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Date:   Tue, 1 Nov 2022 22:52:20 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Beau Belgrave <beaub@linux.microsoft.com>
Cc:     rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        dcook@linux.microsoft.com, alanau@linux.microsoft.com,
        linux-trace-devel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 0/2] tracing/user_events: Remote write ABI
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On Mon, 31 Oct 2022 10:27:06 -0700
Beau Belgrave <beaub@linux.microsoft.com> wrote:

> 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?

Ah, OK. set_bit()/clear_bit() are atomic ops. So the user tracer must
use per-arch atomic ops implementation too. Hmm, can you comment it there?

> 
> > 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

Thanks for pointing!

> 
> 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.

No, that makes things more complicated. We should choose one.

> 
> > 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.

That's helpful for me. We can have the code under tools/tracing/user_events/.

Thank you,

> 
> > > 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


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>