linux-kernel - Re: [PATCH 0/3] Print CPU at segfault time

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Date:   Thu, 1 Sep 2022 20:15:53 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Rik van Riel <riel@...riel.com>, Borislav Petkov <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...el.com>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH 0/3] Print CPU at segfault time

On Wed, Aug 10, 2022 at 07:49:00PM -0700, Ira wrote:
> From: Ira Weiny <ira.weiny@...el.com>

Any feedback on this series?  Or has Rik's patches been accepted instead?

Ira

> 
> Changes from RFC:
> 	Drop patch 1 as I misunderstood the code and it is not needed for this
> 		use case
> 	Combine patch 2 and 5 into a patch 3 which stores the CPU only for page
> 		faults
> 
> Rik reported that the knowledge of which CPU's are seeing faults can help in
> determining which CPUs are failing in a large data center.[1]
> 
> Storing the CPU at exception entry time allows this print to report the CPU
> which actually took the exception.  This still may not be the CPU which is
> failing but it should be closer.
> 
> Dave and Boris recognized that the auxiliary pt_regs work I did for the PKS
> series could help to store this value and avoid passing the CPU throughout the
> fault handler call stack.
> 
> The patches I sent for RFC had a lot more overhead than is needed to report the
> CPU in a page fault.[2]  After the discussions the generic save/restore of the
> auxiliary pt_regs is overkill for the current use case.  Skip that overhead and
> only store the CPU in the page fault path.
> 
> [1] https://lore.kernel.org/all/20220805101644.2e674553@imladris.surriel.com/
> [2] https://lore.kernel.org/lkml/20220805173009.3128098-1-ira.weiny@intel.com/
> 
> Ira Weiny (2):
>   x86/entry: Add auxiliary pt_regs space
>   x86/mm: Store CPU info on exception entry
> 
> Rik van Riel (1):
>   x86,mm: print likely CPU at segfault time
> 
>  arch/x86/Kconfig                 |  4 ++++
>  arch/x86/entry/calling.h         | 19 +++++++++++++++++++
>  arch/x86/entry/entry_64.S        | 22 ++++++++++++++++++++++
>  arch/x86/entry/entry_64_compat.S |  6 ++++++
>  arch/x86/include/asm/ptrace.h    | 19 +++++++++++++++++++
>  arch/x86/kernel/asm-offsets_64.c | 15 +++++++++++++++
>  arch/x86/kernel/head_64.S        |  6 ++++++
>  arch/x86/mm/fault.c              | 18 ++++++++++++++++++
>  8 files changed, 109 insertions(+)
> 
> 
> base-commit: d4252071b97d2027d246f6a82cbee4d52f618b47
> -- 
> 2.35.3
>