linux-kernel - Re: [PATCH 1/2] lockdep: Introduce CROSSRELEASE_STACK

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Date:   Thu, 19 Oct 2017 08:22:12 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     peterz@...radead.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@....com, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] lockdep: Introduce CROSSRELEASE_STACK_TRACE and make
 it not unwind as default


* Byungchul Park <byungchul.park@....com> wrote:

> On Thu, Oct 19, 2017 at 07:57:30AM +0200, Ingo Molnar wrote:
> > 
> > * Byungchul Park <byungchul.park@....com> wrote:
> > 
> > > On Wed, Oct 18, 2017 at 12:09:44PM +0200, Ingo Molnar wrote:
> > > > BTW., have you attempted limiting the depth of the stack traces? I suspect more 
> > > > than 2-4 are rarely required to disambiguate the calling context.
> > > 
> > > I did it for you. Let me show you the result.
> > > 
> > > 1. No lockdep:				2.756558155 seconds time elapsed                ( +-  0.09% )
> > > 2. Lockdep:					2.968710420 seconds time elapsed		( +-  0.12% )
> > > 3. Lockdep + Crossrelease 5 entries:		3.153839636 seconds time elapsed                ( +-  0.31% )
> > > 4. Lockdep + Crossrelease 3 entries:		3.137205534 seconds time elapsed                ( +-  0.87% )
> > > 5. Lockdep + Crossrelease + This patch:	2.963669551 seconds time elapsed		( +-  0.11% )
> > 
> > I think the lockdep + crossrelease + full-stack numbers are missing?
> 
> Ah, the last version of crossrelease merged into vanilla, records 5
> entries, since I thought it overloads too much if full stack is used,
> and 5 entries are enough. Don't you think so?

Ok, fair enough, I missed that limitation!

> > That's very reasonable and we can keep the single-entry cross-release feature 
> > enabled by default as part of CONFIG_PROVE_LOCKING=y - assuming all the crashes 
> 
> BTW, is there any crash by cross-release I don't know? Of course, I know
> cases of false positives, but I don't about crash.

There's no current crash regression that I know of - I'm just outlining the 
conditions of getting all this re-enabled in the next merge window.

Instead of sending two series, could you please send a series that includes both 
these fixing + re-enabling patches, plus the false positive fixes?

In particular I think the cross-release re-enabling should be done as the last 
patch, so that any future bisections of new false positives won't be made more 
difficult by re-introducing the old false positives near the end of the bisection.

Thanks,

	Ingo