Messages in this thread | | | Date | Mon, 12 Dec 2022 11:12:56 -0500 | From | Steven Rostedt <> | Subject | Re: [for-next][PATCH 02/11] tracing: Add __cpumask to denote a trace event field that is a cpumask_t |
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On Mon, 12 Dec 2022 14:53:27 +0000 Douglas Raillard <douglas.raillard@arm.com> wrote:
> On 24-11-2022 14:50, Steven Rostedt wrote: > > From: "Steven Rostedt (Google)" <rostedt@goodmis.org> > > > > The trace events have a __bitmask field that can be used for anything > > that requires bitmasks. Although currently it is only used for CPU > > masks, it could be used in the future for any type of bitmasks. > > > > There is some user space tooling that wants to know if a field is a CPU > > mask and not just some random unsigned long bitmask. Introduce > > "__cpumask()" helper functions that work the same as the current > > __bitmask() helpers but displays in the format file: > > > > field:__data_loc cpumask_t *[] mask; offset:36; size:4; signed:0;
The current parsing tools break the above into:
"field:" "__data_loc" <some-type> "[]" <var-name> ";" "offset:" <offset> ";" "size:" "<size>" ";" "signed:" <signed> ";"
Where the <some-type> really can be anything, and in lots of cases, it is. Thus its only a hint for the tooling, and has never been limited to what they are.
> > > > Instead of: > > > > field:__data_loc unsigned long[] mask; offset:32; size:4; signed:0; > > > > The main difference is the type. Instead of "unsigned long" it is > > "cpumask_t *". Note, this type field needs to be a real type in the > > __dynamic_array() logic that both __cpumask and__bitmask use, but the > > comparison field requires it to be a scalar type whereas cpumask_t is a > > structure (non-scalar). But everything works when making it a pointer.
The above is for the kernel to build.
> > How is tooling expected to distinguish between a real dynamic array of pointers > from a type that is using dynamic arrays as an "implementation detail" > with a broken type description ? Any reasonable > interpretation of that type by the consuming tool will be broken > unless it specifically knows about __data_loc cpumask*[].
I'm curious to what the tool does differently with the above. What tool are you using? Does it just give up on how to print it?
> However, the set of types using that trick is unbounded so forward > compatibilty is impossible to ensure. On top of that, an actual > dynamic array of cpumask pointers becomes impossible to represent.
I never thought about a user case where we print out an array of cpumask pointers.
> > You might object that if the tool does not know about cpumask, > it does not matter "how it breaks" as the display will be useless anyway, > but that is not true. A parsing library might just parse up to > its knowledge limit and return the most elaborate it can for a given field. > It's acceptable for that representation to not be elaborated with the full > semantic expected by the end user, but it should not return > something that is lying on its nature. For example, it would be sane for > the user to assert the size of an array of pointers to be a multiple > of a pointer size. cpumask is currently an array of unsigned long but there is > nothing preventing a similar type to be based on an array of u8. > Such a type would also have different endianness handling and the resulting buffer > would be garbage. > > > To fix that issue, I propose to expose the following to userspace: > 1. The binary representation type (unsigned long[] in cpumask case). > 2. An (ordered list of) semantic type that may or may not be the same as 1. > > Type (1) can be used to guarantee correct handling of endianness and a reasonable > default display, while (2) allows any sort of fancy interpretation, all that while preserving > forward compatibility. For cpumask, this would give: > 1. unsigned long [] > 2. bitmask, cpumask > > A consumer could know about bitmask as they are likely used in multiple places, > but not about cpumask specifically (e.g. assuming cpumask is a type recently introduced). > Displaying as a list of bits set in the mask would already allow proper formatting, and > knowing it's actually a cpumask can allow fancier behaviors. > > From an event format perspective, this could preserve reasonable backward compat > by simply adding another property: > > field:__data_loc unsigned long[] mask; offset:36; size:4; signed:0; semantic_type:bitmask,cpumask; > > By default, "semantic_type" would simply have the same value as the normal type.
The problem with the above is that it adds a new field, and I have to check if that doesn't break existing tooling.
Another possibility is that I can add parsing to the format that is exposed to user space and simply s/__cpumask *[]/__cpumask[]/
Which will get rid of the pointer array of cpu masks.
> > This applies to any type, not just dynamic arrays. >
Let me know if the above does break existing user space and I'll revert it.
-- Steve
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