linux-kernel - RE: [RFC patch 15/15] LTTng timestamp x86

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Date:	Mon, 20 Oct 2008 11:07:05 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"ltt-dev@...ts.casi.polymtl.ca" <ltt-dev@...ts.casi.polymtl.ca>
Subject: RE: [RFC patch 15/15] LTTng timestamp x86

> And what do we say when we detect this ? "sorry, please upgrade your
> hardware to get a reliable trace" ? ;)

My employer might be happy with that answer ;-) ... but I think
we could tell the user to:

        1) adjust something in /sys/...
        2) boot with some special option
        3) rebuild kernel with CONFIG_INSANE_TSC=y

to switch over to a heavyweight workaround in s/w.  Systems
that require this are already in the minority ... and I
think (hope!) that current and future generations of cpus
won't have these challenges.

So this is mostly a campaign for the default code path to
be based on current (sane) TSC behaviour ... with the workarounds
for past problems kept to one side.

> Nope, this is not required. I removed the heartbeat event from LTTng two
> weeks ago, implementing detection of the delta from the last timestamp
> written into the trace. If we detect that the new timestamp is too far
> from the previous one, we write the full 64 bits TSC in an extended
> event header. Therefore, we have no dependency on interrupt latency to
> get a sane time-base.

Neat.  Could you grab the HPET value here too?


> (8 cores up)

Interesting results.  I'm not at all sure why HPET scales so badly.
Maybe some h/w throttling/synchronizing going on???

-Tony
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