linux-kernel - Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

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Date:   Wed, 11 May 2022 10:16:37 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     tytso@....edu
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Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

On Tue, May 10, 2022 at 02:37:40PM +0900, Byungchul Park wrote:
> Ted wrote:
> > On Tue, May 10, 2022 at 09:32:13AM +0900, Byungchul Park wrote:
> > > DEPT is tracking way more objects than Lockdep so it's inevitable to be
> > > slower, but let me try to make it have the similar performance to
> > > Lockdep.
> > 
> > In order to eliminate some of these false positives, I suspect it's
> > going to increase the number of object classes that DEPT will need to
> > track even *more*.  At which point, the cost/benefit of DEPT may get
> > called into question, especially if all of the false positives can't
> > be suppressed.
> 
> Look. Let's talk in general terms. There's no way to get rid of the
> false positives all the way. It's a decision issue for *balancing*
> between considering potential cases and only real ones. Definitely,
> potential is not real. The more potential things we consider, the higher
> the chances are, that false positives appear.
> 
> But yes. The advantage we'd take by detecting potential ones should be
> higher than the risk of being bothered by false ones. Do you think a
> tool is useless if it produces a few false positives? Of course, it'd
> be a problem if it's too many, but otherwise, I think it'd be a great
> tool if the advantage > the risk.
> 
> Don't get me wrong here. It doesn't mean DEPT is perfect for now. The
> performance should be improved and false alarms that appear should be
> removed, of course. I'm talking about the direction.
> 
> For now, there's no tool to track wait/event itself in Linux kernel -
> a subset of the functionality exists tho. DEPT is the 1st try for that
> purpose and can be a useful tool by the right direction.
> 
> I know what you are concerning about. I bet it's false positives that
> are going to bother you once merged. I'll insist that DEPT shouldn't be
> used as a mandatory testing tool until considered stable enough. But
> what about ones who would take the advantage use DEPT. Why don't you
> think of folks who will take the advantage from the hints about
> dependency of synchronization esp. when their subsystem requires very
> complicated synchronization? Should a tool be useful only in a final
> testing stage? What about the usefulness during development stage?
> 
> It's worth noting DEPT works with any wait/event so any lockups e.g.
> even by HW-SW interface, retry logic or the like can be detected by DEPT
> once all waits and events are tagged properly. I believe the advantage
> by that is much higher than the bad side facing false alarms. It's just
> my opinion. I'm goning to respect the majority opinion.

s/take advantage/have the benefit/g

	Byungchul