linux-kernel - Re: [PATCH v14 4/6] tracing: Allow user-space mapping of the ring-buffer

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Message-ID: <d1aa52b9-d2a5-4315-b39d-d2784cece3f4@efficios.com>
Date: Mon, 5 Feb 2024 13:44:47 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: rostedt@...dmis.org, mhiramat@...nel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v14 4/6] tracing: Allow user-space mapping of the
 ring-buffer

On 2024-02-05 13:34, Vincent Donnefort wrote:
> On Mon, Feb 05, 2024 at 11:55:08AM -0500, Mathieu Desnoyers wrote:
[...]

>>
>> How are the kernel linear mapping and the userspace mapping made coherent
>> on architectures with virtually aliasing data caches ?
>>
>> Ref. https://lore.kernel.org/lkml/20240202210019.88022-1-mathieu.desnoyers@efficios.com/T/#t
> 
> Hi Mathieu,
> 
> Thanks for the pointer.
> 
> We are in the exact same problem as DAX. We do modify the data through the
> kernel linear mapping while user-space can read it through its own. I should
> probably return an error when used with any of the arch ARM || SPARC || MIPS,
> until cpu_dcache_is_aliasing() introduces a fine-grain differentiation.

You might want to use LTTng's ring buffer approach instead. See

https://github.com/lttng/lttng-modules/blob/master/src/lib/ringbuffer/ring_buffer_frontend.c#L1202

lib_ring_buffer_flush_read_subbuf_dcache()

Basically, whenever user-space grabs a sub-buffer for reading (through
lttng-modules's LTTNG_KERNEL_ABI_RING_BUFFER_GET_SUBBUF ioctl), lttng
calls flush_dcache_page() on all pages of this subbuffer (I should
really change this for a call to flush_dcache_folio() which would be
more efficient).

Note that doing this is not very efficient on architectures which have
coherent data caches and incoherent dcache vs icache: in that case,
we issue the flush_dcache_page uselessly. I plan on using the new
cpu_dcache_is_aliasing() check once/if it makes it way upstream to
remove those useless flushes on architectures which define
ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE, but do not virtually alias the
data cache.

The equivalent of LTTng's "get subbuf" operation would be
the new TRACE_MMAP_IOCTL_GET_READER ioctl in ftrace AFAIU.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com