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

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Feb 05, 2024 at 01:44:47PM -0500, Mathieu Desnoyers wrote:
> 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()

Thanks!

> 
> 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.

I believe the aim is to use the mapping by default in libtracefs and fallback to
splice whenever not available...  But for those arch, I guess that might be a
mistake. Wonder if then it isn't just better to return ENOTSUPP?

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

That is correct!

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