Re: [PATCH 6/7] tracing/user_events: Use bits vs bytes for enabled status page data

[Beau Belgrave <beaub@linux.microsoft.com>]

On Wed, Apr 20, 2022 at 01:53:47PM -0400, Mathieu Desnoyers wrote:
> 
> 
> ----- On Apr 19, 2022, at 7:48 PM, Beau Belgrave beaub@linux.microsoft.com wrote:
> 
> > On Tue, Apr 19, 2022 at 05:26:20PM -0400, Mathieu Desnoyers wrote:
> >> ----- On Apr 19, 2022, at 2:57 PM, Beau Belgrave beaub@linux.microsoft.com
> >> wrote:
> >> 
> >> > On Tue, Apr 19, 2022 at 10:35:45AM -0400, Mathieu Desnoyers wrote:
> >> >> ----- On Apr 1, 2022, at 7:43 PM, Beau Belgrave beaub@linux.microsoft.com wrote:
> >> >> 
> >> >> > User processes may require many events and when they do the cache
> >> >> > performance of a byte index status check is less ideal than a bit index.
> >> >> > The previous event limit per-page was 4096, the new limit is 32,768.
> >> >> > 
> >> >> > This change adds a mask property to the user_reg struct. Programs check
> >> >> > that the byte at status_index has a bit set by ANDing the status_mask.
> >> >> > 
> >> >> > Link:
> >> >> > https://lore.kernel.org/all/2059213643.196683.1648499088753.JavaMail.zimbra@efficios.com/
> >> >> > 
> >> >> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> >> >> > Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> >> >> 
> >> >> Hi Beau,
> >> >> 
> >> >> Considering this will be used in a fast-path, why choose bytewise
> >> >> loads for the byte at status_index and the status_mask ?
> >> >> 
> >> > 
> >> > First, thanks for the review!
> >> > 
> >> > Which loads are you concerned about? The user programs can store the
> >> > index and mask in another type after registration instead of an int.
> >> 
> >> I'm concerned about the loads from user-space, considering that
> >> those are on the fast-path.
> >> 
> >> Indeed user programs will need to copy the status index and mask
> >> returned in struct user_reg, so adapting the indexing and mask to
> >> deal with an array of unsigned long rather than bytes can be done
> >> at that point, but I wonder how many users will go through that
> >> extra trouble unless there are helpers to convert the status index
> >> from byte-wise to long-wise, and convert the status mask from a
> >> byte-wise mask to a long-wise mask (and associated documentation).
> >> 
> > 
> > Yeah, do you think it's wise to maybe add inline functions in
> > user_events.h to do this conversion? I could then add them to our
> > documentation.
> > 
> > Hopefully this would make more APIs/people do the better approach?
> > 
> >> 
> >> > 
> >> > However, you may be referring to something on the kernel side?
> >> 
> >> No.
> >> 
> > 
> > [..]
> > 
> >> >> Ideally I would be tempted to use "unsigned long" type (32-bit on 32-bit
> >> >> binaries and 64-bit on 64-bit binaries) for both the array access
> >> >> and the status mask, but this brings extra complexity for 32-bit compat
> >> >> handling.
> >> >> 
> >> > 
> >> > User programs can store the index and mask returned into better value
> >> > types for their architecture.
> >> > 
> >> > I agree it will cause compat handling issues if it's put into the user
> >> > facing header as a long.
> >> > 
> >> > I was hoping APIs, like libtracefs, could abstract many callers from how
> >> > best to use the returned values. For example, it could save the index
> >> > and mask as unsigned long for the callers and use those for the
> >> > enablement checks.
> >> > 
> >> > Do you think there is a way to enable these native types in the ABI
> >> > without causing compat handling issues? I used ints to prevent compat
> >> > issues between 32-bit user mode and 64-bit kernel mode.
> >> 
> >> I think you are right: this is not an ABI issue, but rather a usability
> >> issue that can be solved by implementing and documenting user-space library
> >> helpers to help user applications index the array and apply the mask to an
> >> unsigned long type.
> >> 
> > 
> > Great. Let me know if updating user_events.h to do the conversion is a
> > good idea or not, or if you have other thoughts how to make more people
> > do the best thing.
> 
> Usually uapi headers are reserved for exposing the kernel ABI to user-space.
> I think the helpers we are discussing here do not belong to the uapi because they
> do not define the ABI, and should probably sit elsewhere in a proper library.
> 

Makes sense.

That likely means I should remove the enablement helper check from
user_events.h, right?

> If the status_mask is meant to be modified in some ways by user-space before it can
> be used as a mask, I wonder why it is exposed as a byte-wise mask at all ?
> 
> Rather than exposing a byte-wise index and single-byte mask as ABI, the kernel could
> simply expose a bit-wise index, which can then be used by the application to calculate
> index and mask, which it should interpret in little endian if it wants to apply the
> mask on types larger than a single byte.
> 
> Thoughts ?
> 

Yeah, you're right, we can just expose out the bit-wise index at the
ABI.

I'll switch over to that model in the next version.

Thanks,
-Beau

> Thanks,
> 
> Mathieu
> 

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