linux-kernel - Re: [PATCH v9 1/4] Uprobes: Support SDT markers having reference count (semaphore)

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Date:   Tue, 28 Aug 2018 12:07:30 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>, mhiramat@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org,
        open list <linux-kernel@...r.kernel.org>,
        ananth@...ux.vnet.ibm.com,
        Alexis Berlemont <alexis.berlemont@...il.com>,
        naveen.n.rao@...ux.vnet.ibm.com,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
        linux@...linux.org.uk, ralf@...ux-mips.org, paul.burton@...s.com
Subject: Re: [PATCH v9 1/4] Uprobes: Support SDT markers having reference
 count (semaphore)

Hi all,

What's our plan with this work? Will this be routed via Steven's tree?

Thanks,
Song

On Wed, Aug 22, 2018 at 5:39 AM Srikar Dronamraju
<srikar@...ux.vnet.ibm.com> wrote:
>
> * Ravi Bangoria <ravi.bangoria@...ux.ibm.com> [2018-08-20 10:12:47]:
>
> > Userspace Statically Defined Tracepoints[1] are dtrace style markers
> > inside userspace applications. Applications like PostgreSQL, MySQL,
> > Pthread, Perl, Python, Java, Ruby, Node.js, libvirt, QEMU, glib etc
> > have these markers embedded in them. These markers are added by developer
> > at important places in the code. Each marker source expands to a single
> > nop instruction in the compiled code but there may be additional
> > overhead for computing the marker arguments which expands to couple of
> > instructions. In case the overhead is more, execution of it can be
> > omitted by runtime if() condition when no one is tracing on the marker:
> >
> >     if (reference_counter > 0) {
> >         Execute marker instructions;
> >     }
> >
> > Default value of reference counter is 0. Tracer has to increment the
> > reference counter before tracing on a marker and decrement it when
> > done with the tracing.
> >
> > Implement the reference counter logic in core uprobe. User will be
> > able to use it from trace_uprobe as well as from kernel module. New
> > trace_uprobe definition with reference counter will now be:
> >
> >     <path>:<offset>[(ref_ctr_offset)]
> >
> > where ref_ctr_offset is an optional field. For kernel module, new
> > variant of uprobe_register() has been introduced:
> >
> >     uprobe_register_refctr(inode, offset, ref_ctr_offset, consumer)
> >
> > No new variant for uprobe_unregister() because it's assumed to have
> > only one reference counter for one uprobe.
> >
> > [1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
> >
> > Note: 'reference counter' is called as 'semaphore' in original Dtrace
> > (or Systemtap, bcc and even in ELF) documentation and code. But the
> > term 'semaphore' is misleading in this context. This is just a counter
> > used to hold number of tracers tracing on a marker. This is not really
> > used for any synchronization. So we are calling it a 'reference counter'
> > in kernel / perf code.
> >
>
>
> Acked-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
>
> > Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
> > Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>
> > [Only trace_uprobe.c]
> > Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> > ---
>