Re: [RFC PATCH 0/2] tracing/user_events: Remote write ABI

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

On Mon, Oct 31, 2022 at 11:15:56PM +0900, Masami Hiramatsu wrote:
> Hi Beau,
> 
> On Thu, 27 Oct 2022 15:40:09 -0700
> Beau Belgrave <beaub@linux.microsoft.com> wrote:
> 
> > As part of the discussions for user_events aligned with user space
> > tracers, it was determined that user programs should register a 32-bit
> > value to set or clear a bit when an event becomes enabled. Currently a
> > shared page is being used that requires mmap().
> > 
> > In this new model during the event registration from user programs 2 new
> > values are specified. The first is the address to update when the event
> > is either enabled or disabled. The second is the bit to set/clear to
> > reflect the event being enabled. This allows for a local 32-bit value in
> > user programs to support both kernel and user tracers. As an example,
> > setting bit 31 for kernel tracers when the event becomes enabled allows
> > for user tracers to use the other bits for ref counts or other flags.
> > The kernel side updates the bit atomically, user programs need to also
> > update these values atomically.
> 
> I think you means the kernel tracer (ftrace/perf) and user tracers (e.g. 
> LTTng) use the same 32bit data so that traced user-application only checks
> that data for checking an event is enabled, right?
> 

Yes, exactly, user code can just check a single uint32 or uint64 to tell
if anything is enabled (kernel or user tracer).

> If so, who the user tracer threads updates the data bit? Is that thread
> safe to update both kernel tracer and user tracers at the same time?
> 

This is why atomics are used to set the bit on the kernel side. The user
side should do the same. This is like the futex code. Do you see a
problem with atomics being used between user and kernel space on a
shared 32/64-bit address?

> And what is the actual advantage of this change? Are there any issue
> to use mmaped page? I would like to know more background of this
> change.
> 

Without this change user tracers like LTTng will have to check 2 values
instead of 1 to tell if the kernel tracer is enabled or not. Mathieu is
working on a user side tracing library in an effort to align writing
tracing code in user processes that works well for both kernel and user
tracers without much effort.

See here:
https://github.com/compudj/side

Are you proposing we keep the bitmap approach and have side library just
hook another branch? Mathieu had issues with that approach during our
talks.

> Could you also provide any sample program which I can play it? :)
> 

When I make the next patch version, I will update the user_events sample
so you'll have something to try out.

> > User provided addresses must be aligned on a 32-bit boundary, this
> > allows for single page checking and prevents odd behaviors such as a
> > 32-bit value straddling 2 pages instead of a single page.
> > 
> > When page faults are encountered they are done asyncly via a workqueue.
> > If the page faults back in, the write update is attempted again. If the
> > page cannot fault-in, then we log and wait until the next time the event
> > is enabled/disabled. This is to prevent possible infinite loops resulting
> > from bad user processes unmapping or changing protection values after
> > registering the address.
> > 
> > NOTE:
> > User programs that wish to have the enable bit shared across forks
> > either need to use a MAP_SHARED allocated address or register a new
> > address and file descriptor. If MAP_SHARED cannot be used or new
> > registrations cannot be done, then it's allowable to use MAP_PRIVATE
> > as long as the forked children never update the page themselves. Once
> > the page has been updated, the page from the parent will be copied over
> > to the child. This new copy-on-write page will not receive updates from
> > the kernel until another registration has been performed with this new
> > address.
> > 
> > Beau Belgrave (2):
> >   tracing/user_events: Use remote writes for event enablement
> >   tracing/user_events: Fixup enable faults asyncly
> > 
> >  include/linux/user_events.h      |  10 +-
> >  kernel/trace/trace_events_user.c | 396 ++++++++++++++++++++-----------
> >  2 files changed, 270 insertions(+), 136 deletions(-)
> > 
> > 
> > base-commit: 23758867219c8d84c8363316e6dd2f9fd7ae3049
> > -- 
> > 2.25.1
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,
-Beau