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

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Message-ID: <20171019055730.mlpoz333ekflacs2@gmail.com>
Date:   Thu, 19 Oct 2017 07:57:30 +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 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?

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 
and false positives are fixed by the next merge window.

Thanks,

	Ingo