Re: [PATCH v4] dma/swiotlb: decouple high watermark tracking from CONFIG_DEBUG_FS

[Marek Szyprowski]

On 12.08.2026 17:01, Michael Kelley wrote:
> From: Frank Chen <frankchen158@xxxxxxx> Sent: Wednesday, August 12, 2026 12:05 AM
>> From: chenhuguanshen <chenhgs@xxxxxxxxxxxxxxx>
>>
>> Under heavy concurrent DMA traffic on CoCo VMs, inc_used_and_hiwater()
>> performs an atomic_long_add_return() plus a CAS loop on the global
>> used_hiwater, and dec_used() performs an atomic_long_sub() on total_used.
>> All CPUs contend on the same cacheline, causing measurable throughput
>> degradation at scale.
>>
>> Historically these counters were only compiled in under CONFIG_DEBUG_FS,
>> which means production kernels with debugfs paid the atomic overhead
>> unconditionally. Make the tracking boot-time opt-in instead so that it
>> is disabled by default with near-zero overhead via static_call, and can
>> be enabled via "swiotlb=track_hiwater" parameter on demand for debugging.
>>
>> Note that when CONFIG_DEBUG_FS is enabled but hiwater tracking is disabled,
>> the "io_tlb_used" metric reports an approximate value rather than an
>> instantaneously exact one.
>>
>> Suggested-by: Fan Du <fan.du@xxxxxxxxx>
>> Signed-off-by: Jun Miao <jun.miao@xxxxxxxxx>
>> Co-developed-by: Fan Du <fan.du@xxxxxxxxx>
>> Signed-off-by: Fan Du <fan.du@xxxxxxxxx>
>> Tested-by: chenhuguanshen <chenhgs@xxxxxxxxxxxxxxx>
>> Signed-off-by: chenhuguanshen <chenhgs@xxxxxxxxxxxxxxx>
>>
>> ---
>> v1 -> v2:
>>  - Change the patch title.
>>  - Doing the exact hiwater calculation is dynamic and defaults to "off",
>>    dynamic config would replace being under #ifdef CONFIG_DEBUG_FS
>>  - The mechanism used for dynamic config needs to be one that is selectable
>>    on the kernel boot line so that the exact hiwater mark during boot is
>>    easily available.
>>
>> v2 -> v3:
>>  - When track_hiwater is enabled, we keep the original precise hiwater
>>    calculation backed by the global total_used atomic counter. If disabled,
>>    we switch to the approximate approach that sums per-area counters to
>>    derive the total used slot count.
>>
>> v3 -> v4:
>>  - Change commit message and documentation.
> LGTM.
>
> Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Tested-by: Michael Kelley <mhklinux@xxxxxxxxxxx> 

Applied to dma-mapping-for-next, thanks!

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland