linux-kernel - Re: [PATCH v6 1/3] x86/fpu: Measure the Latency of XSAVES and XRSTORS

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Date:   Sat, 2 Sep 2023 12:09:10 -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,
        tglx@...utronix.de, 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,
        dave.hansen@...ux.intel.com, yi.sun@...el.intel.com
Subject: Re: [PATCH v6 1/3] x86/fpu: Measure the Latency of XSAVES and XRSTORS

> Instead of adding overhead to the regular FPU context saving/restoring code 
> paths, could you add a helper function that has tracing code included, but 
> which isn't otherwise used - and leave the regular code with no tracing 
> overhead?
> 
> This puts a bit of a long-term maintenance focus on making sure that the 
> traced functionality won't bitrot, but I'd say that's preferable to adding 
> tracing overhead.

Or just use PT

% sudo perf record --kcore -e intel_pt/cyc=1,cyc_thresh=1/k --filter 'filter save_fpregs_to_fpstate' -a sleep 5
% sudo perf script --insn-trace --xed -F -comm,-tid,-dso,-sym,-symoff,+ipc
[000] 677203.751913565:  ffffffffa7046230               nopw  %ax, (%rax)
[000] 677203.751913565:  ffffffffa7046234               nopl  %eax, (%rax,%rax,1)
[000] 677203.751913565:  ffffffffa7046239               mov %rdi, %rcx
[000] 677203.751913565:  ffffffffa704623c               nopl  %eax, (%rax,%rax,1)
[000] 677203.751913565:  ffffffffa7046241               movq
0x10(%rdi), %rsi
[000] 677203.751913565:  ffffffffa7046245               movq  0x8(%rsi), %rax
[000] 677203.751913565:  ffffffffa7046249               leaq 0x40(%rsi), %rdi
[000] 677203.751913565:  ffffffffa704624d               mov %rax, %rdx
[000] 677203.751913565:  ffffffffa7046250               shr $0x20, %rdx
[000] 677203.751913565:  ffffffffa7046254               xsaves64 (%rdi)
[000] 677203.751913565:  ffffffffa7046258               xor %edi, %edi
[000] 677203.751913565:  ffffffffa704625a               movq 0x10(%rcx), %rax
[000] 677203.751913565:  ffffffffa704625e               testb  $0xc0, 0x240(%rax)
[000] 677203.751913636:  ffffffffa7046265               jz 0xffffffffa7046285    IPC: 0.16 (14/85)
...


So it took 85 cycles here.

(it includes a few extra instructions, but I bet they're less than what
ftrace adds. This example is for XSAVE, but can be similarly extended
for XRSTOR)

-Andi