Re: [PATCH 2/2] Markers Implementation for Preempt RCU Boost Tracing
- From: Ingo Molnar <mingo@xxxxxxx>
- Date: Mon, 31 Dec 2007 11:20:46 +0100
* K. Prasad <prasad@xxxxxxxxxxxxxxxxxx> wrote:
@@ -486,12 +309,12 @@ void rcu_unboost_readers(void)
spin_lock_irqsave(&rcu_boost_wake_lock, flags);
- rcu_trace_boost_try_unboost_readers(RCU_BOOST_ME);
+ trace_mark(try_unboost_readers, "%u", smp_processor_id());
this isnt directed at you or your patch, it's more directed at Mathieu,
but looking at this actual markers patch submitted to me, i'm still
fundamentally worried about the whole marker approach.
Firstly, why on earth does a full format string have to be passed in for
something as simple as a CPU id? This way we basically codify it forever
that tracing _has_ to be expensive when enabled. The latency-tracer
(which i'd love to convert to markers, if only markers were capable
enough) has shown it that tracing _can_ be used to capture performance
data without disturbing the measured system, even at hundreds of
thousands context switches a second per CPU.
Secondly, the inlined overhead of trace_mark() is still WAY too large:
if (unlikely(__mark_##name.state)) { \
preempt_disable(); \
(*__mark_##name.call) \
(&__mark_##name, call_data, \
format, ## args); \
preempt_enable(); \
} \
Whatever became of the obvious suggestion that i outlined years ago, to
have a _single_ trace call instruction and to _patch out_ the damn
marker calls by default? No .state flag checking inlined. No
preempt_disable()/enable() pair. Patching static tracepoints OUT of the
kernel, only leaving a ~5-byte NOP sequence behind them (and some
minimal disturbance to the variables the tracepoint accesses). We've got
all the alternatives.h code patching infrastructure available for such
purposes. Why are we 2 years down the line and _STILL_ arguing about
this?
Thirdly, the patch selects CONFIG_MARKERS:
config RCU_TRACE
- bool "Enable tracing for RCU - currently stats in debugfs"
+ tristate "Enable tracing for RCU - currently stats in debugfs"
select DEBUG_FS
- default y
+ select MARKERS
Which adds overhead (inlined checks for markers) all around the kernel,
even if all markers are deactivated! Imagine a thousand of them and the
kernel blows up measurably.
Sadly, this whole trace_mark() API seems to have gotten much worse since
i last saw it. It's sub-par when it's turned on and it's sub-par when
it's turned off. It gets us the worst of both worlds.
If it continues like this then i'd much rather see people add printks as
tracing, because there you _know_ that it's high-overhead and people
wont start arguing about trace compatibility either, etc.
Ingo
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- From: K. Prasad
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