Message-ID: <YnnAnzPFZZte/UR8@mit.edu>
Date:   Mon, 9 May 2022 21:32:15 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Byungchul Park <byungchul.park@....com>
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Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

On Tue, May 10, 2022 at 09:32:13AM +0900, Byungchul Park wrote:
> Yes, right. DEPT has never been optimized. It rather turns on
> CONFIG_LOCKDEP and even CONFIG_PROVE_LOCKING when CONFIG_DEPT gets on
> because of porting issue. I have no choice but to rely on those to
> develop DEPT out of tree. Of course, that's what I don't like.

Sure, but blaming the overhead on unnecessary CONFIG_PROVE_LOCKING
overhead can explain only a tiny fraction of the slowdown.  Consider:
if time to first test (time to boot the kernel, setup the test
environment, figure out which tests to run, etc.) is 12 seconds w/o
LOCKDEP, 49 seconds with LOCKDEP/PROVE_LOCKING and 602 seconds with
DEPT, you can really only blame 37 seconds out of the 602 seconds of
DEPT on unnecessary PROVE_LOCKING overhead.

So let's assume we can get rid of all of the PROVE_LOCKING overhead.
We're still talking about 12 seconds for time-to-first test without
any lock debugging, versus ** 565 ** seconds for time-to-first test
with DEPT.  That's a factor of 47x for DEPT sans LOCKDEP overhead,
compared to a 4x overhead for PROVE_LOCKING.

> Plus, for now, I'm focusing on removing false positives. Once it's
> considered settled down, I will work on performance optimizaition. But
> it should still keep relying on Lockdep CONFIGs and adding additional
> overhead on it until DEPT can be developed in the tree.

Well, please take a look at the false positive which I reported.  I
suspect that in order to fix that particular false positive, we'll
either need to have a way to disable DEPT on waiting on all page/folio
dirty bits, or it will need to treat pages from different inodes
and/or address spaces as being entirely separate classes, instead of
collapsing all inode dirty bits, and all of various inode's mutexes
(such as ext4's i_data_sem) as being part of a single object class.

> 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.

					- Ted

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