linux-kernel - Re: [RFC PATCH v2 18/20] context_tracking,x86: Defer kernel text patching IPIs

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Message-ID: <xhsmhtttsru2s.mognet@vschneid.remote.csb>
Date:   Tue, 25 Jul 2023 14:36:59 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
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Subject: Re: [RFC PATCH v2 18/20] context_tracking,x86: Defer kernel text
 patching IPIs

On 25/07/23 06:49, Joel Fernandes wrote:
> Interesting series Valentin. Some high-level question/comments on this one:
>
>> On Jul 20, 2023, at 12:34 PM, Valentin Schneider <vschneid@...hat.com> wrote:
>>
>> text_poke_bp_batch() sends IPIs to all online CPUs to synchronize
>> them vs the newly patched instruction. CPUs that are executing in userspace
>> do not need this synchronization to happen immediately, and this is
>> actually harmful interference for NOHZ_FULL CPUs.
>
> Does the amount of harm not correspond to practical frequency of text_poke?
> How often does instruction patching really happen? If it is very infrequent
> then I am not sure if it is that harmful.
>

Being pushed over a latency threshold *once* is enough to impact the
latency evaluation of your given system/application.

It's mainly about shielding the isolated, NOHZ_FULL CPUs from whatever the
housekeeping CPUs may be up to (flipping static keys, loading kprobes,
using ftrace...) - frequency of the interference isn't such a big part of
the reasoning.

>>
>> As the synchronization IPIs are sent using a blocking call, returning from
>> text_poke_bp_batch() implies all CPUs will observe the patched
>> instruction(s), and this should be preserved even if the IPI is deferred.
>> In other words, to safely defer this synchronization, any kernel
>> instruction leading to the execution of the deferred instruction
>> sync (ct_work_flush()) must *not* be mutable (patchable) at runtime.
>
> If it is not infrequent, then are you handling the case where userland
> spends multiple seconds before entering the kernel, and all this while
> the blocking call waits? Perhaps in such situation you want the real IPI
> to be sent out instead of the deferred one?
>

The blocking call only waits for CPUs for which it queued a CSD. Deferred
calls do not queue a CSD thus do not impact the waiting at all. See
smp_call_function_many_cond().