and stays

I did try your patches and I noticed this, i.e I was hitting the point 
at which we start to alloc more entries.

> there - AFAICS it should be sufficient to just expose nr_total_entries
> as-is, since users can then calculate current and maximum occupancy
> based on *_free_entries. Does that sound reasonable to you?
>

Sounds ok. I am just interested to know roughly how many DMA buffers 
we're using in our system.

> That also indirectly reminds me that this lot is documented in
> DMA_API.txt, so I should be good and update that too...

Thanks,
John

>
> Cheers,
> Robin.
>
> .
>