linux-kernel - Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace

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Message-Id: <20180423151222.GO26088@linux.vnet.ibm.com>
Date:   Mon, 23 Apr 2018 08:12:22 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     rostedt <rostedt@...dmis.org>, Joel Fernandes <joelaf@...gle.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        fweisbec <fweisbec@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        kbuild test robot <fengguang.wu@...el.com>,
        baohong liu <baohong.liu@...el.com>,
        vedang patel <vedang.patel@...el.com>,
        kernel-team <kernel-team@....com>
Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you
 can

On Mon, Apr 23, 2018 at 10:59:43AM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 23, 2018, at 10:53 AM, rostedt rostedt@...dmis.org wrote:
> 
> > On Mon, 23 Apr 2018 10:31:28 -0400 (EDT)
> > Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> > 
> >> I've been wanting to introduce an alternative tracepoint instrumentation
> >> "flavor" for e.g. system call entry/exit which rely on SRCU rather than
> >> sched-rcu (preempt-off). This would allow taking faults within the
> >> instrumentation
> >> probe, which makes lots of things easier when fetching data from user-space
> >> upon system call entry/exit. This could also be used to cleanly instrument
> >> the idle loop.
> > 
> > I'd be OK with such an approach. And I don't think it would be that
> > hard to implement. It could be similar to the rcu_idle() tracepoints,
> > where each flavor simply passes in what protection it uses for
> > DO_TRACE(). We could do linker tricks to tell the tracepoint.c code how
> > the tracepoint is protected (add section code, that could be read to
> > update flags in the tracepoint). Of course modules that have
> > tracepoints could only use the standard preempt ones.
> > 
> > That is, if trace_##event##_srcu(trace_##event##_sp, PARAMS), is used,
> > then the trace_##event##_sp would need to be created somewhere. The use
> > of trace_##event##_srcu() would create a section entry, and on boot up
> > we can see that the use of this tracepoint requires srcu protection
> > with a pointer to the trace_##event##_sp srcu_struct. This could be
> > used to make sure that trace_#event() call isn't done multiple times
> > that uses two different protection flavors.
> > 
> > I'm just brain storming the idea, and I'm sure I screwed up something
> > above, but I do believe it is feasible.
> 
> The main open question here is whether we want one SRCU grace period
> domain per SRCU tracepoint definition, or just one SRCU domain for all
> SRCU tracepoints would be fine.
> 
> I'm not sure what we would gain by having the extra granularity provided
> by one SRCU grace period domain per tracepoint, and having a single SRCU
> domain for all SRCU tracepoints makes it easy to batch grace period after
> bulk tracepoint modifications.

I don't see how having multiple SRCU domains would help anything, but
perhaps I am missing something basic.

							thanx, Paul