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