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Date:   Mon, 7 Oct 2019 10:56:58 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Alexey Kardashevskiy <aik@ozlabs.ru>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Matthew Wilcox <willy@infradead.org>, Qian Cai <cai@lca.pw>
Subject: Re: [PATCH v3 1/3] mm: kmemleak: Make the tool tolerant to struct
 scan_area allocation failures
Message-ID: <20191007095658.GF45043@arrakis.emea.arm.com>
References: <20190812160642.52134-1-catalin.marinas@arm.com>
 <20190812160642.52134-2-catalin.marinas@arm.com>
 <2ac37341-097e-17a2-fb6b-7912da9fa38e@ozlabs.ru>
 <20191003084135.GA21629@arrakis.emea.arm.com>
 <ba47fb68-f44c-04c9-7ea8-2705e799937b@ozlabs.ru>
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On Sat, Oct 05, 2019 at 01:08:43PM +1000, Alexey Kardashevskiy wrote:
> On 03/10/2019 18:41, Catalin Marinas wrote:
> > On Thu, Oct 03, 2019 at 04:13:07PM +1000, Alexey Kardashevskiy wrote:
> >> On 13/08/2019 02:06, Catalin Marinas wrote:
> >>> Object scan areas are an optimisation aimed to decrease the false
> >>> positives and slightly improve the scanning time of large objects known
> >>> to only have a few specific pointers. If a struct scan_area fails to
> >>> allocate, kmemleak can still function normally by scanning the full
> >>> object.
> >>>
> >>> Introduce an OBJECT_FULL_SCAN flag and mark objects as such when
> >>> scan_area allocation fails.
> >>
> >> I came across this one while bisecting sudden drop in throughput of a
> >> 100Gbit Mellanox CX4 ethernet card in a PPC POWER9 system, the speed
> >> dropped from 100Gbit to about 40Gbit. Bisect pointed at dba82d943177,
> >> this are the relevant config options:
> >>
> >> [fstn1-p1 kernel]$ grep KMEMLEAK ~/pbuild/kernel-le-4g/.config
> >> CONFIG_HAVE_DEBUG_KMEMLEAK=y
> >> CONFIG_DEBUG_KMEMLEAK=y
> >> CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE=16000
> >> # CONFIG_DEBUG_KMEMLEAK_TEST is not set
> >> # CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF is not set
> >> CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN=y
> > 
> > The throughput drop is probably caused caused by kmemleak slowing down
> > all memory allocations (including skb). So that's expected. You may get
> > similar drop with other debug options like lock proving, kasan.
> 
> I was not precise. I meant that before dba82d943177 kmemleak would
> work but would not slow network down (at least 100Gbit) and now it
> does which is downgrade so I was wondering if kmemleak just got so
> much better to justify this change or there is a bug somewhere, so
> which one is it? Or "LOG_SIZE=400" never really worked?

I suspect LOG_SIZE=400 never worked on your system (you can check the
log for any kmemleak disabled messages).

Thanks for testing the patch.

-- 
Catalin