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

[Michael Kelley]

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>