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Subject: Re: [RFC PATCH v2 18/20] context_tracking,x86: Defer kernel text
 patching IPIs
In-Reply-To: <6EBAEEED-6F38-472D-BA31-9C61179EFA2F@joelfernandes.org>
References: <20230720163056.2564824-19-vschneid@redhat.com>
 <6EBAEEED-6F38-472D-BA31-9C61179EFA2F@joelfernandes.org>
Date:   Tue, 25 Jul 2023 14:36:59 +0100
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On 25/07/23 06:49, Joel Fernandes wrote:
> Interesting series Valentin. Some high-level question/comments on this on=
e:
>
>> On Jul 20, 2023, at 12:34 PM, Valentin Schneider <vschneid@redhat.com> w=
rote:
>>
>> =EF=BB=BFtext_poke_bp_batch() sends IPIs to all online CPUs to synchroni=
ze
>> them vs the newly patched instruction. CPUs that are executing in usersp=
ace
>> 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_pok=
e?
> How often does instruction patching really happen? If it is very infreque=
nt
> 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 fr=
om
>> 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().