linux-kernel - Re: [RFC PATCH 16/22 -v2] add get_monotonic

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Date:	Fri, 18 Jan 2008 18:19:29 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Daniel Walker <dwalker@...sta.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Fri, 18 Jan 2008, Mathieu Desnoyers wrote:
> >
> > But I have not seen a lot of situations where that kind of glue-code was
> > needed, so I think it makes sense to keep markers simple to use and
> > efficient for the common case.
> >
> > Then, in this glue-code, we can put trace_mark() and calls to in-kernel
> > tracers.
> 
> I'm almost done with the latency tracer work, and there are only a total
> of 6 hooks that I needed.
> 
>  - schedule context switch
>  - try_to_wake_up
>  - hard_irqs_off (which is already there for lockdep)
>  - hard irqs on (also for lockdep)
>  - lock_contention (already in for the lock contention code)
>  - lock acquire (also in there for contention code)
> 
> With the above, we could have this (if this is what I think you are
> recommending). For example in the context_switch case:
> 
> 	trace_switch_to(prev, next);
> 	switch_to(prev, next, prev);
> 
> and in sched.h I could have:
> 

Almost.. I would add :

static int trace_switch_to_enabled;

> static inline trace_switch_to(struct task_struct *prev,
> 			struct task_struct *next)
> {
    if (likely(!trace_switch_to_enabled))
      return;
> 	trace_mark(kernel_schedudule,
> 		"prev_pid %d next_pid %d prev_state %ld",
> 		prev->pid, next->pid, prev->pid);
> 
> 	trace_context_switch(prev, next);
> }
> 

And some code to activate the trace_switch_to_enabled variable (ideally
keeping a refcount).

By doing this, we would have the minimum impact on the scheduled when
disabled.

But remember that this trace_switch_to_enabled could be enabled for both
markers and your tracer, so you might need to put a branch at the
beginning of trace_context_switch() too.

Mathieu

> and have the trace_context_switch code be something that is turned on with
> the latency tracing utility (config option). That way production code can
> keep it off.
> 
> -- Steve
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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