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

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Date:   Thu, 19 Oct 2017 15:11:12 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Ingo Molnar <mingo@...nel.org>
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

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?

> But yeah, looks like single-entry-stacktrace crossrelease only has a +0.2% 
> performance cost (with 0.1% noise), while lockdep itself has a +7.7% cost.
> 
> 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.

Thanks,
Byungchul

> and false positives are fixed by the next merge window.
> 
> Thanks,
> 
> 	Ingo