linux-kernel - Re: [PATCH v7 1/3] x86/fpu: Measure the Latency of XSAVE and XRSTOR

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Message-ID: <ZQx01AfyvbJQYPsi@tassilo>
Date:   Thu, 21 Sep 2023 09:52:36 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Yi Sun <yi.sun@...el.com>, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        sohil.mehta@...el.com, ilpo.jarvinen@...ux.intel.com,
        heng.su@...el.com, tony.luck@...el.com, yi.sun@...ux.intel.com,
        yu.c.chen@...el.com
Subject: Re: [PATCH v7 1/3] x86/fpu: Measure the Latency of XSAVE and XRSTOR

> It seems unnecessarily complex: why does it have to measure latency 
> directly? Tracepoints *by default* come with event timestamps. A latency 
> measurement tool should be able to subtract two timestamps to extract the 
> latency between two tracepoints...
> 
> In fact, function tracing is enabled on all major Linux distros:
> 
>   kepler:~/tip> grep FUNCTION_TRACER /boot/config-6.2.0-33-generic 
>   CONFIG_HAVE_FUNCTION_TRACER=y
>   CONFIG_FUNCTION_TRACER=y
> 
> Why not just enable function tracing for the affected FPU context switching 
> functions?

Or use PT address filters to get it even accurately, as described
in [1]. In any case I agree the trace points are not needed.

-Andi

[1] https://lore.kernel.org/lkml/ZPOIVmC6aY9GBtdJ@tassilo/