----- On Apr 20, 2022, at 4:12 PM, Beau Belgrave beaub@xxxxxxxxxxxxxxxxxxx wrote: > On Wed, Apr 20, 2022 at 01:53:47PM -0400, Mathieu Desnoyers wrote: >> >> >> ----- On Apr 19, 2022, at 7:48 PM, Beau Belgrave beaub@xxxxxxxxxxxxxxxxxxx >> 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@xxxxxxxxxxxxxxxxxxx >> >> 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@xxxxxxxxxxxxxxxxxxx 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@xxxxxxxxxxxx/ >> >> >> > >> >> >> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> >> >> >> > Signed-off-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> >> >> >> >> >> >> 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? Yes, I would be tempted to remove it, and document the bitwise index ABI instead. > >> 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. Allright ! Thanks, Mathieu > > Thanks, > -Beau > >> Thanks, >> >> Mathieu >> > > [..] -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
