linux-kernel - Re: x86/smp: adding new trace points

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Date:   Tue, 12 May 2020 12:39:17 +0100
From:   Wojciech Kudla <wk.kernel@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, x86@...nel.org
Subject: Re: x86/smp: adding new trace points

Hi all,

I was trying to trace some IPIs (remote tlb shootdowns in this case) and noticed that:

1) irq_vectors:x86_platform_ipi_entry and irq_vectors:x86_platform_ipi_exit are not hit at all for my case. The backtrace on the receiving CPU:

0xffffffff81079535	flush_tlb_func_common.constprop.10+0x105/0x220 [kernel]
0xffffffff81079681	flush_tlb_func_remote+0x31/0x40 [kernel]
0xffffffff8111f76c	flush_smp_call_function_queue+0x4c/0xf0 [kernel]
0xffffffff81120253	generic_smp_call_function_single_interrupt+0x13/0x30 [kernel]
0xffffffff81a030c6	smp_call_function_single_interrupt+0x36/0xd0 [kernel]
0xffffffff81a02679	call_function_single_interrupt+0xa9/0xb0 [kernel]

I would expect that we would hit those trace point somewhere around call_function_single_interrupt()

2) there is no equivalent of ipi:ipi_raise for x86. For the following call stack:

0xffffffff81055d10	native_send_call_func_single_ipi+0x0/0x20 [kernel]
0xffffffff8111f86f	generic_exec_single+0x5f/0xc0 [kernel]
0xffffffff8111f9a2	smp_call_function_single+0xd2/0x100 [kernel]
0xffffffff8111fe3c	smp_call_function_many+0x1cc/0x250 [kernel]
0xffffffff8107982c	native_flush_tlb_others+0x3c/0xf0 [kernel]
(...)

I would expect to have a irq_vectors:x86_platform_ipi_raise (or similar) tracepoint.

Are there any reasons my expectations are wrong?
I'd love to submit a patch that addresses these issue but I'd rather get some more context (history maybe) before that.

Thanks,

Wojtek