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Date:   Thu, 14 Apr 2022 14:40:21 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     andrey.konovalov@linux.dev
Cc:     Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev@googlegroups.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        linux-arm-kernel@lists.infradead.org,
        Peter Collingbourne <pcc@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Florian Mayer <fmayer@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
Subject: Re: [PATCH v3 0/3] kasan, arm64, scs: collect stack traces from
 Shadow Call Stack
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References: <cover.1649877511.git.andreyknvl@google.com>
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On Thu, Apr 14, 2022 at 01:36:59PM +0100, Mark Rutland wrote:
> As I suspected, you're hitting a known performance oddity with QEMU TCG
> mode where pointer authentication is *incredibly* slow when using the
> architected QARMA5 algorithm (enabled by default with `-cpu max`).

> This overhead has nothing to do with the *nature* of the unwinder, and
> is an artifact of the *platform* and the *structure* of the code.
> There's plenty that can be done to avoid that overhead

FWIW, from a quick look, disabling KASAN instrumentation for the
stacktrace object alone (with no other changes) has a significant impact
(compounded by the TCG QARMA5 slowdown), and I note that x86 doesn't
both instrumenting its stacktrace code anyway, so we could consider
doing likewise.

Atop that, replacing set_bit() with __set_bit() brings the regular
unwinder *really* close to the earlier SCS unwinder figures. I know that
the on_accessible_stack() calculations and checks could be ammortized
with some refactoring (which I'd planned to do anyway), so I think it's
plausible that with some changes to the existing unwinder we can bring
the difference into the noise.

> generic kasan w/ `-cpu max`
> ---------------------------
> 
> master-no-stack-traces: 12.66
> master:                 18.39 (+45.2%)
> master-no-stack-depot:  17.85 (+40.1%)
> up-scs-stacks-v3:       13.54 (+7.0%)

master-noasan:            15.67 (+23.8%)
master-noasan-__set_bit:  14.61 (+15.5%)

> Generic KASAN w/ `-cpu max,pauth-impdef=true`
> ---------------------------------------------
> 
> master-no-stack-traces: 2.69
> master:                 3.35 (+24.5%)
> master-no-stack-depot:  3.54 (+31.5%)
> up-scs-stacks-v3:       2.80 (+4.1%)

master-noasan:            3.05 (+13.0%)
master-noasan-__set_bit:  2.96 (+10.0%)

> Generic KASAN w/ `-cpu max,pauth=false`
> ---------------------------------------
> 
> master-no-stack-traces: 1.92
> master:                 2.27  (+18.2%)
> master-no-stack-depot:  2.22  (+15.6%)
> up-scs-stacks-v3:       2.06  (+7.3%)

master-noasan:             2.14 (+11.4%)
master-noasan-__set_bit:   2.10 (+9.4%)

Thanks,
Mark.